Download Intraday Variability in Quasars and BL LAC Objects

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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
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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
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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
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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
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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.
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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
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INTRADAY
VARIABILITY
IN AGN
169
16.0
14.0
12.0
~°°
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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).
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WAGNER
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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
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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.
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172
WAGNER
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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.
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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.
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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
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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).
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176
WAGNER& WITZEL
1.8
1.4
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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
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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).
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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.
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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.
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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
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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.
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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
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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
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184
WAGNER& WITZEL
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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
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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.
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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
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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.
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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
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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
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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).
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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).
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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
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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).
AnyAnnual
Review
chapter,
as wellas anyarticlecitedin anAnnual
Review
chapter,
mayI~ purchas~l
fromthe Annual
Reviews
Preprints
andReprinls
service.
1-800-347-8007;
415-259-5017;
[email protected]
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