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The transient universe • Hill: Detections by Fermi Explosive sources of the highest energy radiation The transient universe Adam Hill reports on the vast variety of transient systems detected by NASA’s Fermi Gamma-ray Space Telescope. A stronomers cannot help but be famil- board Fermi is the Large Area Telescope (LAT), iar with electromagnetic radiation: it an electron–positron pair production telescope is our bread and butter, the primary using solid-state silicon trackers and caesium source of information we have for discovering iodide calorimeters, sensitive to photons from how the universe ticks. The bulk of the radia- ~20 MeV to greater than 300 GeV (Atwood et al. tion observed is produced through thermal pro- 2009). Essentially it is a particle physics deteccesses; “hot things” produce it, typically with tor in space, sensitive to photons with energies a blackbody continuum spectrum. However, 100 billion times greater than optical light. the extremes of the EM spectrum Figure 1 shows the sky as seen by offer the potential to explore the the Fermi LAT at energies above Fermi 1 GeV, in which the large highnon-thermal universe. And, in was launched contrast to the majority of energy source population is in 2008 and has the EM spectrum, it is only self-evident. in the past couple of decades The primary channels to discovered around that sensitive gamma-ray produce gamma-rays at these 2000 distinct telescopes have opened our energies can be split into hadgamma-ray eyes to how the sky looks at ronic and leptonic processes. sources the highest possible energies. In the hadronic scenario, proNASA’s Fermi Gamma-ray Space tons are accelerated to high energies and then collide with each other, Telescope is one such facility. The telescope was launched in June 2008 and has been producing neutral pions that subsequently decay operating successfully for over five years, mak- into two gamma-ray photons or a gamma-ray ing numerous exciting discoveries including the photon and an electron–positron pair. In the detection of ~2000 distinct gamma-ray sources leptonic scenario, electrons are the accelerated (Nolan et al. 2012). The primary instrument on- particles that then inverse Compton scatter off ‘‘ ’’ A&G • December 2013 • Vol. 54 1: A map of the gamma-ray sky at energies above 1 GeV incorporating five years of survey observations from the Fermi Large Area Telescope. The map is oriented in galactic coordinates with the galactic plane clearly running through the centre of the image. (NASA/DOE/Fermi LAT Collaboration) photons (e.g. from a nearby star or the cosmic microwave background), boosting the photons to gamma-ray energies. In both scenarios particle acceleration is required, which necessitates extreme environments such as the jets of a black hole, colliding shock fronts or extreme magnetic fields such as those found around pulsars. These conditions can be produced in a wide variety of astrophysical objects: from the Sun, to binary systems in our galaxy, to extragalactic objects such as gamma-ray bursts. Flaring high-mass X-ray binaries There is a small population of high-mass X-ray binary (HMXB) systems that are known to be high-energy gamma-ray sources. The majority of these systems are persistent emitters, but two systems have been observed by Fermi to be flaring sources that are, most of the time, undetectable by current instruments. Incidentally, 6.31 The transient universe • Hill: Detections by Fermi 2: This diagram illustrates the view from Earth of the LS 2883/PSR B1259-63 binary system as well as key events in the pulsar’s December 2010 periastron passage. (NASA/Goddard Space Flight Center/Francis Reddy) these two systems are the only members of the detected HMXB population for which the nature of the object is known: the microquasar Cyg X-3 and LS 2883, a binary system that hosts the pulsar PSR B1259-63 and a main-sequence companion. Cyg X-3 is a well-known, powerful HMXB with a short orbital period of 4.8 hours. The system comprises a compact object accreting matter from a Wolf–Rayet companion star (van Kerkwijk et al. 1992). It regularly becomes the brightest radio source among known binary systems, with large flares that are attributed to its relativistic jets. The system exhibits a complex X-ray spectrum that fluctuates between two main states: “soft” and “hard”. The source is known to flare in radio when entering the “soft” state with associated relativistic jet ejection events (Fender et al. 2006). Historically there were reported detections at energies between MeV and PeV in the 1970s and 1980s. However, the detections were typically of low significance and although some experiments claimed detections, others could not confirm them. Hence, claims of high energy and very high energy emission from this microquasar were controversial and highly contested. The identification of Cyg X-3 as a source of gamma-rays was put beyond a shadow of a doubt by the independent detection of flares 6.32 from the source by the Fermi and AGILE missions. The LAT detected a 29σ point source at a location consistent with that of Cyg X-3 and at the same time as a radio outburst was detected. The association of the gamma-ray flaring source was confirmed through the detection of the 4.8-hour orbital period in the LAT light curve (Abdo et al. 2009). The gamma-ray light curve of the source flux indicates that the source is highly variable. The initial Fermi detection reports on the identification of two specific active periods that comprise one or more flares: 11 October to 20 December 2009 and 8 June to 2 August. Assuming a distance of 7 kpc, the average luminosity of Cyg X-3 in outburst at energies above 100 MeV is ~3 × 1036 erg s –1. The AGILE mission also reported detecting gammaray activity coincident with the location of Cyg X-3 during these epochs as well as activity from 16–17 April 2008, before the launch of Fermi (Tavani et al. 2009). The correlation between the gamma-ray and radio emission specifically links the gammaray activity to periods of relativistic ejection events. In this case, the gamma-ray emission could be explained by inverse Compton scattering of UV photons from the Wolf–Rayet star off high-energy electrons. However, this scenario requires that the emission region is not too close to the accretion disc, otherwise the gamma-ray emission would be absorbed through pair production on soft X-ray photons coming from the disc. Gamma-ray emission was again detected from Cyg X-3 in 2011 during a giant radio flare (Corbel et al. 2012). However, analysis of this outburst also identified detectable gamma-ray activity associated with much weaker radio flaring immediately prior to the system entering into a radio-quenched (ultrasoft X-ray) state. This has been interpreted to indicate that transitions into and out of the radio-quenched state trigger gamma-ray emission, implying a connection to the accretion process and the production of relativistic jets. LS 2883/PSR B1259-63 is a very different type of system. The binary consists of a ~47 ms pulsar orbiting a massive Be star in a highly eccentric orbit with a period of ~3.4 years (Johnston et al. 1994). Unpulsed radio, X-ray and TeV gamma-ray emission has been observed from the system during periastron passage when the pulsar is closest to the Be star and can interact with the dense circumstellar disc. Although this source was observed by EGRET in the 1990s, there was no GeV detection prior to the launch of Fermi. Consequently the December 2010 periastron passage of this system, the first in the Fermi era, was hotly anticipated. While the system was far from periastron A&G • December 2013 • Vol. 54 The transient universe • Hill: Detections by Fermi 3: Integral flux above 100 MeV as a function of time during the April 2011 Crab flare; mean bin duration is 9 minutes. The dotted line indicates the sum of the 33-month average fluxes from the inverse Compton nebula and the pulsar. The dashed line shows the flux of the average synchrotron nebula, the inverse Compton nebula and the pulsar. (Figure 5 from Buehler et al. 2012) there was no detection with the Fermi LAT. surprise when the Fermi and AGILE missions From ~28 days prior to periastron through to reported detections of powerful flares from the 18 days after periastron, the LS 2883 system was Crab Nebula at energies above 100 MeV (Abdo observed to brighten in gamma-rays as expected et al. 2011a, Tavani et al. 2011). The Crab Nebula is the remnant of a superand yielded a ~5σ detection (Abdo et al. 2011b). During this period the average >100 MeV flux nova explosion reported by Chinese astronoof the system was 0.9 × 10 –10 erg cm –2 s –1. What mers in AD 1054. At the heart of the nebula is was completely unexpected was that 30 days a pulsar that loses rotational energy through a after periastron passage the system exhibited a relativistic electron/positron wind that injects much more powerful flare, peaking at ~10 times energetic particles into the nebula. The Crab higher than the integrated average flux observed Nebula can be detected at wavelengths from during the first disc passage. This flare contin- radio all they way up to TeV energies; below ued for seven weeks, with an average 400 MeV the spectrum is dominated flux of 4.4 × 10 –10 erg cm–2 s–1. Figure by synchrotron emission, whereas This 2 illustrates the system geometry the highest energy radiation is produced via inverse Compand the location of the pulsar is an exciting during the two gamma-ray result as it breaks the ton scattering of photons off active periods. relativistic particles. axiom that pulsars The gamma-ray emission Since the original reports of are steady gammafrom the system is believed flares by Fermi and AGILE, ray sources to be produced in the shock the Crab has been observed to flare at high energies on front formed from the interacaverage once a year. Remarktion of the pulsar wind with the stellar wind of the Be companion. ably, there is no sign of simultaneous While the detection of gamma-ray emission variability at any other wavelengths despite by Fermi was anticipated, the highly variable numerous multi-wavelength follow-up probehaviour was not predicted in any model of grammes. The brightest reported Crab flare gamma-ray emission expected from this system. to date occurred in April 2011 (Buehler et al. The unexpected, strong flare seen at GeV ener- 2012); the source was seen to double its flux in gies was not detected at any other wavelength the space of 8 hours and reached a peak flare despite a highly organized multi-wavelength flux of 1.86 × 10 –10 erg cm –2 s –1, a factor of 30 campaign, and it took place 30 days after peri- increase in the average value of the nebula flux astron passage and after the pulsar had passed (a factor of 7 when including both pulsar and through the dense equatorial disc for the second nebula flux)! Figure 3 shows the dramatic rise in time. Possible explanations for the behaviour the Crab flux during the April 2011 flare. Upon include anisotropy of the gamma-ray emission, further analysis it has been shown the gammaan abrupt change in the physical environment ray emission from the pulsar during the flare is or the emitting region of the emergence of a new constant, indicating that it is some part of the nebula that is variable. emission component (Abdo et al. 2011b). Isolated rotation-powered pulsars observed One of the most amazing results from these at high energies are renowned for their steady, flares, other than the complete lack of detectnon-variable emission. And in the case of the ability at other wavelengths is the rapid timeCrab Nebula and pulsar it is one of the earli- scales of variability and the associated power of est and best-studied high-energy objects whose the emission. From simple causality arguments, constancy has been relied upon as a “standard a doubling time of eight hours implies a comcandle” to calibrate generations of high-energy pact emission region of less than 2.8 × 10 –4 pc space missions. Consequently, it came as a huge and the isotropic power at the peak of the flare ‘‘ ’’ A&G • December 2013 • Vol. 54 is ~4 × 1036 erg s –1, which corresponds to ~1% of the total spin-down power of the Crab pulsar, the power source for the nebula emission. Despite the observation of multiple gamma-ray flares of the Crab, there is still no clear picture of the origin of this behaviour and the observed gamma-ray properties present great challenges to current theoretical models of particle acceleration within the nebula. Recent studies by the LAT of the pulsar PSR J2021+4026 in the Gamma Cygni region have discovered that this system is also variable (Allafort et al. 2013). The variability is much less spectacular than in the case of the Crab, however, and in this instance the variability is associated with the pulsar itself. During the analysis of 52 months of Fermi observations, the researchers unveiled a sudden decrease in flux above 100 MeV; around 16 October 2011, the pulsar flux decreased from 8.33 × 10 –10 erg cm –2 s –1 by ~20% over less than a week. At the same time, the spin-down rate of the pulsar increased by ~4%. Allafort et al. (2013) interpret these results as indicating that the observed change in the pulsed gamma-ray emission is driven by changes in the emission beaming and speculate that it may be precipitated by a shift in the magnetic field structure. This is an exciting result as it breaks the axiom that pulsars are steady gamma-ray sources. The discovery of gamma-ray novae Astronomers have observed novae for thousands of years, but it is only in more modern times that the processes behind these cosmic eruptions have become clear. Classical novae (CNe) are a subset of cataclysmic variables, binary systems which host a white dwarf that accretes material from the secondary star. In the vast majority of these systems the orbital period ranges between 1.4 and 8 hours and the secondary star is a low-mass, main sequence star; there are a few longer-period systems in which for the secondary star to fill its Roche lobe it has to have evolved off the main sequence (Bode 2012). The white dwarf accretes material off the secondary star, building up H-rich material on 6.33 The transient universe • Hill: Detections by Fermi 4: Japanese amateur astronomers discovered Nova Cygni 2010 in an image taken at 19:08 UT on 10 March (4:08 a.m. Japan Standard Time, 11 March). The erupting star (circled) was 10 times brighter than in an image taken several days earlier. The nova reached a peak brightness of magnitude 6.9, just below the threshold of naked-eye visibility. (K Nishiyama and F Kabashima/H Maehara, Kyoto Univ.) 5: Fermi’s Large Area Telescope saw no sign of a nova in 19 days of data prior to 10 March (left), but the eruption of Nova Cygni 2010 is obvious in data from the following 19 days (right). The images show the rate of gamma-rays with energies greater than 100 million eV (100 MeV); brighter colours indicate higher rates. (NASA/ DOE/Fermi LAT Collaboration) its surface. Once sufficient material has accumulated for the critical pressure/temperature to be achieved at the base of the accreted envelope, then a thermonuclear runaway explosion is triggered, producing a CN event. The outburst increases the luminosity to ~10 000 solar luminosities and large amounts of mass are ejected at high velocities (Bode 2012), typically ~10 –5 –10 –4 solar masses at velocities of ~102 –103 km s –1. The inter-outburst period of CN explosions is believed to be 1000–10 000 years. Conversely, recurrent novae (RNe) have inter-burst timescales of the order of decades and may host more evolved red giant companions. Only ten such systems have been identified within our galaxy. Gamma-ray line emission at ~MeV energies has long been predicted from novae as a consequence of the decay of radioactive elements produced in the nova explosion (Hernanz 2013). However, prior to the discovery of GeV emission from V407 Cyg by the Fermi LAT in 2010 (Abdo et al. 2010), there had been no predictions of continuum emission at such high energies. On 10 March 2010, Japanese amateur astronomers reported the discovery of a new 8th magnitude nova in the Cygnus constellation (figure 4). The nova was identified as originating from the known symbiotic binary V407 Cyg. This 6.34 system comprises a hot white dwarf accreting from a Mira-type variable red giant and consequently the white dwarf is embedded in a particularly dusty environment generated by the heavy wind of the donor star. The discovery of a classical nova event in this system was completely unexpected and was compounded by the surprising discovery of gammaray emission above 100 MeV from the nova by the LAT (Abdo et al. 2010). The gamma-rays were detectable for about two weeks after the optical nova onset, with an average spectral energy distribution in the form of a power law with an exponential cutoff and flux above 100 MeV of 4.4 × 10 –7 ph cm–2 s –1 (figure 5). A natural explanation for the origin of the gamma-ray emission appeared to be a result of the nova ejecta shell expanding outwards and colliding with the stellar wind of the red giant companion and forming a shock front where particles could be accelerated to high energies. The line connecting the white dwarf with the donor star contains the largest local density enhancement attributable to the red giant wind in which the nova shell can sweep up material in a fashion similar to what is ascribed in supernova explosion models. Taking the estimated parameters of the system (wind density, ejecta velocity, etc) and the measured spectral energy distribution, it was shown that this model could feasibly produce gamma-ray emission through the decay of neutral pions produced in proton collisions or inverse Compton scattering off accelerated electrons (Abdo et al. 2010). More detailed modelling of the environment has suggested that leptonic processes may be dominant (Martin and Dubus 2013). The direct link between the dense local wind environment and the gamma-ray production mechanism and the rarity of symbiotic and RS Oph-type RNe systems led to the suggestion that gamma-ray novae would be exceptionally rare events (Nelson et al. 2012). From 16–30 June 2012, Fermi identified a new gamma-ray source, Fermi J1750-3243, which was not consistent with any of the known 2FGL catalogue sources (Nolan et al. 2012). The location of the new LAT transient was consistent with the report of a newly discovered optical nova, MOA 2012 BLG-320 (Nova Sco 2012), which had entered into optical outburst 1.77–2.15 June 2012 (Cheung et al. 2012b). It appeared that Fermi had discovered another gamma-ray nova. However, this system appeared to be more like traditional CNe systems, with no indication of a dense stellar environment, and its behaviour at other wavelengths was quite different to that observed in A&G • December 2013 • Vol. 54 The transient universe • Hill: Detections by Fermi 6: These images show how the sky looks at gamma-ray energies above 100 MeV with a view centred on the north galactic pole. The first frame shows the sky during a three-hour interval prior to GRB 130427A. The second frame shows a three-hour interval starting 2.5 hours before the burst, and ending 30 minutes into the event. (NASA/DOE/Fermi LAT Collaboration) V407 Cyg (Hill et al. 2012). On 22 June 2012, another new unidentified gamma-ray source was detected by the LAT – Fermi J0639+058 (Cheung et al. 2012a) – but its proximity to the Sun prohibited follow-up observations at other wavelengths. In August 2012, a late-stage optical nova, Nova Mon 2012, was discovered within the LAT error circle. The first optical spectra were taken about 55 days after the gamma-ray peak and identified the white dwarf as being a member of the ONe class (Shore et al. 2013), i.e. the more massive class of white dwarfs. This system has also been associated with the more common CNe systems and has been identified as having a tight binary orbital period of ~7.1 hours (Osborne et al. 2013). In August 2013, Fermi discovered gamma-rays from Nova Del 2013 (Hays et al. 2013), increasing the gamma-ray nova sample to four distinct objects. The LAT light curves of the novae are remarkably similar and indicate that each object lasts as a gamma-ray source for ~2–3 weeks. However, there are also several distinct differences in their more general multi-wavelength characteristics. Most notable is that, with the exception of V407 Cyg, all the other gamma-ray novae appear to be CNe in the traditional sense and so the gamma-ray production mechanism invoked to explain the emission in V407 Cyg cannot be applied; it is likely that there is another mechanism in action in these systems. This raises the potential for many other CNe and RNe to be gamma-ray bright, although it is equally evident that numerous other optically detected CNe have not been reported by the LAT (there have been more than 40 reported optical novae since Fermi launched). Only with further study and analysis will we be able to identify the gamma-ray production channels and determine what fraction of the novae population of the galaxy can produce emission at these high energies. Gamma-ray bursts Gamma-ray bursts (GRBs) are in many respects the epitome of transient astronomy. They are A&G • December 2013 • Vol. 54 rapid flaring events typically lasting from less than one second to tens of seconds and are the most luminous explosions in the universe, detectable across the electromagnetic spectrum. There are two main progenitor models used to explain GRBs. “Long” GRBs, those with durations longer than 2 s, are believed to be the result of the collapse of a massive star when it exhausts its nuclear fuel. As the core collapses into a black hole, jets of material shoot outward at nearly the speed of light. “Short” GRBs, those with durations less than 2 s, are believed to be a result of the merger of a pair of neutron stars or a neutron star with a black hole. Both types of system are extragalactic in origin. The Fermi LAT has detected several GRBs since its launch in 2008, but none as spectacular as GRB 130427A, which erupted on 27 April 2013. This GRB is associated with Type Ic supernova SN 2013cq, at a redshift of z ~ 0.34 (Xu et al. 2013), placing it in the closest 5% of bursts. This close proximity makes it an exceptionally bright source of gamma-rays; it was detectable by the LAT for almost an entire day. Figure 6 shows how bright the GRB appeared to the LAT. Not only was it the longest GRB to be detected by the LAT, but it also holds the record for the highest energy photon detected at 94 GeV. The brightness and long duration of detectability of the GRB allowed many telescopes across all wavelengths to observe it while active. As the results are published in coming months it is hoped that there will be many exciting discoveries reported. Summary It is clear that the universe is a violent and variable place at high energies. A wide variety of object classes are capable of variations on a range of timescales from seconds to months and involve tremendous amounts of power and extreme environmental conditions that test and extend our understanding of fundamental physics. Presented here is only a taste of the diverse range of astrophysical systems capable of producing variable levels of the most energetic high-energy radiation; others include solar flares, terrestrial gamma-ray flashes (gammarays produced in thunderstorms in the Earth’s atmosphere), and the vast population of flaring active galactic nuclei. These are all sources of gamma-rays that Fermi has and is observing. The Fermi mission is answering many questions about the high energy universe and raising new ones that challenge our understanding of how high-energy particle physics processes operate in astrophysical environments. ● A B Hill, KIPAC, Dept of Physics and SLAC National Accelerator Laboratory, Stanford University, USA and School of Physics & Astronomy, University of Southampton, UK; [email protected]. A B Hill acknowledges support by a Marie Curie International Outgoing Fellowship within the 7th European Community Framework Programme (FP7/2007-2013) under grant agreement no. 275861. References Abdo A A et al. 2009 Science 326 1512. Abdo A A et al. 2010 Science 329 817. Abdo A A et al. 2011a Science 331 739. Abdo A A et al. 2011b Astrophys. J. 736 11. Allafort A et al. 2013 Astrophys. J. Letts 777 2. Atwood W et al. 2009 Astrophys. J. 697 1071. Bode M 2012 Proc. 11th Asian–Pacific Regional IAU Meeting 2011 NARIT Conference Series Vol. 1 eds S Komonjinda, Y Kovalev and D Ruffolo arXiv:1111.4941. Buehler R et al. 2012 Astrophys. J. 749 26. Cheung C C et al. 2012a Astronomers Telegram #4224. Cheung C C et al. 2012b Astronomers Telegram #4284. Corbel S et al. 2012 Mon. Not. R. Astron. Soc. 421 2947. Fender R P et al. 2006 Mon. Not. R. Astron. Soc. 369 603. Hernanz M 2013 Proc. Stella Novae: Future and Past Decades eds P A Woudt and V A R M Ribeiro ASPCS arXiv:1305.0769. Hays E et al. 2013 Astronomers Telegram #5302. Hill A B et al. 2012 on behalf of the Fermi–LAT collaboration in Proc. 4th Fermi Symposium Monterey CA arXiv:1304.2427. Johnston S et al. 1994 Mon. Not. R. Astron. Soc. 268 430. van Kerkwijk M H et al. 1992 Nature 355 703. Martin P and Dubus G 2013 Astron. Astrophys. 551 37. Nelson T et al. 2012 Astrophys. J. 748 43. Nolan P et al. 2012 Astrophys. J. Supp. 199 31. Osborne J P et al. 2013 Astronomers Telegram #4727. Shore S et al. 2013 Astron. Astrophys. 553 123. Tavani M et al. 2009 Nature 462 620. Tavani M et al. 2011 Science 331 736. Xu D et al. 2013 Astrophys. J. in press arXiv:1305.6832. 6.35