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Annual Reviews www.annualreviews.org/aronline Annu. Rev. Astron, Astrophys. 1995. 33:163-97 Copyright (~1995 byAnnual Reviews Inc. Allrights reserved Annu. Rev. Astro. Astrophys. 1995.33:163-197. Downloaded from arjournals.annualreviews.org by PURDUE UNIVERSITY LIBRARY on 01/16/07. For personal use only. INTRADAYVARIABILITYIN QUASARSAND BL LAC OBJECTS S. J. Wagner Landessternwarte, Ktnigstuhl, 69117 Heidelberg, Germany A. Witzel Max-Planck-Institut fiir Radioastronomie,Auf demHiigel 69, 53121Bonn, Germany KEY WORDS: activegalacticnuclei,interstellarscintillation,jets, radiationmechanisms ABSTRACT Activegalacticnucleiwithflat radiospectraexhibitsignificantvariationsof continuum flux densityontimescales of daysor less throughoutthe entire wavelengthrange.Theserapid variationsdghtlyconstrainthe diametersof the emitting regionsandimplyextremelyhighphotondensitiesif the variationsare intrinsic. At radio frequenciesthe flux densities of compactobjects mayflicker due to interstellar scintillation, andin all frequency bandsmicrolensing bystars in interveninggalaxiesmayintroducevariationsthat are not intrinsic to the source. Wereviewthe characteristicsof these variations in the variouselectromagnetic bands.Extrinsicmechanisms mayaffect the light curvesof compact extragalactic sources,butclose correlationsbetween flares recordedin differentbandsstrongly supportthe assumption that intradayvariability is an intrinsic phenomenon. Theapparentbrightnesstemperaturesin the radio regimeexceed1017K and implyrelativistic beamingwith veryhigh Dopplerfactors, coherentradiation mechanisms, or special geometriceffects. INTRODUCTION Emissions from radio-loud active galactic nuclei (AGN),which are produced predominantlythrough nonthermal processes, vary on time scales from years to less than a day in all frequencyregimes.Causality argumentslimit the radius of the emitting region to r < c. At, with the light travel time At corresponding 163 0066-4146/95/0915-0163505.00 Annual Reviews www.annualreviews.org/aronline Annu. Rev. Astro. Astrophys. 1995.33:163-197. Downloaded from arjournals.annualreviews.org by PURDUE UNIVERSITY LIBRARY on 01/16/07. For personal use only. 164 WAGNER ’& WITZEL to the time scale of variations. Variations on time scales of a day thus provide constraints on the source structure on linear scales smaller than the Solar System. Variability in AGN has been reviewedin this series by Kellermann&PaulinyToth (1968, 1981) and Stein et al (1976). Further reviews by Altschuler (1989, 1990), Bregman(1990), Witzel (1990), Wagner&Witzel (1992), Wiita and Melrose (1994) have incorporated the considerable information gathered in the meantime.This review concentrates on intraday continuumvariations in radio-loud AGN.Most of the sources have been classified as an HPQ(Highly Polarized Quasar), OVV(Optically Violently Variable), or as a BLLac object. Theseclasses are often collectively addressed as blazars, following the suggestion of E Spiegel at the Pittsburgh Conferenceon BLLac objects (Wolfe 1978). Angel & Stockman(1980) describe the properties of blazars. Recent reviews are given by Kollgaard(1994) and in various contributions to related conferences(Maraschiet a11989,Valtaoja &Valtonen1992), the latter focusing entirely on blazar variability. To illustrate the scales probedby fast variability one maycomparethemto the correspondingangular scales obtained with the resolution ofinterferometric -1 imaging. Weassume redshifts z to be cosmological and use Ho= 50 kms -1, q0 = 0.5 for all calculations. For a source at z = 1.0, observed time Mpc scales of 50 h correspond to intrinsic time scales and diameters of 25 h and 3 x 1015 cm(i.e. 10-4 mas) respectively. Ground-basedVLBIhas successfully achieved resolutions of 1 mas and 0.05 mas at 5 and 90 GHzrespectively, corresponding to 8.6 and 0.4 pc at z = 1.0. VLBIobservations have not yet resolved any blazar cores, but give upper limits to the diameters of < 50-100p~as(i.e. 1 pc at z = 1.0). It is importantto note that the small diameters inferred fromthe time scales do not necessarily implya central location of the emitting plasma. Wecover only rapid intensity variations (as defined below) and refer reviews by Altschuler (1989, 1990) and Bregman(1990) for discussions of variability. Wedo not discuss structural variability, whichhas beenaddressedby e.g. Witzel (1992) and Readhead(1993) and in dedicated conferences (Zensus & Pearson 1987, 1990; Zensus & Kellermann 1994). Wealso exclude the rapid variations of Seyfert galaxies seen in the X-ray, UV,and optical regimes. Althoughour survey of the literature extends to August1994, somepreprints and PhDtheses have been included, where appropriate, prior to publication. After introducing somedefinitions, we begin with a few historical remarks. Wethen discuss the state of observational results in the next section. The following two sections address extrinsic and intrinsic mechanismsto explain rapid variations. Weclose by sketching someimplications and future trends. Definitions Intraday variability (IDV) is commonlyused to describe variations on time scales of about one day or less. Herewe use the term liberally (i.e. we refer to Annual Reviews www.annualreviews.org/aronline Annu. Rev. Astro. Astrophys. 1995.33:163-197. Downloaded from arjournals.annualreviews.org by PURDUE UNIVERSITY LIBRARY on 01/16/07. For personal use only. INTRADAY VARIABILITYIN AGN 165 time scales in the observers’ frame as being less than 50 h), because redshifts of several sources are still unknown,prohibiting a conversionof observedtime scales tobs into the sourceframetsouree = tobs/(1 + Z). Variability time scales have been defined in different ways. Typical investigations of rapid variations are rarely long enoughto probe the entire range of temporal frequencies of the power spectrum. Both the most commondefinition of the variability time scale t It = F/(IAF/Atl), whereF is the flux density] and the moreconservative approach (t = IAt/A In FI) (e.g. Burbidge et al 1974)have the advantageof weightingfluctuations by their amplitude. In cases of small amplitudeswe do not use extrapolations but refer to the interval betweensubsequent minimaand maxima.This requires sufficiently dense sampiing with atime step no longer than half the shortest time scale. A conservative approachignoring individual outliers is appropriate. Usingtime scales as a measureof the size of the source, the observedflux maybe converted into photon densities or a brightness temperature ¯ Tb=(4"5× lol°K’F[tob:~ld-+ 2 "z)] wherethe flux density F, wavelengthL, distance d, and time scale t are measured in Jy, cm, Mpc,and days, respectively. IDVin radio-loud sources has only beenseen in objects with flat spectra, i.e. So ~x va, ot > -0.5. Wefollow the nomenclature of radio terminology: The term "low-frequencyvariability" (LFV)is used for radio variability at frequencies below1 GHzwith typical time scales of monthsto years and amplitudesin the range of 3-30%.Theterm ~’flickering" refers to a stochastic variation. In the radio regime, flickering has a typical time scale of days to weeksand amplitudes of a few percent (Heeschen1984). "ESE"(extreme scattering events) radio flares with high amplitudesof up to 100%and a characteristic frequency dependence. These typically showa minimumbracketed by maximaon either side (Fiedler et al 1987, 1994). Historical Remarks Variability in AGNha~ served as a tool to investigate the emission processes in these objects since the ¯discovery of quasars. Matthews& Sandage(1963) derived the first size dstimates of the emitting regions from observations of variable optical intensitieS. Causality argumentsconstrained these sizes to be smaller than several lightweeks (deduced from the variability time scales knownat that time). This implied high energy densities and immediatelyruled out a stellar (i.e. fusion powered)origin of the optical luminosity. Subsequent observations found intraday variations in 3C 279 (Oke 1967). Following the initial identification of the radio source VRO 42.22.01with the "variable star" BLLac (Hoffmeister 1929, Schmitt 1966), similar radio sources were found be violently variable (DuPuyet al 1969, Strittmatter et al 1972)on time scales Annual Reviews www.annualreviews.org/aronline Annu. Rev. Astro. Astrophys. 1995.33:163-197. Downloaded from arjournals.annualreviews.org by PURDUE UNIVERSITY LIBRARY on 01/16/07. For personal use only. 166 WAGNER& WITZEL from days to months. Biraud (1971) identified APLib as a BLLac object and using measurementsfrom Ashbrook(1942) reported variability on time scales of about a day (cf Miller et al 1975). Rapid variations were soon found be a frequent phenomenonin quasars and BLLac objects (cf Kinman1975). Typical variabilities wereon a time scale of the order of days, but there were reports of even faster changesdownto time scales of hours and minutes (e.g. Racine1970, Bertaud et al 1973, Miller et al 1975, Grauer1984). IR variations were reported by Epstein et al (1972) and Rieke et al (1976). Early multicolor data had indicated that the color-luminosity relationship found on long time scales (sources getting redder during periods of decreasingflux) is also valid for intraday variations (Racine 1970, Bertaud et al 1973, Andrewset al 1974). Riekeet al (1977) reports intraday polarization variability in O1090.4. In the radio regime,the discoveryof variations in the intensity of the emission of extragalactic objects by Sholomitskii (1965) and Dent (1965) showed the synchrotronradiation results from a nonstationary emission process. These variations implied that the photondensities in the sources were so high that Compton scattering of the relativistic electrons with the low-energyphotons emitted by them wouldquickly lead to catastrophic cooling and unobserved high X-ray fluxes (Hoyle et al 1966) unless the objects were close by. This led to the suggestion of bulk relativistic motion(Rees 1966, Woltjer 1966). VLBIobservations proved the existence of the postulated compactregions, and the apparent superluminal motionthat was found gave direct observational evidencefor the existence of relativistic motion. Basedon their radio spectra, variations in quasars were attributed to plasmonsthat are initially opaqueat long wavelengthsand becometransparent at higher frequencies. Subsequent adiabatic expansion with constant magnetic flux increases the transparency of the cooling cloud. Whilebecomingoptically thin the plasma emits most of its radiation in the expanding "photosphere" where the optical depth r approaches unity (Shklovsky 1962, Pauliny-Toth Kellerman1966, van der Laan 1966). These models implied that the volumes emitting at radio frequencies must be larger than a few lightweeks (about 1016 cm)and could thus not showintrinsic variability on time scales as short as those present at shorter wavelengths. The long-term radio variations were actually foundto be correlated in the optical/IR/radio bandswith significant time lags (months)(e.g. Bregmanet al 1990), indicating that the radio emission largely comesfrom a different, muchmore extended volume(cf Bregman1990). Radio variability on time scales of 2 days was observed at cm and mm frequencies as early as 1971 (Kinman& Conklin 1971, Andrewet al 1971). Kinman& Conklin (1971), Hackney et al (1972), and Andrewet al (1974) found probable correlations betweenvariations in the.radio regimeand optical range on time scales of days and months, respectively, in OJ 287 and BLLac. MacLeodet al (1971) confirmed rapid radio flares in BLLac but could not Annual Reviews www.annualreviews.org/aronline Annu. Rev. Astro. Astrophys. 1995.33:163-197. Downloaded from arjournals.annualreviews.org by PURDUE UNIVERSITY LIBRARY on 01/16/07. For personal use only. INTRADAY VARIABILITYIN AGN 167 confirmany correlations with optical variability. In a statistical investigation, only a few cases showedpossible correlations with short lags (Balonek 1982). Shapirovskaya(1978) attributed variability at low frequenciesto propagationinducedeffects, i.e. refractive interstellar scintillation, similar to the LFVphenomenafound by Hunstead(1972) and later studied extensively by e.g. Fanti et al (1983). Low-amplitudevariability on time scales of weekswas found in manycompact extragalactic radio sources (Heeschen 1984). Heeschen Rickett (1987) also explainedthis flickering by refractive scattering effects the interstellar medium.The search for flickering on shorter time scales led to the discoveryof rapid intensity variations in extragalactic radio sourceson time scales shorter than or comparableto a day (Witzel et al 1986, Heeschenet al 1987). Quirrenbach (1990) confirmed IDVfor a larger sample of compact, fiat-spectrum radio sources (Figure 1). Variations on intraday time scales have also been established in the X-ray regime(Snyderet al 1980). Variations on time scales of minutessuggested that the X-ray emission is beamedrelativistically as well. Changesin radio-loud sources were successfully modeledin inhomogeneous jet-models with a radial changein cut-off frequency(Georgeet al 1988). I ~ 0 2.0 t I’ I f ,_¢07 I I ~ I I I 0 I 0 ° N ~, m 1.0 ~ 10.o .~ 0.0 ~ -10.0 -20.0 I 1 7328 1 I 7329 I I I ~ 7330 7331 JD - 2,440,000 I ~ 7332 I 7333 FigureI Variations of residualfluxdensity(in percent,filled circles)andpolarization (open circles)in 0917+ 624(seealsoQuirrenbach et al 1989b,Krichbanm et al 1992).Theerrorsare smallerthanthe size of the symbols. In additionto 10% variations ona timescaleof about1 day, fast (undersampled) changes occur,Thereality of spikes(as onJD2447331.1) has beenproven byindependent observations fromdifferenttelescopes. Annual Reviews www.annualreviews.org/aronline Annu. Rev. Astro. Astrophys. 1995.33:163-197. Downloaded from arjournals.annualreviews.org by PURDUE UNIVERSITY LIBRARY on 01/16/07. For personal use only. 168 WAGNER& WITZEL Manyearly reports about rapid variability were received with skepticism. Instrumentalinstabilities (Beall et al 1988)and the inherent nonrepeatability time-critical observationsrequire independentconfirmationsof rapid variations. Independentmeasurementswith different telescopes and instruments or truly simultaneousobservations of constant calibrators (Witzel 1990, Wagner1991) have nowclearly demonstratedthe reliability of changeson time scales of hours. The reproducible detections of IDVtriggered a newdebate about the origin of this type of variability. The most detailed modelsfor the explanation of radio variations are basedon the scenario of shocks propagatingwithin the jet plasma (e.g. Ktnigl & Choudhuri1985, Marscher 1992, Hugheset al 1989). Asthe modelswere originally suggestedfor slow variations, shorter time scales could only be explainedfor optically thin regions if the shocksenter turbulent regions. Source-intrinsic radio IDVseemedimplausible owingto the implied high values of the brightness temperatures Tb, whichare far in excess of the "Comptonlimit" (Kellermann &Pauliny-Toth 1969). Extrinsic (propagation-induced)variability, such as interstellar scintillation (ISS) in the low-frequency regime and gravitational microlensing (e.g. Paczyfiski 1986) have been suggested, although no dependence for the IDV on the galactic latitude could be established (Quirrenbachet al 1992). Also, the frequencydependenceand polarization characteristics could not easily be explained by ISS. Todiscriminate against refractive interstellar scattering (RISS), simultaneous optical and radio campaignswere organized since RISSonly affects low frequencyradio radiation. RESULTS IDV Across the Electromagnetic Spectrum Flat-spectrumradio sources have very broad energy distributions and emit comparable powerper logarithmic bandwidthover a ver_y wide range in frequency. The energy distributions hardly showany humps,which is generally taken as an indication of a unique emission mechanism.One wouldhence expect variations to be observable over a wide range in frequencies as well. Below, we summarizerapid variability in the various frequencybands. GAMMA-EMISSION The gamma-ray regime has become accessible with the advent of the ComptonGammaRay Observatory (CGRO). One of the early results was the discovery that most of the sources detected at energies of 100 MeVto 1 GeVare radio-loud objects. The all-sky survey led to the discovery of 25 blazars (Fichtel et al 1994), most of which are knownto be intraday variable sources in the optical and radio regimes. Conversely,a large fraction of the prominent radio-IDV sources (e.g. 0716 + 714, .0804 + 499) are strong emitters of high-energy gammarays. For manyof them the gamma-ray Annual Reviews www.annualreviews.org/aronline INTRADAY VARIABILITY IN AGN 169 16.0 14.0 12.0 ~°° Annu. Rev. Astro. Astrophys. 1995.33:163-197. Downloaded from arjournals.annualreviews.org by PURDUE UNIVERSITY LIBRARY on 01/16/07. For personal use only. 10.0 e 8.0 6.0 t) 4.0 0.0 ’ 9000 9005 9010 9015 9020 9025 9030 JD - 2,440,000 Figure2 A rapidflare of 100MeV-10 GeV gamma-ray emission(opencircles) and650nm opticalradiation (filled circles)observed in PKS 1406-076. Theflareshavesimilarshapes and timescales.They lastfor18h in thesource frame anddecay asfastas theyrise.Theopticalflare clearlyprecedes thegamma-ray flareby22h (seeWagner et al 1995b). emissions were also found to be variable (Michelsonet al 1994). The early discoveryof strong emission from3C 279 showedan increase of a factor of 2 within2 daysanda subsequentdecaythat wasevenfaster (factor of 2 within24 h) (Kniffenet al 1993). Similar behaviorwasfoundin PKS0528+ 134 (Hunter et al 1993), PKS2251 + 158(= 3C 454.3) (Hartmanet al 1993), and 1633 382, whichall showedflares that rise anddecaywithin 48 h, corresponding to 0.8 day in the sourceframe(Mattoxet al 1993). Noconnectionto variations other wavelengthscould be established. In the case of PKS1406-076,a flare lasting for 53 h in the rest frameof the quasarwasobservedin January,1993. Simultaneous optical monitoringregistered a comparable flare that preceded the gamma-ray outburst by 22 h (Figure 2; Wagner et al 1995a). OtherradioIDVsources showgamma-ray variability on time scales of days (e.g. 0716 714, JR Mattox,private communication). Evenin the bright sources only abouta dozenphotonsare recordedper day, prohibiting studies of IDVunless the amplitudesexceeda factor of a few. In several objects, unusualoptical IDVhas beenreportedat longer wavelengths duringthe detection of gamma-ray radiation (e.g. 0836+ 710, von Lindeet al 1993). Flares mayeven occur at higher photonenergies. Mkn421 was claimed to haveexhibiteda factor of 2.3 increasewithin24 h at 1 TeV(Kerricket al 1995). Annual Reviews www.annualreviews.org/aronline 170 WAGNER & WITZEL Annu. Rev. Astro. Astrophys. 1995.33:163-197. Downloaded from arjournals.annualreviews.org by PURDUE UNIVERSITY LIBRARY on 01/16/07. For personal use only. While none of these blazars were detected from low-energygammarays, the radio galaxy Cen A has been seen to vary on time scales of 12 h by as muchas 50%in the 50-500 keV band (Kinzer et al 1994). x RAYSRapid variations of AGNhave frequently been reported in the Xray regime, where radio-loud and radio-weak objects have shownsubstantial variations on time scales shorter than one day. Rapid X-ray variability in Seyfert galaxies is different fromthat of blazars in its spectral and temporal characteristics. Thesecharacteristics are reviewed by Mushotzkyet al (1993) and are not discussed here. In contrast to the variability characteristics of emission-line AGN, the rapid X-ray variability of blazars has usually been described by low-amplitudefluctuations around a steady meanwith occasional flares (but no sudden drops) superposed (Giommiet al 1990). As seen in the EXOSAT long exposures, BL Lac objects spend only 10%of their time in spectacular flares; the rest of the time they stay within a 50%range of their long-term average. Only a few studies have been carded out for a sufficiently long time to showoscillatory variations with smoothtrends on time scales of about a day, similar to the behavior seen at lower frequencies (Snyderet al 1980). Thesehave been reported in 0716+ 714 by Wagner(1992b) and Witzel et al (1993) and studied in detail in PKS2155-304by Brinkmannet al (1994). Spectral variability is rare within the soft range probedin the recent ROSAT PSPCstudies, but has been seen in the EXOSAT and GINGAobservations. The variability amplitudeis larger at higher photonenergies, implyinga spectral hardeningduring flares (Giommiet al 1990, Sambrunaet al 1993). The scatter arounda linear relationship betweenintensity and spectral index is larger than the errors. In well-studied cases a more complexspectral evolution has been found. Usingall of the high quality data of rapid X-rayvariability of blazars taken by GINGA,Tashiro (1994) suggests systematic spectral evolution with time in the 1-10 keVband. He finds that the sources first becomebrighter and then get a flatter spectrumwith a delay comparableto the rise time before the source becomesfainter and, again delayed by several hours, regains a steeper spectrum. He derives a soft lag of about 1.2 h. Studies of PKS2155-304 by Sembayet al (1993) using GINGAin the 2-45 keV band and observations of Mrk 421 by Thomas& Fink (1994), however, indicate a more complex evolution. Thepowerdensity spectra are not sampledsufficiently well to decide whether the oscillatory variationsare related to the fast flickering that occurswithina few hours or less. Both phenomena occur in the same objects (e.g. PKS2155-304, 0716÷ 714). Early reports about variations on very short time scales (1 s) Agrawal& Riegler (1979) were not confirmed in later experiments by Snyder et al (1980). Variations on time scales of 30 s were found by Feigelson et (1986) in H0323+ 022. Similar results have been reported by Kohmuraet Annual Reviews www.annualreviews.org/aronline Annu. Rev. Astro. Astrophys. 1995.33:163-197. Downloaded from arjournals.annualreviews.org by PURDUE UNIVERSITY LIBRARY on 01/16/07. For personal use only. INTRADAY VARIABILITYIN AGN 171 (1994), Morini et al (1986) (a factor of 4 increase in 4 h in PKS2155-304), Remillard et al (1991) (a 60%flare in 200 s in PKS0558-504), and in 0716 714 by Wagneret al (1995c) (a 60%increase within 500 s). The variations those four objects (including the radio-weak H0323+ 022 and PKS2155-304) are so fast that relativistic beamingof the X-ray emissionis required in order not to violate the Eddingtonlimit (Elliot &Shapiro 1974). The extremelyfast changesmaybe related to the injection and acceleration of particles (e.g. Lesch 1992, Kirk 1992, Kirk & Mastichiadis 1992). uv Ultraviolet-variability has been studied from short campaignsand from combined,unrelated observations on longer time scales. Blazar measurements were compiledby Edelson (1992). Maraschiet al (1986) reported variations PKS2155-304and OJ 287 on time scales of 1 and 2 days, respectively. Edelson et al (1991) reported variations of PKS2155-304on time scales of a few hours in the 1600-2600/~band. Evidence for spectral variations remainedmarginal in most IUE campaigns. Low-amplitudevariability (15%) on time scales 0.04 dayswasfound in the far UVat 100/~ by Marshall et al (1993). Urry et al (1993) discuss quasi-periodic (-’~0.7 d) oscillations in PKS2155304, reminiscentof the behaviorof 0716+ 714in the radio and optical regimes, and suggest spectral evolution with a lag between1400to 2800/~shorter than the samplinginterval. Again,an increase in flux is accompaniedwith (delayed) spectral hardening. OPTICAL Early monitoring efforts on short time variations have been reviewed by Kinman(1975) and Angel & Stockman(1980). For long-term monitoring campaignssee e.g. Webbet al (1988). Mooreet al (1982) found photometric and polarimetric variations of BLLac over a period of one week, with 0.1 mag variations throughout individual and betweensubsequent nights. PKS2155-304 showed oscillations of about 1 day (Smith et al 1992, Courvoisier et al 1995). In four-week campaigns, 0954 + 658 showedwelldefined, rapid and symmetricflares with exponential slopes (Wagneret a11993), whereas0716 + 714 exhibited oscillatory variations (Wagner1991). With few exceptions (e.g. PKS2155-304;Smith et al 1992), BLLac objects showspectral changes(toward the blue) during brighter stages (but see Miller 1981). Miller et al (1989), Carini (1990), and Carini et al (1990) report flares time scales of a few hours (downto rise times of 10 min in 0.05 magflares). Searchesfor periodicities are discussed below. Xie et al (1994and references therein) studied intraday variations in miscellaneous BLLac objects and radio-loud quasars. Heidt & Wagner(1995) find optical IDVin 80%of the sources in a complete sample of BLLac objects (Stickel at al 1991). While time scales do not correlate with luminosity redshift, the fractional IDV-amplitudes increase with luminosity. Annual Reviews www.annualreviews.org/aronline Annu. Rev. Astro. Astrophys. 1995.33:163-197. Downloaded from arjournals.annualreviews.org by PURDUE UNIVERSITY LIBRARY on 01/16/07. For personal use only. 172 WAGNER & WITZEL Thereis only a little evidencefor rapid variations in radio-quiet QSOs(e.g. Gopal-Krishna et al 1993) and steep-spectrum radio sources. Lyutyi et al (1989), Dultzin-Hacyan et al (1992), and Miller &Noble (1994) found variations in radio-weakSeyfert galaxies. It is unclear whetherthis is related to the rapid variations in radio-loud objects, or whetherit represents the high (temporal) frequencyend of the continuumvariations in Seyfert galaxies that are related to thermalvariability of the accretion disk. The polarization properties and their temporalvariations have been reviewed by Angel &Stockmah(1980) and Saikia &Salter (1988). The polarization OVV and BLLac objects is generally high (>3%).Variability of polarization the optical domainhas beenfound for almost all well-studied, rapidly variable blazars. Total and polarized flux are not correlated (Smith et al 1992). The existence of preferred polarization angles is controversial. Hagen-Thorn& Yakovleva(1987) concludedthat preferred polarization angles are visible during quiescent periods, while the overall polarization during active periods varies randomly.The ratio of steady polarized flux (probably from the moreextended jet component)to variable polarized flux during outbursts dictates the chances of discovering preferred polarization angles in individual sources. Mooreet al (1982, 1987) have investigated polarimetric variations in Lac downto time scales of a few minutes. Theyconclude that the path in the Stokes plane is generally a randomwalk downto time scales of 30 min. They were also able to trace a fast swing in the Stokes plane (and back). Similar observations in other sources imply that the random-walkcharacteristics may be due to undersamplingof rapid polarization variability. Spectral variations of the polarization and the polarization angle as well as temporal variation of the frequency dependenceof the polarization have also beenreported (e.g. Holmeset al 1984). For additional references, see Saikia Salter (1988). Further workon variations of frequency-dependentpolarization have been reported by Takalo(1991), Meadet al (1990), and Sillanp~i~i et (1991). IR Extensionsof optical studies to the near infrared showsimilar amplitudes and time scales, and in somecases a trend of sources getting bluer as they brighten (e.g. Lorenzetti et al 1990, Takaloet al 1992, Kidgeret al 1994). Reports of very rapid (1 min) IR flares could not be confirmed(Wolstencroft et al 1982; A Sillanp/iii, private communication). In the mid-IR, IRASmeasurements demonstrated variability time scales of 15-233 days (Impey & Neugebauer 1988 and references therein). IDVsearches in the mid-IRare planned for the ISOsatellite. RADIO Early reports for short time scale radio variability were madefor BL Lac (elg. Harvey et al 1972), 3C 273 (e.g. Efanovet al 1977), Cen A (e.g. Kellermann 1974), IIIZw 2 (Huchtmeier & Wright 1973), and OJ 287 (e.g. Annual Reviews www.annualreviews.org/aronline Annu. Rev. Astro. Astrophys. 1995.33:163-197. Downloaded from arjournals.annualreviews.org by PURDUE UNIVERSITY LIBRARY on 01/16/07. For personal use only. INTRADAY VARIABILITYIN AGN 173 Andrew et al 1974, Epstein et al 1972, Kikuchiet al 1973, Dreheret al 1986, de Bruyn1988). The observations up to 1980are discussed in Epstein et al (1980). Simonettiet al (1985) reported the most rapid low-frequencyvariability. Witzel et al (1986) and Heeschenet al (1987) established radio IDV frequent phenomenon amongthe sources from the S5-radio sample (Kiihr et al 1981a). This sampleis confined to a small region on the sky (3 > 70°), but an extension to a larger range of galactic coordinates showedno dependenceon bn (Quirrenbachet al 1992). Subsequentefforts of the Bonngroup focused on the completesampleof 75 flat-spectrum sources from the 1-Jy catalog (Kiihr et al 1981b), which are north of 3 = 30° (Wagner& Witzel 1992, Wegner1994). Of these, 47 have thus far been monitored for IDV, at least once. Thirteen additional objects, selected as IDVcandidates based on compactnesshave been added. Repeated observations have been performed for 17 of the 18 sources north of 60° (0454 + 844 is missing because it has been too weakduring the last ten years). Below,wesummarizethe observational status of this project. IDVprevails in objects containing80%of their flux density in a central region, which is unresolved even on milliarcsecond-scales. Multi-epoch observations have shownthat IDVseems to occur repeatedly during a time span of at least six years, whereasthe amplitudeof variations maychange(e.g. Witzel 1992). Thefirst-order structure function of the variations Ol(r) = ([R(t + r)- 2), wherethe average is taken over pairs with timelag r and R(t)=lOOx [e(t) L(F) -1% is the residual flux density, can be used to distinguish betweentwo types of variable objects: TypeI varies on scales significantly longer than 50 h and type II varies on scales less than 50 h (Heeschenet al 1987). About25%of the compactfiat-spectrum radio sources can be expected to exhibit type II IDVin the radio regime, while one third will not vary (Quirrenbachet al 1992). The largest amplitudes(up to 35%)in this samplewere found in 0804+499 6 cm wavelength(Quirrenbach et al 1992). 0716 + 714 and 0917 + 624 varied at the 10-20%level in the cm-regime(Witzel 1992). The highest variability amplitudes found to date (45%, with a time scale of 2.5 h) were claimed Romeroet al (1994) for the blazar 0537-441at 21 cm wavelength. 0917 + 624 and 0716 + 714 showquasi-periodic oscillations. Transitions from one dominantIDVtime scale to another were found in 0716 + 714 (e.g. Quirrenbachet al 1991). The fastest changes have been seen in 0917 + 624 on time scales <1 h (Quirrenbach et al 1989b). 0716 + 714 and 0804 + 499 have repeatedly shownsignificant jumpsat the samplingrate of 2 h. Anexampleof a very fast change, which has been proven by independent observations from different telescopes, is shownin Figure 1. Annual Reviews www.annualreviews.org/aronline Annu. Rev. Astro. Astrophys. 1995.33:163-197. Downloaded from arjournals.annualreviews.org by PURDUE UNIVERSITY LIBRARY on 01/16/07. For personal use only. 174 WAGNER& WITZEL The amplitudes of IDVare radio-frequency dependent. In the quasar 0917+ 624 the maximum variability amplitude occurs at 11 cm without significant time lags (Krichbaumet al 1992). In contrast, the blazar 0716 ÷ 714 showed variability with similar amplitudesin the wavelengthrange 3.6-11 cm(Wegner 1994) in five observing epochs, with a significant time lag of 0.2 day in one epoch measured between 3.6 cm and 6 cm. The variability at the observed frequencies was obviously correlated. IDVat mmfrequencies has recently been found in PKS0405-385 by Wagner (1995). Rapidradio polarization variability has been found in long-term monitoring programsby Aller et al (1981). Theyreported swingsin polarization angle BLLac in the cmregime within a few days. Even more rapid changes of the polarized flux density--by about 15%within less than one day--at constant polarization angle were found in 0716 + 714 by Turlo et al (1985) at 6 cm. In the BLLac object 0735 + 178, Gabuzdaet al (1989) found a drop in the polarized flux density of the VLBIcore by 40%within 24 h. Saikia & Salter (1988) provide a reviewof polarization variations. Systematic investigations of polarization-IDV began in 1988 (Quirrenbach et al 1989a). To date, the total numberof sources observed is 24, with the majority of observations performed at 6 cm wavelengthand a few additional campaigns at 2.8 and 11 cm (e.g. Wegner1994). In general, the sources showingintraday variability of the total intensity also showvariations of their polarized flux density and of polarization angle. The percentage changein the polarized intensity seemsto be larger than in the total intensity. Generally, the polarization is below 5%with the exceptions of 0836 + 710 (’-9%) and 0954 + 658 (’,~8%), whereas the best-observed sources 0716 + 714 (1-4%) and 0917 + 624 (1-2%) are weakly polarized. During all epochs of observations between 1988 and 1993, 0716 ÷ 714 showedvariations of its polarized flux density. In each case the variations of the polarization intensity werecorrelated with those of the total intensity at all wavelengths(Wegner&Witzel 1993, Wegner1994). In contrast, the variations of the polarized intensity in 0917+ 624 were anticorrelated with those of the total intensity variability in four out of five cases. Once(in May1989), both quantities showedcorrelated variations (Wegner1994). The variations of the polarization angle once showeda 180°-swing(Quirrenbach et al 1989a) at the time of minimalpolarized intensity. Wegner(1994) claimed that the frequency dependenceof the polarization angle X in the 2-10 cmregime is variable on time scales of the order of one day. This implies that dx/d~, is not due to Faradayrotation. Healso finds that d~(/d~, changessign during these oscillations. BROAD-BAND PROPERTIES The close similarities between variations in different frequency bands imply very similar radiation mechanisms.Physical Annual Reviews www.annualreviews.org/aronline Annu. Rev. Astro. Astrophys. 1995.33:163-197. Downloaded from arjournals.annualreviews.org by PURDUE UNIVERSITY LIBRARY on 01/16/07. For personal use only. INTRADAY VARIABILITYIN AGN 175 correlations are expected from most modelsof radiation mechanismsin blazars (see e.g. Bregman1992, Marscher1993, and references therein). Similarities betweendifferent radio bandson long time scales are well established and can be explained in the adiabatic shockmodel(see e.g. Hugheset al 1989). Correlations with variations on shorter wavelengthshave been incorporated successfully in these modelsby Bregmanet al (1990 and references therein). Rapidvariations are closely correlated throughoutthe radio frequency range. Bridging the IR gap, correlations betweenradio and optical variations have already been reported by Kinman&Conklin (1971). Wagneret al (1990) found a statistical correlation of radio and optical IDVin simultaneousobservations. In 1990, close radio-optical correlations were found in 0716+ 714 (Figure 3; Quirrenbach et al 1992, Wagneret al 1995c). Due to the oscillatory nature of the variations, neither the lag nor the sign of the lag can be determined unambiguously. The simultaneous change between a fast modeand a slower one (Figure 3) suggests a short lag. The lag betweenoptical and radio variations at any given frequency has, however, only limited physical meaning. Wagneret al (1995c) also find a close correlation betweenthe optical variations and the changes of the radio spectral index 6 em ¯ t~3.6 cm (Figure 4). Thedirect correlation betweenoptical flux and either of the radio bands showsthat the correlation with radio spectral index cannot be explained by a superposition of one constant radio componentand one that changes only in flux but not in spectral index. Thespectral index variations are due to a component that either varies rapidly in optical depth or has a variable cut-off. The radio spectrum 7925 7930 7935 7940 Jib - 2,440,000 7945 Figure3 Correlated radioandopticalvariabilityin $507164- 714.The6 cmradiodata(open circles)and650nmopticallight curve(filled circles)show nearlyidenticalpatterns.Oscillatory IDVontimescalesof about1 dayprecedes variabilityonlongertimescales(about6 days).The change in modes occursnearlysimultaneously, indicatinga closecorrelationover5 decadesin wavelength in the low-frequency regime(seeQuirrenbach et al 1991,Wagner et al 1995c). Annual Reviews www.annualreviews.org/aronline 176 WAGNER& WITZEL 1.8 1.4 Annu. Rev. Astro. Astrophys. 1995.33:163-197. Downloaded from arjournals.annualreviews.org by PURDUE UNIVERSITY LIBRARY on 01/16/07. For personal use only. 1.0 O O 0 O0 QQQo0 00000 O0 00 0 O0 O0 Q O 0 0 0 0 OOo - 7925 0 O 0 -0.4 000 O 0 o Q I I : I "1 I , ~ I 7926 7927 JD - 2,440,000 0 o - 7928 7929 ? Figure 4 Correlations of the radiospectral-index %6.~mcm (opencircles)withopticalflux(650rim; filled circles)in 0716÷ 714.Thevariationsare in phasewithoutanymeasurable lag (seeWagner et al 1995c). of 0716 + 714 is remarkably fiat out to 1 mmon average (Bloomet al 1994). The variations in spectral index are confined to a small frequency band. IDV of the 6 cmflux is common in this object, but it is not alwayscorrelated with similar variability of the spectral index - 6em tg3.6 cm" If the spectral changesare due to variable optical depth, secular variations of the transparencymaychangethe turnover frequencyvt. This mayimply that the lowest frequencythat is still correlated with optical variations maybe different in different campaignsand that correlations of optical flares with those at frequenciesas low as 5 GHzmay be rare. Sources with pronouncedindividual flares such as 0954 + 658 have also exhibited variations of radio spectral index (Wegner1994). Correlations with optical flares are indicated in short campaignsbut could not be claimed unambiguouslyduring longer runs of simultaneous observations (Wagneret al 1993). Most blazars showsimple combinationsof pure powerlaws and contributions from a host galaxy throughout the optical/near-IR window(e.g. Falomoet al 1993). It is therefore not surprising that variations at optical and near-IR frequencies are usually well correlated (Kidger & de Diego1992). A similarly close correlation has been suggested in the optical-UVwindow in OJ 287 (Hanson &Coe 1985) and established for PKS2155-304 (Edelson Annual Reviews www.annualreviews.org/aronline Annu. Rev. Astro. Astrophys. 1995.33:163-197. Downloaded from arjournals.annualreviews.org by PURDUE UNIVERSITY LIBRARY on 01/16/07. For personal use only. INTRADAY VARIABILITYIN AGN 177 et al 1991, 1995; Urry et al 1993). A close correlation betweenUVand X rays in this source was reported by Brinkmann et al (1994). For a large fraction their multiwavelength-observingcampaignthe X-ray variations lead the UVby 2 hours. Additionaldeviations wereseen at the very early and very late stages of the campaign(Figure 5). In high energy bands, correlations have mostly been studied on longer time scales, but Wagneret al (1995a)found a rapid simultaneousflare in PKS1406076 during optical and gamma-raymonitoring. In additionto direct correlationsof flux densities in different frequencybands, a statistical increase of the amplitudeof optical variability with radio spectral index has also been found (Hall &Usher1973, Pollock et al 1974). Kikuchiet al (1988) reported broad-bandcorrelations of polarization variations. Aclose link of optical polarization to the unresolvedradio core is also indicated by the identical polarization angles foundin simultaneousoptical and VLBIpolarization measurementsby Gabuzda&S itko (1994). Characteristics of Variability TIMESCALES Blazars showdramatic variations on all time scales, but few objects have been studied well enoughto determine the powerdensity spectra or structure functions over morethan two decades in time. The range covered Figure5 Simultaneous intradayvariabilityin PKS2155-304 at optical(500nm;crosses),UV (140nm;circles), andX-ray(2.5 rim;boxes)wavelengths. Aclosecorrelationover2.5 decades in energyin the high-frequency regimeis obvious.Notethe delayof 2.5 h between the UVand X-rayvariations(seeUrryet a11993,Brinkmann et a11994, Edelson et a11995). Annual Reviews www.annualreviews.org/aronline Annu. Rev. Astro. Astrophys. 1995.33:163-197. Downloaded from arjournals.annualreviews.org by PURDUE UNIVERSITY LIBRARY on 01/16/07. For personal use only. 178 WAGNER& WITZEL is different in the various frequencyranges. In long-term radio studies IDVis usually at the extreme high (temporal) frequency end, whereasX-ray studies rarely provide structure functions on longer time scales. Exceptionsare a few radio monitoring programs(e.g. Waltmanet al 1991) and X-ray studies that combinedata fromdifferent satellites. At optical wavelengths IDVusually reaches a large fraction of the total historical range in brightness. Thestructure function levels off on scales longer than 0.1-0.5 years. In 0716-t- 714 IDVamountsto a few tens of percent of the total historical range in luminosity (over 10 years) in radio, optical, and X-ray studies. ManyIDVsources reach a broad maximum in the powerdensity spectra on scales of a fewdays. Thereis no statistical study on the wavelength dependenceof the maximaof powerdensity spectra. Theshortest well-resolvedfeatures havebeen discoveredin the X-ray regime, and havetime scales as short as 100s. At optical frequenciesvariations as rapid as 1000 s have been reported. In the radio regime the shortest well-resolved variations have been of the order of a few 103 s (e.g. de Bruyn 1988). all cases, the shortest variations have beeneither limited by the samplingtime scale or by the amplitudesreaching the noise level (if a smoothextrapolation of the structure function to higher temporal variations applies). The apparent frequencydependenceof the most rapid time scales is due to photonstatistics and measurementaccuracies. It is not clear whetherIDVis closely related to long-term variations. The close correlations with very small lags claimed in multifrequencystudies argue against IDVbeing the high-frequency end of the same physical mechanism responsible for long-term variations. This maybe investigated by searching for breaks in the structure functions. Althoughthese are less sensitive to the particular observing windowsthan power-density spectra, they can give misleading results if the samplingrate dependson the brightness of the object (as is the case in manymonitoring programs). Strictly periodic variations also have been claimed. In OJ 287periodicities of about 30 min were repeatedly reported in optical and radio measurements (e.g. Visvanathan &Elliot 1973) but not confirmed by Kiplinger (1974) Dreheret al (1986). Kidgeret al (1992) listed the periodicities found in 287. They range from about 360 s to 11.5 years. Hardly any study claiming a periodicity is sampledboth well enoughto resolve an individual event and long enoughto cover several periods. Intraday variability in 0917+ 624, 0716 + 714, and PKS2155-304has been foundto be in the form of quasi-periodic oscillations, whichare well resolved and occur at constant amplitudesfor 5 to 8 cycles. Theseoscillations introduce clear maximain the powerdensity spectra but are not strictly periodic. It is possible, however, that fundamentalperiodicities are superposed by random variations or subject to secular changes. Annual Reviews www.annualreviews.org/aronline Annu. Rev. Astro. Astrophys. 1995.33:163-197. Downloaded from arjournals.annualreviews.org by PURDUE UNIVERSITY LIBRARY on 01/16/07. For personal use only. INTRADAY VARIABILITYIN AGN 179 The amplitudes of IDVvary considerably. Extreme cases amount to 30% variations at 5 GHz,and factors of 2 at optical and X-ray wavelengthswithin hours and minutes, respectively. Quasars of high absolute luminosity do not seem to show IDVas often as BLLac objects. There is at present no strong indication that IDVhappens preferentially during specific states of averageintensity. Well-observedsources have shownperiods with and without IDVboth during phases of bright or faint averagelevels of flux. Defininga duty cycle of variability as the fraction of time a source spends morethan 3tr awayfrom its weeklyaverage, the rate of activity can be quantified. Statistical investigations of the IDVsources in the $5 samplein radio and optical domainsyielded duty cycles varying from 1 in 0716 + 714, which is never constant, to 10-3 in 0836+ 710. Lowerduty cycles are possible but the coverage obtained by ongoing monitoring campaignswould not have detected the IDVof such sources. FLARESHAPESHigh duty cycles introduce considerable blending, which complicates the analysis of flare shapes. In a fewcases individual outbursts are well separated and resolved and the shape can be studied unambiguously.Wagner et al (1993) found that in 0954 + 658 such individual optical flares have exponential rise and decay with equal e-folding times. In manyother cases individual events are blendedto the extent that the light curve resemblesrandomvariations around somemeanbrightness level. In all well-sampledcases, however,the distribution of deviations AI from a meanintensity (I) is skewed toward positive values, demonstrating that outbursts are more common than drops in intensity. This is true in the radio, optical, and X-ray regimes. The more symmetricdistributions of steady comparisonsources rule out noise biases. Clear, negative excursions have only been found in campaigns with insufficient samplingor total lengths of less than a fewtimes the characteristic time scale. Wagneret al (1995c) showthat in 0716+ 714 the distribution of AI/(I) is skewedin different bins of At, whichindicates that the individual flares are asymmetricin time. This methodcan be applied in spite of the blending of different outbursts. The characteristics oflDVexhibit a large variety in individual objects. Since few studies have been carried out in well-defined samples, it is unclear at present whetherthe different temporaland spectral characteristics are due to peculiarities of individual sources; whetherthey are correlated with e.g. jet speed or viewing angle; whether the whole variety may be due to secular variations (e.g. long-term changes of the duty cycle); or whetherthey merely reflect the different temporalextent, samplingrate, and accuracyof individual investigations. Furthermore,different processes (both intrinsic and extrinsic) maydominatein different sources. Annual Reviews www.annualreviews.org/aronline Annu. Rev. Astro. Astrophys. 1995.33:163-197. Downloaded from arjournals.annualreviews.org by PURDUE UNIVERSITY LIBRARY on 01/16/07. For personal use only. 180 WAGNER & WITZEL SPECIFICSOURCES Intraday variability has been discovered in more than 50 objects in the optical and radio regimes. Fromthe abovedescriptions it is clear, however, that only a few sources have received adequate coverage in multifrequencystudies. Statistical investigations do not yet allow us to judge whetherthese objects are really extraordinaryin their characteristics or whether their prominenceis a pure coincidence. Interestingly, 0716+ 714, whichis one of the brightest BLLac objects throughoutthe entireelectromagnetic spectrum, had hardly been studied until the late 1980s. While a numberof objects have been observed in great detail in someparts of the electromagnetic spectrum (BLLac, Mrk421, 0954+ 658, 0917+ 624), three objects have an outstanding record of multifrequency coverage: $5 0716 + 714 $5 0716 + 714 is the only blazar with IDV throughout the entire wavelengthregime. It has a very compactVLBIstructure and is a prototypical radio-selected BLLac object. Rapid variations on time scales of a few hours have been observed at 5 GHzby Quirrenbach et al (1989b). The object has a very high duty cycle of variability and providedthe clearest case yet for correlated radio and optical variations (Quirrenbachet al 1991). Its variability characteristics are discussedin detail in Wagner et al (1995c). OJ 287 Takalo (1994) collected the various investigations of this prominent source, which was one of the first BLLac objects to be studied extensively for variability properties. Becauseit is such a weaktarget for high-energy observations, variability studies of it concentratedmostly on frequencies lower than 1015Hz. PKS2155-304 First detected as a radio source, PKS2155-304 soon became the first BLLacobject to be identified on the basis of its strong X-rayemission, and is a prototypical X-ray dominatedBLLac object. Its low radio flux has not been investigated for IDV. Moststudies have been carried out throughout the optical/UV/X-rayregime. A series of papers discuss a detailed multifrequencycampaignorganized in 1991: Urry et al (1993), Brinkmannet al (1994), Courvoisieret al (1995), and Edelsonet al (1995). EXTRINSIC EXPLANATIONS Rapidvariations require very extremeconditions within sources if the variations are indeed intrinsic. Alternatively, the variations maybe caused by sourceextrinsic mechanisms.These maynot only introduce variability or modifyits pattern but also alter the spectral behavior. Bothvariable absorption along the line of sight as well as deflection of light maycausesuch variations. In the latter case the deflection mayeither be due to the gravitationally distorted metric or Annual Reviews www.annualreviews.org/aronline Annu. Rev. Astro. Astrophys. 1995.33:163-197. Downloaded from arjournals.annualreviews.org by PURDUE UNIVERSITY LIBRARY on 01/16/07. For personal use only. INTRADAY VARIABILITYIN AGN 181 to refractive or diffractive effects of the galactic plasmaalongthe line of sight. Motionof the source, the observer, or the medium causing the deflection leads to temporalvariations of the flux density registered by the observer. Beforeembarking on a discussion of possible source-intrinsic explanations of the rapid variations, extrinsic causes have to be investigated. Variable absorbingscreens have been discussed by Marscher(1979). High-frequencyradio variations cannot be explained by variable absorption. Likewise,the spectral characteristics discussed in the previous section wouldbe inconsistent with variable absorption at optical or shorter wavelengths. Discrimination against other propagation-inducedorigins of the variations are mostly based on the frequencydependenceof these effects: While gravitational microlensingis achromatic,interstellar scintillation only affects observations at lowerradio frequencies. Interstellar Scintillation Density inhomogeneitiesof electrons of the interstellar medium(ISM)on different spatial scales introducediffractive and refractive scattering, resulting in angular broadeningof compactsources and apparent variability (interstellar scintillation). The principles of interstellar scintillation (ISS) havebeen viewedrecently by Rickett (1990). Melrose(1994) reviews previous attempts to explain radio variability by scattering. The size of the Fresnel scale determines whether scattering is weak(with vanishing fluctuations in phase) strong. In the strong regime both diffractive (DISS) and refractive (RISS) interstellar scintillation mayoccur. Dramaticcases oflSS are the extremescattering events discussed by Fiedler et al (1987); these events are most likely due to isolated, compactregions of enhancedelectron density. Shapirovskaya (1978) pointed out that low-frequencyvariables lie preferentially behindlargescale galactic features, and suggested that their behavior could be explained by strong (refractive) scattering. Specific searches for DISSproducednegative results (Condon& Dennison 1978, Dennison & Condon1981, and references therein). Signatures for RISSmaycomefrom the statistical occurrenceof variability, whichshould be related to the distribution of scattering matter as well as to the spectral shape. Interstellar scintillation has a significant frequencydependence. The scaling dependson the spectrum of fluctuations in the refracting ISMand follows a v-2’2 decrease for a Kolmogorov spectrum of the correlation scale. Testing the hypothesisthat radio variations are due to interstellar scattering is therefore most easily done by studying the frequency dependencedownto very short wavelengths. Followingthe first reports of flickering by Heeschen(1984), dedicated searches towardlow galactic latitude (e.g. Dennisonet al 1987) were carried out. Heeschen&Rickett (1987) confirmedthat flickering is correlated with galactic latitude. Annual Reviews www.annualreviews.org/aronline Annu. Rev. Astro. Astrophys. 1995.33:163-197. Downloaded from arjournals.annualreviews.org by PURDUE UNIVERSITY LIBRARY on 01/16/07. For personal use only. 182 WAGNER & WITZEL Several sets of observations of rapid variability were modeledin terms of RISS. At least in the case of 0954+ 658, RISSin the form of extremescattering events (At ,~ 5.d) is undoubtedlypresent (Fiedler et a11987).Wambsganss et (1989) were able to reproduce the observed time scales at 6 cmand 11 cm normal IDVin a numerical simulation within the single-screen approximation of B landford &Narayan(1992). S imonetti (1991 ) reproducedrapid swings ¯ .~ the polanzatl0n angle of 0917 + 624 by assuming a two-componentstructure with different, polarization properties. Theseapproachesconsidered isolated, highly refractive structures. Rickett et al (1995) emphasizethat at centimeter frequenciesthe distinction betweendiffractive and refractive scattering vanishes and discuss a detailed model of the variations of 0917 + 624 by "normal" RISSdue to turbulence. They have been successful in fitting the frequency dependence of 0917 ÷ 624 during a short campaign with a two-component model. Anumberof problemswith ,the hypothesisof interstellar scintillation remain: Simonetti et al (1985) confirmedthe existence of flickering on time scales a few days in manycompactsources but conclude that the intensity ratios at 1.4 and 2.4 GHzcannot be fitted by simple modelsof refractive or diffractive scintillation. The modelof Rickett et al (1995) does not fit the amplitude the polarization variations, without invoking several independentcomponents. Likewise,the correlation of total and polarized fiux seen in 0716+ 714 cannot be reproduced. 0917 ÷ 624 showeddifferent frequency dependencies during different campaignsand a changeof sign of d x/d)~, whichcannot be explained by a two-componentmodel. In general, detailed predictions of interstellar scattering havebeenunable to reproducethe observedcharacteristics. This implies that our understandingof the interstellar medium is still rather poor (Spangleret al 1993). Problemsoccur in the case of variations at higher frequencies. Certainly, IDVin the mmregime (Wagner 1995) cannot be caused by RISS. The most convincing argumentto exclude RISSas the dominantmechanismfor the variations is the intimate correlation betweenthe radio and optical bands. Refractive interstellar scattering cannot produceany variations at optical wavelengths.The close correlation and identical powerspectra at optical and radio frequencies wouldrequire a source that varied intrinsically at 1014 Hz with a preferred time scale, while at the same time being subject to RISS-inducedvariability at radio frequencies, such that the latter had the samepowerspectrumas seen in the optical regime (Wagner&Witzel 1992). Whereascross-correlations mayyield accidental similarities if IDVwere a common property without very source-specific characteristics, the very fine-tuned correlations foundin wellresolved studies are unlikely to be a coincidence. Althoughthe correlation is hamperedby the quasi-periodic nature of the variations, the case against scintillation-induced variations is strongest in 0716 + 714, which shows a Annual Reviews www.annualreviews.org/aronline Annu. Rev. Astro. Astrophys. 1995.33:163-197. Downloaded from arjournals.annualreviews.org by PURDUE UNIVERSITY LIBRARY on 01/16/07. For personal use only. INTRADAY VARIABILITYIN AGN 183 correlation betweenradio spectral index and optical brightness (Figures 3 and 4; Wagneret al 1995c). If the variations even at 10 GHzwere due to RISS, the angular broadening of the interstellar scattering wouldlimit the spatial resolution achievablewith space VLBIat these frequencies. Nevertheless, RISS-inducedvariations are undoubtedly present in the IDVsource 0954 ÷ 658 (Fiedler et al 1987). Any source of the high degree of compactnessimplied by the rapid intrinsic variability is boundto be subject to RISS-induced variability as well. If the source is sufficiently compact,it is expectedto be susceptibleto diffractive scattering, whichhas not beenobservedso far. However, a statistical analysis of other fiatspectrumsources exhibits clear indications for a minorcontribution of IDV,as described by Witzel &Quirrenbach (1993). Thestrong case for close correlations betweenradio and optical variations in at least one sourcerules out the possibility that all of the rapid radio variations are due to interstellar scattering. Separating themfromother variations is a major problemand can only be tackled by campaignsthat are well sampled in time and frequency coverage. Further searches for DISSand rigid constraints on the properties of the ionized interstellar mediumare required. MicroIensing Chang&Refsdal (1979) suggested that gravitational lensing of distant quasars wouldnot onlybe causedby the deflection of light by the potential of intervening galaxies, but mayalso be causedby the fine-scale structure of the potential well, i.e. fromindividual stars within galaxies that are sufficiently close to the line of sight. Relative motionof source, lens, or observer will result in variations of the amplificationfactor of gravitational lensing, and hencecausevariability. Nottale (1986) and Schneider&Weiss(1987) had suggested that BLLac objects mayactually be nonvariable backgroundquasars, whosecontinua are amplified by gravitational lensing. This increases the continuumflux; microlensingthen introduces variations (see also Barnothy 1965). The stronger enhancement of the continuumwith respect to the line emission of the quasar (caused by the muchsmaller size of the continuum-emitting region) wouldalso explain the small equivalent line widths of BLLac objects. The state of theoretical understanding and empirical evidence is illustrated by various contributions in Surdej at al (1993). Since the effects of microlensingare achromatic,it temptingto investigate whetherthe variations occurring throughouta wide span in frequency could be caused by microlensing. Wagner(1992a) has argued that the intraday variations of sources such 0716÷ 714 are unlikely to be caused by microlensingfor several reasons: 1. The variability has a very high duty cycle. ManyIDVsources are almost always active. Lensing can only introduce permanent variations in the optically thick case. In optically thick gravitational lenses the combined Annual Reviews www.annualreviews.org/aronline 184 WAGNER& WITZEL Annu. Rev. Astro. Astrophys. 1995.33:163-197. Downloaded from arjournals.annualreviews.org by PURDUE UNIVERSITY LIBRARY on 01/16/07. For personal use only. potential of the microlensingstars is so large that it mustcause macrolensing. Optically thick lensing can hence only occur in objects that have multiple macroimages.This has not been observed in any IDVsource. 2. The variations are too fast to be causedby stars movingin front of a small, steady continuum source. In the microlensing theory the observed time scales are related to the relative transverse velocity and the lens massby °’5 5 × 1016cm. A t = v-ltrans (MIMe3) 3. In order to explain the observed variability on the very short time scales (At < 1 d), the required velocities V~ansare relativistic if the lenses have M> 0.01Mo.Such velocities are only possible in the source-plane where relativistic motions have been found (Gopal-Krishna& Subramanian1991). However,the required multiple ejection of superluminal knots within the source implies intrinsic variations, whichreduce the potential role of microlensing to an amplification of intrinsic events (see also Schramm et al 1993). 4. Other information--such as the absence of obvious lensing foreground galaxies; the statistical asymmetry of light curves; the particular shape of individual, unblendedevents; and lags betweendifferent bands comparable to the intrinsic widths of flares--cannot be reconciled with the microlensing scenario. While none of them is conclusive alone, the overall indications argue against microlensingplaying a major role in IDV. Giventhe high rate of occurrenceof rapid variations in blazars, it is obvious that any discrimination betweenintrinsic and lensing-inducedeffects will be very difficult. Whatevermechanismcauses IDV, its power density spectrum is likely to extend to lower temporal frequencies where microlensing could becomeimportant. Note that most of the sources reported as candidates for microlensing have shownintraday variations. Dueto the compactsource structure, microlensingmayalso occur in IDVsources, but such objects are not the ideal targets to searchfor this effect. If microlensingby objects of substellar massesdominatesin a particular case, simultaneousmultifrequencyobservations wouldalso be a useful wayto study quasar structure at very high spatial resolution. Rauch&Blandford(1991) and Wambsganss & Paczyfiski (1991) have demonstratedthe potential diagnostics of multifrequencyobservations in such cases. Several rapidly variable sources have been suggested as candidates for various propagation-induced variations. PKS0537-441, claimed to be a source with extreme brightness temperatures (1021 K), has been discussed in terms of interstellar scattering by Romero et al (1994) and in terms of gravitational lensing by Stickel et al (1988). However,the lensing hypothesishas been questioned by Narayan& Schneider (1990) and Falomoet al (1992). This compact Annual Reviews www.annualreviews.org/aronline Annu. Rev. Astro. Astrophys. 1995.33:163-197. Downloaded from arjournals.annualreviews.org by PURDUE UNIVERSITY LIBRARY on 01/16/07. For personal use only. INTRADAY VARIABILITYIN AGN 185 southernblazar exhibits all the characteristics of intrinsic variations and has been knownto be rapidly variable at optical frequencies for over 20 years (Eggen 1973). It is an IDVsource at mmand optical wavelengths (Heidt Wagner1995) and variable in the X-ray regime (Treves et al 1993). Thompson et al (1993) report high gamma-ray flux densities. Neither interstellar scattering nor microlensingcan causeall of these characteristics. Intrinsic variability implies very compactsource structures, which wouldincrease the amplitudes of additional contributions by extrinsic mechanisms. INTRINSICVARIABILITY" If the variations are intrinsic to the source, changesin observedflux correspond to significant changes in luminosity. For a typical fiat-spectrum IDVradio source of a quiescent luminosity of 1046.5 erg/s, a factor 2 flare lasting for 24 h requires a total energy of 1051 erg (assumingisotropic emission). The variations haveslightly different amplitudesin different wavelengthregimesbut correspond to the broad-bandenergy distributions of the average state within less than an order of magnitude(see Figure 6). The changes in intensity maybe comparedwith the brightness temperatures Tbderivedin the radio regime,whichare up to 1021K in the case of the flares of 0537-441reported by Romero et al (1994), 1019K in individual rapid variations 12.0 11.0 10.0 $5 0716+714 9.0 8.0 , , 1 I I I I 15 log . [Hz] i I i 20 I I I I 25 Figure 6 Broad-bandenergy distribution of $5 0716+ 714. Thick lines indicate flux densities and spectral slopes in the various energy bands in a contemporaneousset of measurements.The shadedrangesindicate the typical amplitudeof IDV.Thebars indicate the total rangeof variability recorded throughout 1980-1993. Annual Reviews www.annualreviews.org/aronline Annu. Rev. Astro. Astrophys. 1995.33:163-197. Downloaded from arjournals.annualreviews.org by PURDUE UNIVERSITY LIBRARY on 01/16/07. For personal use only. 186 WAGNER & WITZEL on time scales of hours at a few GHz(de Bruyn1988, Quirrenbachet al 1992), and 1017 K in the oscillatory variations of the IDVobject 0917 ÷ 624 and other northern fiat-spectrum radio sources (Quirrenbachet al 1989a, Witzel Quirrenbach 1993). Variations in the mmregime on scales of weeksimplied brightness temperatures up to 1014K (Ter~ranta &Valtaoja 1994), comparable to limits from low-frequencyvariables (Singal &Gopal-Krishna1985). Space VLBIexperiments have been used to infer T0 as high as 3 x 1012 K (Linfield et al 1990). All of these are muchin excess of the brightness temperatures inferred from the fast optical variations, whichare of the order of 10a K. Since short time scales imply small emitting volumes, it has often been assumedthat the flares arise from the central region. Anobvioussite wouldbe the accretion disk postulated in the standard AGN model. Thermaldisk models fail to explain the close similarities of the variations seen in the optical, UV, and X-ray regimes, as demonstrated in the case of PKS2155-304(Urry et al 1993, Edelsonet al 1995). Modelsbased on spots, flares, or disturbances in accretion disks have been suggested by Abramowiczet al (1991), Mangalam & Wiita (1993), and Chakrabarti &Wiita (1993). Details of those models have been reviewed by Wiita (1994). The accretion disk models are able explain someof the phenomenaseen in the optical to X-ray range but cannot explain radio intraday variablility. TheX-rayspectral variability and the convex spectral shape in the X-ray regimestrongly support the hypothesis that the Xray emission up to a few keVis dominatedby synchrotron emission and hence likely to vary with synchrotronflux at lowerfrequencies in a coherent manner. Theclose correlations call for a modelthat explains the broad-bandvariations. RadioIDVintroduces the strongest constraints to such an explanation. The high brightness temperatures inferred from the small diameters violate the Compton limit of 1012K of incoherent synchrotron emission (Kellermann&Pauliny-Toth 1969) by a large amount. Followingthe original suggestion of Rees (1966) Woltjer (1966) that relativistic beamingintroduces anisotropic emission, one can derive a minimumDopplerfactor 73 = [1" (1 - fl cos0)] -1 (with Lorentz factor 1~) that wouldbe required to reduce the observedbrightness temperatures by ~)-3. Relativistic beamingis suggested in these sources also on other grounds. Flat-spectrum radio sources often showsuperluminal motion, implying Doppler factors as high as 20. The X-ray luminosities are lower than predicted from Compton scattering in isotropic sources (Marscheret al 1979, Ghisellini et al 1993). X-ray variability has occasionally been found with AL/At values in excess of the Eddington limit (Fabian 1979). Gamma-ray fluxes are too high to be compatiblewith ), y pair absorption, if they were emitted isotropically (Jelley 1966, McBreen1979). In these cases, moderate Dopplerbeaming(79 20) can explain the observations. This wouldconfirm the upper limit on the Dopplerfactors suggested from several considerations discussed by Phinney Annual Reviews www.annualreviews.org/aronline Annu. Rev. Astro. Astrophys. 1995.33:163-197. Downloaded from arjournals.annualreviews.org by PURDUE UNIVERSITY LIBRARY on 01/16/07. For personal use only. INTRADAY VARIABILITYIN AGN 187 (1987). Radio IDV, however, wouldrequire Doppler boosting up to 79 = 200 to explain Tb = 1019K. The high Dopplerfactors derived abovesuggest that the plasmaemitting the variable flux flows with relativistic speeds in the synchrotronjet. Correlations betweenstructural variability within the jet (i.e. the emission of newknots) and flux-density variations have been suggested by Babadzhanyants&Belokon (1986) in 3C 345 and were later found in other sources as well by Mutelet (1990) and Krichbaumet al (1990). A close link of intensity variations radio structure is also suggestedby the similarities betweenpolarization characteristics found in VLBIstudies and preferred polarization angles of monitoring observations. Establishing correlations betweenvariability in flux and radio structure in IDVsources will be difficult, but maybe possible if the duty cycle is lowenoughto separate knots related to separate outbursts. The closest jet in M87has been resolved on scales of a few lightweeks. The structure seen there is compactenoughto vary coherently within a month. Shocks in Jets In-situ acceleration within jets is required to explain the existence of highly relativistic particles at radii wherethe light travel timefromthe nucleusis orders of magnitudelarger than the radiative lifetime of the particles. This provides strong constraints especially for fast simultaneousoptical/X-rayvariations. The absence of strong spectral evolution in the X-ray regime, wherethe radiative lifetimes are shorter than a second, implies continuous in-situ acceleration (Celotti et al 1991). The most commonsuggestion involves acceleration in shock fronts (Blandford & K6nigl 1979). This model has been workedout in further detail by Jones and coworkers(1988 and references therein), Marscher(1980), Marscher&Gear (1985), and Melia &Kt~nigl (1989). Shocksin turbulent will lead to variations of the local emissivity and produce outbursts. Dueto compressionof magneticfields in the shocks, these variations will also lead to variations in polarization and polarization angle, as discussed by Hugheset al (1985) and K6nigl &Choudhuri (1985). These models have been used explain the simultaneous swing in radio and optical polarization by Kikuchi et al (1988). Rosen(1990) modeledthe variations caused by an axisymmetric shockintersecting a helical filament within the jet to explain slowvariations. Thin shocks can lead to a very rapid rise. The frequency-dependentduration of the flare corresponds to the energy-dependentcooling length behind the shock front. As illustrated e.g. by Marscher(1992), the flare will rise at high frequencies and then subsequently moveto lower frequencies before decreasing again. Valtaoja (1991) has remarkedthat the turnover frequencies at which the flares reach maximum developmentvary over more than a factor of 100. He distinguishes high-peakingflares and low-peakingflares, dependingon the observing frequency and the underlying source spectrum. Annual Reviews www.annualreviews.org/aronline Annu. Rev. Astro. Astrophys. 1995.33:163-197. Downloaded from arjournals.annualreviews.org by PURDUE UNIVERSITY LIBRARY on 01/16/07. For personal use only. 188 WAGNER & WITZEL These shock-in-jet modelssuffer from a numberof problems in explaining IDV.It is not clear whetherthin, planarshockscan actually travel for a significant length within a turbulent, highly relativistic flow. Turbulenceis suggestedto explain the high duty cycle of someof the objects, but cannot be reconciled with the self-similarity of flares observed e.g. in 0954+ 658 (Wagneret al 1993). Interference maylead to spurious periodicities for a few cycles, but cannot explain the oscillations seen in 0917 + 624 and 0716 + 714. Neither the time scales nor the frequency dependenceof the radio variations can be explained. This is especially a problemfor close correlation of the optical and radio variations, whichare hard to reconcile with the expected difference in cooling length behind the shock front. As pointed out by Wagneret al (1995c), the identical width of the autocorrelation functions from radio and optical light curves implies a similar spatial extent of the optical- and radioemitting volumes. This discrepancycould be resolved in several ways: If the short time scales are due to light-echo or lighthouse effects, the emitting volumesare larger and the brightness temperatures are lower than deducedfrom observations. If geometrydoes not play an important role, the constraints on the Dopplerfactor could be relaxed if the observedradiation is coherent, thus avoiding the Compton limit of 1012K. If the emissionis indeed due to incoherent radiation, then those parts of the jets causing the rapid variations maybe Poynting-flux dominated. This wouldavoid the Comptondrag and could explain larger Doppler factors. Modifications due to Geometry Geometricaleffects mayintroduce variations or alter amplitudes, time scales, and flares considerably. Wagner(1991) pointed out that the symmetricshapes of flares in 0954 + 658 indicate a geometrical modulation. The outbursts are self-similar with exponential slopes of constant e-folding time (Wagneret al 1993). It is importantto understandthe accidental (geometric) modulations separate them from physical mechanisms. The simplest geometry-inducedvariations are light-echo effects as suggested e.g. by Lynden-Bell(1977). Theselead to phasevelocities larger than the speed of light and large underestimatesof the dimensionsof the illuminated emitter. Light-echo effects in the context of shock-in-jet modelshave been suggested by Qian et al (1991) to describe the rapid variations in 0917+ 624. Marscher (1992) discusses the variations due to pattern speeds in general and illustrates the severe constraints imposedby the suggestions of Qian et al, whichrequire an extraordinary focusing and a very special geometrical configuration. Since IDVwith comparablebrightness temperatures is common in fiat-spectrum radio sources, a moregeneral modelis required. Camenzind(1991) argues that shocks are unlikely to travel along the jet axis. Camenzind(1992) and Camenzind& Krockenberger(1992) contend Annual Reviews www.annualreviews.org/aronline Annu. Rev. Astro. Astrophys. 1995.33:163-197. Downloaded from arjournals.annualreviews.org by PURDUE UNIVERSITY LIBRARY on 01/16/07. For personal use only. INTRADAY VARIABILITYIN AGN 189 knots of enhancedparticle density are injected at a finite jet radius. There, every volumeelement of plasma has finite angular momentum and moveson a helical trajectory due to the superpositionof the outflowand a circular motion. In knots movingrelativistically on helical trajectories, the direction of forward beamingvaries with time. For an observer close to the jet axis the beam sweepsacross the line of sight, introducingflares due to the lighthouse effect as in pulsars. This will lead to quasi-periodicvariations of a fewoscillations, and maycause complexlight curves if the contributions of several knots are superposed.A similar effect results from the purely geometricmodelof GopalKrishna &Wiita (1992). VLBIobservations of curved trajectories of individual knots, sometimesaccompaniedby changes in angular velocity, are interpreted as an indication that the plasmais indeed movingon three-dimensionalhelical trajectories (e.g. Krichbaumet al 1993). Schrammet al (1993) and Wagner et al (1995b)used these mod.els to explain long-term variations of 3C 345 and PKS0420-014.It is unclear whetherthe quasi-periodic oscillations of IDVcan also be explainedby this model.In particular, it is unclear whetherthe assumed constant acceleration can be provided. Radiation Mechanisms In the early 1970sit was demonstratedthat the characteristics of radio-loud quasars could be explained by incoherent synchrotron emission (Jones et al 1974, Burbidgeet al 1974). Relativistic (anisotropic) induced Comptonscattering (Sincell &Krolik 1994, Coppi et al 1993) and additional refinements (Ghisellini et al 1993) have nowbeen included into the workingmodel(e.g. Hughes 1991). Coherent emission from plasma oscillations (Ginzburg Ozernoy1965) is not limited to brightness temperatures of 1012 K and has been suggested as an alternative radiation process (Baker et al 1988, Benford 1992, Lesch &Pohl 1992). Melrose (1991) reviews collective plasma radiation processes; application to IDVis also discussed in Melrose(1994). Other alternative radiation processes havebeen discussed by Cockeet al (1978), who studied incoherent synchrotron emission in the small-pitch-angle approximation and ripple radiation. Coherentcurvature radiation has been investigated by Cocke & Pacholczyk (1975) and Colgate &Petschek (1978 and references therein). Someof the early problems, such as strong (unobserved) circular polarization could be solved (see Benford1992), but induced Comptonscattering mayprevent coherent radiation from escaping (Rees 1992, Coppi et al 1993). Slysh (1992) suggested that high brighmess temperatures mayoccur during flares from monoenergeticelectrons, but his modelneg!ects Compton scattering (see Ghisellini et al 1993). Constraints on the radiation mechanismsare given by the broad-bandnature of IDV. The smoothvariation of the broad-bandpolarization from centimeter wavelengthsto the optical regime indicates that the entire spectrum of flatspectrumradio sources is due to one single radiation mechanism (Rudnicket al Annual Reviews www.annualreviews.org/aronline Annu. Rev. Astro. Astrophys. 1995.33:163-197. Downloaded from arjournals.annualreviews.org by PURDUE UNIVERSITY LIBRARY on 01/16/07. For personal use only. 190 WAGNER & WITZEL 1978,Bregman 1992). CorrelatedIDVas seen in simultaneousradio/optical andoptical/gamma-ray campaigns indicates that the variable contributionsare as broad-band as the averageenergydistributions. Sinceit is unclearwhetherintradayvariability is related at all to the very fast flares (timescalesless thanhours)or to long-termvariations,the radiation mechanisms responsible for the different phenomena need not be the same. Thefastest variationson timescales of a fewminutesup to an hourhavelower amplitudesthan the oscillatory IDV.Brightnesstemperaturesderivedfromthe fast changesdonot exceedthose of the IDV.Nevertheless,suchvariationspoint towardsignificant changesof emissivity within volumesof a few AU.Such rapid flares maybe directly linkedto the injection andaccelerationprocesses. In 0716+ 714 fast flares havebeenseen in all frequencybands, but these havenot beenstudied in simultaneouscampaigns.If fast optical flares are counterpartsto the rapid X-rayflickering, it is unclearwhethertheycanbe used to constrainaccretingblackhole models on the basis of timescale vs luminosity relationships(Elliot &Shapiro1974,Fabian1979). Variationson long time scales (months)showdelayedcorrelations between optical and radio bands (Hufnagel&Bregman 1992, Bregman 1992and referencestherein). Thelow-frequency emissionlags the optical changesby several months.In BLLac and OJ287 radio-optical correlations havebeenclaimed with short (IDV)and long lags. Derivinglags on time scales of months rapidly variable sourcesis, however,not unambiguous. Bothcorrelations may be operatingin a jet that is either deceleratingoutwardsor bendingawayfrom the line of sight. Correlatedvariationson differenttimescales wouldbe dueto volumes at differentdistancesalongthe jet. Compact Jets Theclose correlation betweenthe structure of the M87jet in the optical and radio regimeson scales of 20 to 100pc (Sparkset al 1994)wouldcorrespond to similarities in the variationson scales of a fewyearsif this jet werefast enoughand seen underan angle correspondingto a Dopplerfactor of 20. If the similaritiesstill exist onsmallerscales(Junor&Biretta1994)in faster jets, they couldcorrespondto correlationson IDVtime scales. In deceleratingjets veryhigh Dopplerfactors mayoccurclose to the nucleus providedthat they are Poynting-fluxdominated. Begelman et al (1994)explain the high brightness temperaturesby refinementsof the general relativistic jet modelandfind that Dopplerfactors as high as 79 ,~ 100maybe possible. Thejets needto be very inefficient synchrotronemitters, carryinglarge amountsof kinetic energyandPoyntingflux, andwouldrequire hydromagnetic acceleration.This modelfits the spectral indexvariationsseenin 0716+ 714qualitativelyandpredictsspecific polarization properties, whichhavenot beentested so far. Theyalso predict large gammaray fluxes fromCompton scattering of ambientradiation (Sikora etal 1994). Annual Reviews www.annualreviews.org/aronline INTRADAY VARIABILITYIN AGN 191 Problemswith this scenario include the upperlimit of the brightness temperature (1017K), whichis violated by the mostrapid flares at 5 GHzby 0804+ 499 (Quirrenbach et al 1992) and the observed correlations of optical/gamma-ray variations (Figure 2). Annu. Rev. Astro. Astrophys. 1995.33:163-197. Downloaded from arjournals.annualreviews.org by PURDUE UNIVERSITY LIBRARY on 01/16/07. For personal use only. CONCLUSIONS AND PROSPECTS Intraday variability is a common phenomenon in blazars and has been detected in different sources in all frequencybands from1 GHzup to energies of 1 GeV. This implies that a significant fraction of the total luminosityis emitted from within a volume that is muchsmaller than 200 AUor 0.1 mas. IDVmaybe influenced by extrinsic mechanisms,which are achromatic (microlensing), have a steep frequency dependence(interstellar scattering). The small diameters implied from the variability time scales indicate that somecontribution to variability from extrinsic mechanisms is very likely---especially at low frequencies. At least one IDVsource (0954 + 658) has varied on time scales weeksdue to an extreme scattering event. However,the correlations between optical and radio bandsimplythat intraday variability in general cannot be due to extrinsic mechanisms, A class of very compactsources exists, which showscorrelated, intrinsic IDVthroughout the entire spectrum (e.g. 0716 + 714). Althoughthe optical depth approachesunity in the GHzregime, IDVis still recorded. The resulting correlation of spectral index with flux in the optically thin rangeunderlines the intrinsic interpretation. It also showsthat the close correlation betweenthe 6 cmand the optical data dependssensibly on the actual photosphereand cannot be expected to be seen in every campaign.Quirrenbachet al (1989b) had noted that the relationship of variations between11 cmand 6 cmdiffers in campaigns separated by a few months. The physical mechanismsresponsible for intrinsic IDVare not yet known. High Dopplerfactors and a very special geometric situation are probably required to explain brightness temperaturesup to 1019K, but realistic treatment of the radiative processes is also important. The most rapid variations may be directly caused by acceleration mechanisms of the radiating particles. The radiation and reprocessing mechanismsare nonstationary. The relationship to rapid variations in the gamma-ray regimeis of particular importance. Toclarify the low-frequencylimit whereintrinsic IDVis dominatedby variations due to interstellar scattering, long and well-sampledmultifrequencycampaigns throughout the optical, (sub)mm,and cmregimes are required. Insight into the physical processes is expected particularly from broad-bandmonitoring throughout the optically thin part of the entire wavelengthrange. This requires concertedefforts with spacecraft and provides unique opportunities to study quasar structure at high resolution, In somecases intensity variability will probemuchsmaller spatial scales than structural variability studied with Annual Reviews www.annualreviews.org/aronline Annu. Rev. Astro. Astrophys. 1995.33:163-197. Downloaded from arjournals.annualreviews.org by PURDUE UNIVERSITY LIBRARY on 01/16/07. For personal use only. 192 WAGNER& WITZEL VLBI, as is illustrated by the close link between optical and radio morphology in the M87jet on larger scales. Intraday variability studies also have implications for unification scenarios, Quasars and BL Lac objects both show IDV, but with different duty cycles. Radio-loud AGNexhibit IDV, while radio-weak ones do not show this behavior (~vith the exception of a few Seyfert galaxies, but these are most likely due to another mechanism). Classical BL Lac objects and X-ray selected BL Lac sources have similar X-ray variability properties (Giommiet al 1990) but differ in the optical band (Heidt 1993). Irrespective of the detailed mechanism, intr~day variability in quasars and BL Lac objects provides an important tool for studying variations on very small spatial scales. There is currently no other means at hand to achieve similar resolution, so increasing our reliance on this tool depends solely on improving our understanding of the phenomena involved. Intraday variability does not only enable studies at high resolution: The photon densities, spectral energy distributions, and polarization properties of flares are very similar to the characteristics of the average state of emission from blazars and hence provide clues for a deeper understanding of the radiation processes in these extreme incarnations of active galactic nuclei. ACKNOWLEDGMENTS Weare grateful for comments, preprints, and advice from manycolleagues. The help of JA Zensus and A Kraus was invaluable. We would like to acknowledge discussions with M Camenzind, A Quirrenbach, A Marscher, TP Krichbaum, IIK Pauliny-Toth, JR Mattox, P Schneider, U Borgeest, S Kikuchi, and A Sillanp~i~i. Our ownobserving programs benefited from the help of the technical staff at the Landessternwarte Heidelberg, the 100-m telescope of the MPIfR in Effelsberg, the VLA, and the Calar Alto Observatory. SWacknowledges support from the DFG(SFB 328). 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