Active galactic nuclei horizons from the gamma-ray perspective

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Active galactic nuclei horizons from the gamma-ray perspective Andrew M. Taylor Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, Dublin 2, Ireland

a r t i c l e

i n f o

a b s t r a c t

Article history: Accepted 7 June 2017 Available online xxx

Recent results in the field of high energy active galactic nuclei (AGN) astrophysics, benefiting from improvements to gamma-ray instruments and observational strategies, have revealed a surprising wealth of unexpected phenomena. These developments have been brought about both through observational efforts to discover new very high energy gamma-ray emitters, as well as from further in-depth observations of previously detected and well studied objects. I here focus specifically on the discovery of repeated temporal structures observed in AGN lightcurves, and new hard spectral components within the spectral energy distributions of other AGN systems. The challenges that these new features place on the modeling of the sources are highlighted, along with some reflections on what these results tell us about the underlying nature of the emission processes at play.

MSC: 00-01 99-00 Keywords: Gamma-ray astronomy Active galactic nuclei New frontiers

© 2017 Elsevier B.V. All rights reserved.

Contents 1. 2. 3. 4. 5. 6.

Introduction . . . . . . . . . . . . . . . . High redshift AGN . . . . . . . . . . . Gravitationally lensed AGN . . . . AGN quasi-periodicity . . . . . . . . Fast variability of FSRQ . . . . . . . New hardening features at VHE 6.1. Mrk 501 results . . . . . . . . 6.2. Centaurus A results . . . . . 7. Conclusion. . . . . . . . . . . . . . . . . . Acknowledgement . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . .

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1. Introduction The past several decades have seen extragalactic high energy

γ -ray astronomy develop from an emerging discipline into a fully fledged research field. Starting in the 1990s with the first AGN discoveries of Mrk 421 and Mrk 501 (Punch et al., 1992; Quinn et al., 1996), the field now boasts of more than 60 AGN having been detected at very high energies (VHE) by ground-based gamma-ray instruments 1 . Following these observational achievements, a considerable array of different AGN subclasses, believed to represent various manifestations of a single (few) AGN type(s) (Urry and Padovani, 1995), are now identified. These range from the bright

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E-mail address: [email protected] See http://tevcat.uchicago.edu for an up-to-date list.

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1 2 2 4 4 6 6 6 8 9 9

beamed blazars of both BL Lac and flat spectrum radio quasar (FSRQ) type, the most numerously observed AGN subclass at VHE, to their dimmer weakly beamed counterparts, radio galaxies. The radio galaxy members of the AGN family observed at VHE, although much dimmer, offer the potential to provide direct spatial information about their emission site due to their locality. Contrary and perhaps complementary to this, the jet-beamed blazar family members, are observed as point-like sources. For these, information about the spatial extent of the emission site may instead be encoded into the temporal structure of the flux that they emit. Indeed, the most challenging/enlightening results from observations of such temporal structure information, come from the most intense outbursts (such as that of PKS 2155-304 (The HESS collaboration, 2007) in 2006). Such extreme bright episodic emission has lead to tight constraints being placed upon the size of the emission region and the jet Doppler factor.

http://dx.doi.org/10.1016/j.newar.2017.06.001 1387-6473/© 2017 Elsevier B.V. All rights reserved.

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At present, three principal stereoscopic Cherenkov telescope instruments are currently in operation, H.E.S.S. based in Namibia, MAGIC based on the Spanish island of La Palma, and VERITAS based in Arizona in the USA. These sensitive stereoscopic Cherenkov telescope instruments collectively cover both the northern and southern hemisphere regions of the sky. Together, the achievements of these these instruments have brought about the present flourishing status of the field. Furthermore, an upgrade of each of them carried out around 2012, has resulted in significant improvements in their sensitivities, and a lowering of their threshold energies (Holler et al., 2016; D. B. Kieda for the VERITAS Collaboration, 2014; Sitarek et al., 2016). Recent years have also seen the arrival of new monitoring instruments, with FACT (The FACT collaboration, 2015), and the now completed HAWC-300 (Pretz and for the HAWC Collaboration, 2016), collectively able to provide wide field of view and sensitive effective AGN monitoring. The complementarity provided by the monitoring and follow-ups through both the broad sky coverage, and the in-depth low energy threshold targeted observations, make promising the prospects for further growth in the coming years. Such collaborative efforts allow the maximum to be obtained from the present generation of instruments before the arrival of the next generation CTA north and south instruments (Actis et al., 2011). In the following, several of the key recent observational developments in AGN gamma-ray astrophysics will be covered. In Section 2 the highest redshift AGN observed to-date will be focused on. Following this, in Section 3 the recently discovered first gamma-ray lensed AGN system will be discussed. In Section 4 the recent evidence for quasi-periodicity in a gamma-ray bright AGN will be highlighted. In Section 5, fast variability in FRSQ systems will be addressed. Lastly, in Section 6 the discovery of new unexpected spectral hardening features in local AGN will be covered. The conclusions to this discourse will be provided in Section 7. 2. High redshift AGN At VHE, the propagation of photons is attenuated by the extragalactic background light (EBL) through pair production interactions. The strong energy dependence of this process, brought about by the required threshold energy for pair production, results in extragalactic space becoming optically thick τ > 2 at redshift z = 0.5 (2) at for photon energies of 30 0 GeV (10 0 GeV) (Franceschini et al., 2008). It is therefore apparent that the threshold energy for an air Cherenkov telescope (ACT) instrument, has significant implications on the size of the Universe open to it for extragalactic observations. The recent lowering of the threshold energy of ACTs to energies below 100 GeV, following upgrades back in 2012, has opened up the high redshift window to the Universe. Prior to these upgrades, the highest redshift AGN observed by ACTs had been 3C 279 (The MAGIC collaboration, 2008) (z =0.54), KUV 00311-1938 (Becherini et al., 2012) (z > 0.51) and PKS 1424+240 (Acciari et al., 2010) (z =0.60). Furthermore, with bright AGN, particularly FSRQ, whose outbursts are frequently brightest down at these energies, the opening up of the low energy domain allows for a rich ensemble of phenomena to be probed with the potential of high statistic results for bright outbursts. Direct proof that access to the high redshift Universe has indeed been achieved through these upgrades of the instruments comes from the recent successful detection, both by MAGIC and VERITAS, of a flaring outburst from the FSRQ PKS 1441+25 in April 2015 (The VERITAS collaboration, 2015; The MAGIC collaboration, 2015). This AGN sits at a redshift of 0.94 (Shaw et al., 2012), making it the highest redshift VHE blazar detected to date. The spectral energy distributions (SED) obtained by the observations from these instruments, which both achieved threshold energies down well

below 100 GeV, are both shown in Fig. 1. In this figure, the spectral values of both the observed, and EBL deabsorbed points are shown. Note the reference EBL model in the two plots are different, with the VERITAS SED adopting the model (Gilmore et al., 2012) and the MAGIC SED adopting the model (Domínguez et al., 2011). A comparison of the two observational result sets show striking agreement, validating both the detection and the robustness of the spectrum obtained by the two independent experiments. Although remarkable simply for its high redshift value, the detection of this FSRQ at VHE brings with it additional new information. Perhaps one of the most striking such results, however, actually comes from what is not seen. The continuation of the spectral slope in the derived SED, particularly apparent through the comparison of the Fermi-LAT and deabsorbed VHE spectrum, amounts to a lack of evidence of internal absorption being present in the intrinsic spectrum output by the source. In turn, this result can be used to place strong constraints on the position of the emission site with respect to the broad line region location (Tavecchio and Ghisellini, 2012). With gamma-ray emission up to 200 GeV detected from this source, the emission site is found to sit at a distance be be constrained1to /2 yond rBLR ≈ 1017 cm Ldisk /1045 ergs−1 (Ghisellini and Tavecchio, 2009), where Ldisk is the thermal luminosity of the accretion disk which is spectrally dominated by the output in the optical energy range. This point will be returned to later in Section 5. The detection of this high redshift AGN, out at a redshift of 0.94, carries other implications related to the subsequent tranparency of space outside the source in the extragalactic environment, and the corresponding EBL constraint. The rule-of-thumb is that a VHE photon of energy Eγ , emitted by a source at redshift z, attenuates off background photon of wavelength λ such that, (Eγ /TeV )/(1 + z )2 ≈ (λ/μm ). Following this guideline, the detection of VHE < 100 GeV flux from a source at redshift z ≈ 1 predominantly provides EBL information for the small wavelength component (∼ 0.6 μm), as shown in left-hand panel of Fig. 2. Under the assumption that the intrinsic spectral shape follows a simple mathematical functional form, the constraints on the normalisation of a fiducial reference model, (Domínguez et al., 2011), are shown in the right-hand panel of Fig. 2. These high redshift AGN observations clearly demonstrate the successful reduction in energy threshold of both MAGIC and VERITAS instruments. In a similar manner, the H.E.S.S. collaboration demonstrated the successful reduction of the H.E.S.S.-II energy threshold through the detection of the FSRQ 3C 279, at a redshift of z ≈ 0.54, in a flaring state back in 2015 (Cerruti et al., 2017). The implications of these observations, which achieved an energy threshold of ∼ 66 GeV, will be discussed further in Section 5. This collective reduction in the energy threshold of the different ACT instruments, therefore, has quickly borne fruit. 3. Gravitationally lensed AGN The recent upgrades of the stereoscopic ACT instruments which have opened up the high redshift window, have led to the discovery of an ensemble of new unexpected phenomena. One prime example is the detection of the first VHE gamma-ray gravitationally lensed system, B0218+357 (The MAGIC collaboration, 2015). Within this system, the source AGN sits at a redshift of ∼ 0.94 (Cohen et al., 2003), and the lens at a redshift of z ≈ 0.68 (Browne et al., 1993). This system was detected several years ago at GeV gamma-ray energies by the Fermi-LAT satellite (The Fermi collaboration, 2014). Since then, the lensed AGN has exhibited several GeV bright flaring episodes, along with the subsequent detection of their corresponding delayed counterparts (see top-panel of Fig. 3). The delay times between these flares and their counterparts have been repeatedly

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Fig. 1. The spectral energy distribution of PKS 1441+25, taken from (The VERITAS collaboration, 2015) and (The MAGIC collaboration, 2015).

Fig. 2. Left-panel: the EBL component constrained by the observations of PKS 1441+25, taken from (The VERITAS collaboration, 2015). Right-panel: the constraint on the normalisation of a fiducial EBL model (Domínguez et al., 2011) for a variety of functional forms for the intrinsic spectra, (The MAGIC collaboration, 2015).

Fig. 3. Top-panel: lightcurves at GeV energies of 3 flares and delayed counterparts from the lensed system containing B0218+357. The lower panel shows the flux ratio between the flare and delayed emission. This figure has been taken from (The Fermi collaboration, 2014). Bottom-panel: a schematic describing the distortion of the delayed flare due to the addition of microlensing effects.

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measured, with a value of ∼ 11.5 ± 0.3 days being obtained. Interestingly, such a delay time may be somewhat longer than that measured at radio energies, for which a delay time of ∼ 10.5 ± 0.4 days has been obtained (Biggs et al., 1999). However, the status of any discrepancy, and the possibility that the radio and gammaray emission sites are not colocated, is presently a topic hotly debated (The Fermi collaboration, 2014). Following naive geometrical arguments, the calculation of the delay times follows the form,

δt ≈ (δθ )2

L c

(1)

where L is the distance to the lensing object, and θ describes the geometrical angular deflection of the image. By describing the delay times in this form, at least qualitatively, highlights the ability to measure the distance to objects using collectively the angular and delay time information. Indeed, measurements from the lensed system, with B0218+357 as the source, have led to the inference of Hubble constant values of H0 = 61 ± 7 km s−1 Mpc (York et al., 2005). Interestingly, such a value is noted to sit somewhat below the preferred range inferred from the Hubble space telescope measurements (Freedman et al., 2001; Riess et al., 2009; 2011; 2016). It is worth highlighting, however, that tensions between these Hubble space telescope measurements and the CMB inferred value from the recent Planck mission (Planck Collaboration et al., 2016), indicate a broader lack of consensus. This suggests that the issue of the Hubble constant value remains open, and that new insights from independent avenues such as gravitational lensing are of considerable scientific merit. The effects due to substructure within the lens, once also factored into account, alters the simple lensing picture put forward. A depiction of these effects is provided in the bottom-panel of Fig. 3 2 The addition of such micro-lensing effects, introduced by the lensing galaxy’s individual stars, leads to a distortion factor (labeled μ in bottom panel of Fig. 3) in the amplitude of the delayed emission. Investigations into historic outbursts of B0218+357, have revealed evidence for the time variation of this distortion factor, allowing an estimation of the caustic crossing time introduced by movement of the stars (Vovk and Neronov, 2016). If this interpretation is correct, the measurement of the distortion factor provides a constraint on the emission region size, with values on the order of the Schwarzschild radius being obtained. This result demonstrates the remarkable potential such a lensed system offers, for accurately locating the VHE emission site, orders of magnitudes beyond that typically considered achievable for a z = 1 AGN using present instruments. 4. AGN quasi-periodicity The temporal aspects of AGN activity remains poorly understood for even the brightest and most studied objects to date. This situation, in part, has been driven by both limitations in instrumentation and the duration/incompleteness of observational temporal coverage. Within the extragalactic VHE gamma-ray community, predictability within the time domain is an unfamiliar characteristic. However, in parallel with the gravitationally lensed system discussed previously, such new characteristics are now starting revealing themselves. A growing body of evidence now exists that some level of predictability is present in the multi-wavelength lightcurves following long-term observations of the blazar PG 1553+113. Since searches for such periodicity carry potential shortfalls (Vaughan et al., 2016), a considerable threshold of robustness for the presence of any signature is demanded. Such a threshold now appears to be reached 2

provided via private communication by I. Vovk

by across-the-board lightcurves from radio, optical, X-ray through to gamma-rays (The Fermi collaboration, 2015), such that the existence of periodicity within its lightcurve now appears robust. From these observations, a periodicity timescale of ∼ 2 years has been determined. Furthermore, comparing the phases of these different wavelength components, there are preliminary signs that the radio emission is mildly delayed relative to the other components. At VHE, PG 1553+113 was detected previously during relatively quiescent (The HESS collaboration, 2006; The MAGIC collaboration, 2007a), and outburst (Abramowski et al., 2015) episodes. A lowering of the threshold energy of ACT instruments in combination with its soft spectral index at energies of ∼ 200 GeV resulted in its becoming bright at lower energies close to the new post 2012 upgrade energy threshold. These improvements have now allowed for its regular monitoring at VHE, which have resulted in evidence for quasi-periodicity in the VHE flux also being discovered (Prandini et al., 2016). Although the driver causing this periodic variability behaviour remains unclear, (Rieger, 2004), orbital motion, jet precession, and an internally rotating jet flow have all been discussed as valid candidates. With each of these processes predicting a distinct variability time-scale, the time period for the oscillations offers itself as a natural discriminatory variable between these possibilities. With PG 1553+113 indicating a quasi-perdiodic time-scale of ∼ 2 yrs (The Fermi collaboration, 2015), the favoured driver most naturally able to account for such a period is the precession of the jet. Such an explanation leaves the cause for the jet precession unanswered. Any discussion on this point is therefore speculation. However, one exciting possibility is that the central compact massive object actually consists of a binary pair of supermassive blackholes. Such a possibility relies on a relatively high abundance of binary supermassive blackhole systems, whose value is presently largely unknown. Although the existence of such systems is naturally expected from hierarchical structure formation (Begelman et al., 1980), their ubiquitousness remains unclear, with few spatially resolved confirmations of similar such systems possessing a comparable level of blackhole separation (eg. J0402+379 (Rodriguez et al., 2006)). The abundance of these systems, however, will be probed in the future by space-based gravitational wave detectors such as LISA (Hughes, 2002), through its sensitivity to their merger rates. Thus, indirectly, gravitational wave instruments may be able to shed new light on the nature of this quasi-periodic AGN activity. 5. Fast variability of FSRQ Despite the two AGN focused upon in the previous sections (B0218+357 and PG 1553+113) demonstrating temporally recurring structures in their brightness, such phenomena are far from usual for blazars at VHE. Indeed, the bulk of blazar activity shows more complex time domain structure, exhibiting variability on all time scales down to the smallest temporal scales that can presently be probed (H. E. S. S. Collaboration et al., 2017; The MAGIC collaboration, 2007b) (i.e. the smallest temporal variability time-scale is limited solely by current instrument sensitivities). Information about the spatial size of the emission site, from where the outburst originates, is obtained from the minimum temporal variability time-scale. One of the most constraining such results to date has come from the flares of PKS 2155-304 (The HESS collaboration, 2007). Following a major outburst back in 2006 from this BL Lac type blazar, the constraint on the size of emission site from the ∼ minute scale variability observed set the constraint R/δ < 5 × 1012 cm (0.3 A.U.), where δ describes the Doppler factor of the emitter. As means of a comparison, the Schwarzschild radius for this system is estimated to be RSchwarz. = 2GMBH /c2 = 5 × 1014 cm (30 A.U.), where MBH refers to the blackhole mass, which for PKS 2155-304 is estimated to be 1 − 2 × 109 M (The

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Fig. 4. Left-panel: Preliminary H.E.S.S. and Fermi-LAT EBL-deabsorbed spectral data points of PKS 1510-089 in three different time intervals. The solid line shows an unabsorbed power-law extrapolation of a fit to the Fermi-LAT data into the VHE range. The dashed line shows an unabsorbed log-parabola fit to the Fermi-LAT data into the VHE range. Right-panel: estimation of the absorption of gamma-rays by the broad line region for emission from different distances from the blackhole. Both figures have been taken from (Zacharias et al., 2017).

HESS collaboration, 2007). Note, however, that considerable uncertainty exists on the blackhole mass value, and that the actual value could be as much as a factor of 10 smaller than this (Rieger and Volpe, 2010). A reduction in the threshold energy of ACTs, now allows similar such constraints to be provided for flaring FSRQs. Although bright, the emission from these AGN typically peaks at lower energies than HBL blazars. Indeed, the brightest AGN outburst observed down at GeV energies are from the FSRQs 3C 454 and 3C 279 (Romoli et al., 2017), whose SED emission peaks at an energy of tens of GeV. For the FSRQ subset of blazars, other information about the emission site is also released during flaring episodes. For these objects, intense thermal radiation fields produced by the broad line region (BLR) are observed to be present, at distances close to or just beyond its accretion disk. Should the emission site sit within this intense radiation field radius, the VHE produced must traverse this intense radiation field, allowing for gamma-gamma absorption to occur. Estimates of the intensity of the radiation field within the BLR suggest that significant VHE gamma-ray attenuation should occur if their emission site sits at radii within this region (Tavecchio and Ghisellini, 2012). The spectrum emitted during flaring outbursts by FSRQs, therefore, provides additional information about the location of the central emitter, complimentary to the probe provided in the temporal structure of the emission. In recent years, both PKS 1510-089 and 3C 279 have been observed to undergo bright outburst episodes. During these outbursts, unexpectedly short time-scale variability have been revealed for both objects. Specifically, for PKS 1510-089, ∼ tens of minutes time-scale structure has been found in the HE lightcurve (Foschini et al., 2013). Likewise, for 3C 279, ∼ minute time-scale structure in the HE lightcurves was discovered (The Fermi collaboration, 2016). Such short/extremely short temporal structure has only been seen previously in BL Lac type blazar objects. Spectrally, observations by Cherenkov telescopes during such outbursts provide further key insights. Preliminary such spectral results have been recently provided for both objects during flares observed in 2015 (Zacharias et al., 2017; Cerruti et al., 2017). Together, both the lack of internal absorption features in the flaring FSRQ spectra, and the short variability time-scales observed during the flare, make for rather challenging constraints. Evidence for the lack of internal absorption present in the spectra obtained for PKS 1510-089 is shown in left-hand panel of fig 4. On the one hand, the lack of absorbtion signatures in the spectra place the emission site outside the BLR. On the other, the short variability time-scale demands that the size of the emission region is con-

strained to scales significantly below the Schwarzschild radius for a 5 × 108 M mass blackhole. Collectively, these results are suggestive that compact emission sights exist external to the central accreting objects. Furthermore, the growing abundance of such challenges for FSRQ (see similar such results of PKS 1222+216 (The MAGIC collaboration, 2011)) suggest that this is a general challenge that must be faced for this source class. The brightness of the 2015 flare of the FSRQ 3C 279 within the Fermi-LAT energy range, sets it as one of the brightest AGN outburst events recorded by the Fermi-LAT instrument. Such a level of statistics allows for acute in-depth investigations into the underlying spectral shape. Furthermore, with a redshift of ∼ 0.54 (Burbidge and Rosenberg, 1965), a negligible degree of EBL attenuation of the spectrum is calculated to be expected within the part of the Fermi-LAT energy band where the flux has been detected. Such observations, therefore, give unique insight into a high quality unadulterated spectra intrinsic to the source. In-depth modeling of the spectrum, with an unprecedented level of statistics spanning a broad energy range, allows a rigorous consideration of the functional form of the underlying spectrum produced during the outburst. Motivated by both shock acceleration (Zirakashvili and Aharonian, 2007) and stochastic acceleration scenarios (Schlickeiser, 1985; Akharonian et al., 1986), a class of electron spectra with four free parameters is considered, of the form,

  (Ee ) = (E0 )(Ee /E0 )−e exp −(Ee /Ec )βe .

(2)

The subsequent gamma-ray spectrum produced by such a distribution of electrons through their electromagnetic energy-loss interactions follows an analogous distribution of the form,

   −γ βγ  (Eγ ) = (E0 ) Eγ /E0 exp − Eγ /Ec .

(3)

The relation between the corresponding electron and photon parameters in these distributions is dependent on the specifics of the emission process at play (Fritz, 1989; Lefa et al., 2012). To exemplify this point, Inverse Compton emission in the Thomson regime leads to the cutoff stretching parameters for the photons and electrons being related by, βγ = βe /(βe + 2 ). The tentative constraint on the photon curvature parameter, βγ = 0.36 ± 0.03 (see righthand plot in fig. 5), is therefore suggestive that a β e ≈ 1 (ie. simple exponential) type cutoff exists in the underlying electron spectrum. Related to the above constraint, it is also worth noting what the detected spectra from such a bright flare did not reveal. A comparison of the multi-wavelength spectrum in both the Fermi-LAT and H.E.S.S. energy bands, shown in the left-panel of Fig. 5, reveals

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Fig. 5. Left-panel: a comparison of preliminary H.E.S.S. and contemporaneous Fermi-LAT spectral data points of 3C 279 made from a period during the giant outburst on 16th June 2015, along with historic MAGIC spectral data points of 3C 279 from 2008 (The MAGIC collaboration, 2008). This figure has been taken from (Cerruti et al., 2017). Right-panel: a fit to the Fermi-LAT data points during the 2015 outburst, adopting an emission spectrum of the form described by eqn 3. This figure has been taken from (Romoli et al., 2017).

no evidence for any spectral upturn feature. Indeed, an extrapolation of the spectral slope from the highest data points within the Fermi-LAT energy band appear to match on to the H.E.S.S. detected spectral points. Such a result emphasises the remarkable lack of evidence for internal absorption features in the bright spectra of this FSRQ. A similar lack of evidence of such absorption features is found in spectra of the flaring FSRQ PKS 1510-089, shown in Fig. 4. 6. New hardening features at VHE 6.1. Mrk 501 results Although over two decades since its discovery as a VHE emitter (Quinn et al., 1996), and one of the closest blazars available to study, our understanding of the non-thermal emission from Mrk 501 remains far from complete. Rapid growth in this understanding has been possible in recent times with the advent of the Fermi-LAT era, able to provide considerably more uniform, though less sensitive on short time-scales (Funk et al., 2013), coverage than that obtained from ground-based ACTs, whose much smaller field-of-views and limited observational periods constrain the amount of time that may be dedicated to any single source. A further improvement in the VHE coverage has been brought about by the arrival of the now complete HAWC-300 array (Pretz and for the HAWC Collaboration, 2016). Complementing this coverage, FACT, a monitoring ACT, also regularly observes this AGN. Indeed, it was from such monitoring by FACT that H.E.S.S. was alerted to a giant outburst in 2014 whose flux level matched that of the record level, observed by HEGRA back in 1997 (Djannati-Atai et al., 1999). The obtained spectra both during the flare, and in the quiescent state, are shown in the left-panel of Fig. 6. The shape of the spectrum observed during the flare, once EBL absorption had been accounted for, showed no signs a cutoff, continuing as a hard spectrum up to the highest energy data point (∼ 20 TeV). Perhaps most singular of all, however, has been the further discovery of “challenging” spectral slopes, within the temporally resolved spectra in the 10–300 GeV Fermi-LAT energy band (see right-panel of fig 6 3 ) (Shukla et al., 2016). These have been found to be present during some of the brightest flaring episodes in the

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provided via private communication by A. Shukla

past 8 years, during which Fermi-LAT coverage has been available. Tentative evidence, following the large outburst caught by both Fermi-LAT and VERITAS in 2009 (Abdo et al., 2011), had previously been claimed to show such a feature (Neronov et al., 2012). However, the further mounting of evidence for this effect, seen in the Fermi-LAT data within a larger observational time period, warrants a further reflection on the implications of such peculiarly hard spectral indices. The renewed evidence for the existence of a hard spectral component calls upon more involved scenarios in order to be able to naturally produce such a feature within the sources. Presently, a primary candidate for the generation of such an index are leadingblob stochastic acceleration scenarios (Lefa et al., 2011), which utilise the development of an ensemble of relativistic Maxwellian ´ distributions (Katarzynski et al., 2006). Alternatively, such a feature could be developed outside the source zone through either the production of electromagnetic cascades within weak extragalactic magnetic fields (Abdo et al., 2011) or through absorption on radiation fields local to the source (Aharonian et al., 2008). The required consideration of less run-of-the-mill type scenarios highlight the challenges that the presence of a hard spectral component within the Mrk 501 SED during flaring outbursts place on the emission site, whose presence appears wholly incompatible with conventional one-zone emission scenarios.

6.2. Centaurus A results One of the most local AGN, at a distance of only 3.8 Mpc (Harris et al., 2010), which was only relatively recently discovered as a VHE source (The HESS collaboration, 2009), Centaurus A continues to reveal new aspects about itself. This nearby FR1 radio galaxy is one of the few AGN at gamma-ray energies to also have morphology information. Fermi-LAT observations of this AGN have revealed that the gamma-ray emission is produced from both its core and giant lobe regions (The Fermi collaboration, 2010). The underlying nature of the acceleration process, and the spatial position of the source of the high energy particles present within these regions, however, presently remains less clear. From the nuclear core gamma-ray emission region, located within ࣠ 0.1° from the nucleus core itself, analysis of the emission observed by Fermi-LAT continues to provide weak evidence for the

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7

Fig. 6. Left-panel: the observed spectral data points of Mrk 501 above 300 GeV, from observations taken both before and during the extremely bright outburst in June 2014 (Cologna et al., 2016). Right-panel: (Above) a table noting three temporal regions in the long-term 10–300 GeV lightcurve of Mrk 501, measured by Fermi-LAT. (Below) the 10–300 GeV Fermi-LAT long-term lightcurve of Mrk 501 with an indication of the 3 regions noted in the table, each bounded by vertical green dashed lines. Analysis of the spectra within these 3 regions indicate the presence of extremely hard spectral components. This is an altered figure from a version taken from (Shukla et al., 2016). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 7. Left-panel: indications for the presence of a hard spectral component within the core (࣠ 0.1°) emission Centaurus A spectrum at GeV energies. This figure is a modified version of that shown in (Brown et al., 2016). Right-panel: weak evidence (∼ 2.6σ ) for variability in the Fermi-LAT > 0.1 GeV lightcurve of Centaurus A between 2008 and 2015 in 45 day bins.

presence of flux variability (see right-panel of fig 7 4 ). Indeed, a hint of such variability in the flux from the core emission, with a ∼ 2σ evidence for departure from the hypothesis of a constant flux level from 4 years of Fermi-LAT data, had previously been suggested (Sahakyan et al., 2013). The revised estimate of this statistical strength, after seven years of data, sits√at a level of ∼ 2.6σ . If genuine, and a constant growth rate of σ / t holds, a robust 3σ signal of variability can be expected from a 10 year data set. Accompanying this accumulating evidence of variability in the core emission’s gamma-ray flux, the Fermi-LAT data from this region now robustly further confirms the presence of a new hard spectral component in Centaurus A’s core emission, with an on-

4

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set energy of ∼ 3 GeV (see left-panel of fig. 7 5 ). It should be noted, however, that the present level of statistics from the FermiLAT core emission observations are insufficient for a statement to be made on the level of variability associated with the new hard spectral component. Such an aspect is particularly relevant since a potential difference in variability of the new harder would lend support to the idea that it originates from a separate cosmic ray population (Brown et al., 2016). Taking advantage of the spatially resolved lobes both at radio and gamma-ray energies, evidence for spatial variation of the spectral index across the giant lobes has also been looked into (Sun et al., 2016). This work built further on earlier findings (Yang et al., 2012), and revealed evidence for energy dependent

5

provided via private communication by J. Graham

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Fig. 8. Left-panel: A breakdown of Cen A’s giant radio lobe structures into 3 northern lobe and 3 southern lobe regions. Right-panel: A comparison of the energy spectra within the 3 southern lobe regions labeled “S1”, “S2”, and “S3”. Both plots have been taken from (Sun et al., 2016).

differences in the lobe morphologies. In particular, observations of the northern and southern lobes at radio, microwave and gamma-ray energies have indicated that the gamma-ray emission extends spatially beyond the emission region in both radio and microwave bands. Furthermore, morphological spectral differences across the lobes have also been recently discovered. Due to the low gas densities in the lobes, the low energy (< GeV) gamma-ray spectral component appear most likely of leptonic origin, produced through IC interactions on the CMB by in situ accelerated electrons. Applying a description of the underlying electron spectrum using expression eqn 2, this gamma-ray emission was found to originate from electrons within the cutoff energy range for all lobe regions bar the southern most one (referred to “S1” in the left-panel of fig. 8). This highlights the fact that the spectrum in this part of the lobe is found to be markedly harder than the spectra in all the other regions, providing no evidence for a cutoff. This result potentially motivates the presence of strong (∼ 10 μG) magnetic fields at this location, which would be considerably larger than that suggested to be present in the other regions. The appearance of a new hard spectral component at energies above a GeV, and its corresponding spatial dependence, can potentially provide a diagnostic of its origin. However, attempts to fit the multi-wavelength spectrum in the regions across the lobes reveal challenges for both hadronic and leptonic models. On the one hand, the lack of evidence of a π 0 bump at GeV energies is suggestive against a hadronic origin of the sub-GeV component of the spectrum. On the other, the emergence of a hard spectral component can be difficult to reconcile under a leptonic origin only scenario under a pure power-law model. Specifically, an interpretation in which the new hard spectral component, which reveals itself at GeV energies, relates to the changeover of target photons for IC scattering from CMB to EBL photons, requires a spectral break or cutoff to exist in the electron spectrum at an energy of tens of GeV (Sun et al., 2016). 7. Conclusion The present epoch in gamma-ray astrophysics is one of a maturing field. Although an increase in the number of sources detected

continues, there has been an evolution of focus in the observational frontier. Indeed, the research highlights covered in this review demonstrate this trend. New source discoveries continue to play an important role, primarily thanks to recent improvements in the instrumentation which have lowered their energy threshold. However, subsequent deeper observations of already detected objects are now playing an increasingly important part in leading the field through the new insights provided. Within the time domain results covered, the discovery of the lensed AGN system, B0218+357, reveals an entirely new phenomenon in VHE astrophysics. The further potential provided by this system for probing both the nature of the lens itself, as well as providing a natural magnifying glass for probing the emission region, is considerable. Moreover, this system provides a connection to an even broader range of astrophysics, through the possibility of probing the length scale to the lens, and subsequently providing input on the Hubble constant value debate. The discovery of quasi-periodicity in the emission of PG 1553+113, provides an archetypal example of discoveries brought about through deeper observations of already discovered VHE emitters. Although previously observed to demonstrate outburst activity, the recent realisation of an underlying regular quasi-periodic structure in the gamma-ray emission is once again entirely new to extragalactic VHE astrophysics. The possible implication that such cyclic behaviour results from the presence of a binary supermassive blackhole system is an exciting one. Interestingly, new information about such systems, whose abundance has mostly been hypothesised until now, are set to be probed in the near future by upcoming gravitational wave detectors such as LISA (Hughes, 2002). Beyond structure within the temporal domain, AGN variability is a well known and familiar phenomena. Despite this, however, the short-time variability results observed from recent FSRQ outbursts at VHE are challenging. The blazar 3C 279, which underwent a giant outburst in June 2015, demonstrated minute-scale variability at GeV energies. Such short-time variability at gamma-rays energies approaches the shortest level caught from the BL Lac PKS 2155-304. Since considerable internal absorption for FSRQ are expected should the emission zone be located close to the BLR, both the compactness of the emission zone suggested by the short time-

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scale structure, and the large distance from the BLR region, are collectively rather challenging to reconcile. In association with the information provided in the timedomain structure, AGN spectra at VHE provide complimentary information about the nature of the emission process. The 2015 flare of 3C 279, one of the two brightest such events ever observed by the Fermi-LAT satellite over the past eight years, provided a unique opportunity to probe the nature of the spectral shape. Investigations into the shape of the spectral cutoff, found the spectrum to be describable by a stretched exponential cutoff, (Romoli et al., 2017). This shape, under the assumption of emission through IC scattering in the Thomson regime, is suggestive of an underlying exponential cutoff in the electron spectrum (ie. βe = 1, see eqn. 2). Bright flaring events from the blazar Mrk 501 and long duration observations of the radio galaxy Centaurus A have also turned up spectral surprises. Namely, evidence has been revealed for the onset of an unexpected hard spectral component at ∼ GeV energies in these two very different systems. In both the blazar and radio galaxy cases, the presence of this new component cannot be explained within the confines of the simplest models previously put forward to explain their emission. Indeed, in the case of the Mrk 501, the hardness of the spectrum observed, with an index of  ≈ 1, poses a problem whose solution promises to provide exciting new insights into the acceleration and emission process at work. Accompanying the information encoded in their energy spectra, additional further information is available for nearby resolved objects from their spatial morphology. Centaurus A, a local radio galaxy, has been discovered to possess lobe structures at gammaray energies, from observations made with the Fermi-LAT satellite (The Fermi collaboration, 2010). Such features are consistent with those observed previously at radio energies. However, comparisons between the radio and gamma-ray morphologies of the giant radio lobes now reveal peculiar differences between them. Specifically, the gamma-ray emission region has been found to extend beyond that of the radio emission region in both the northern and southern lobes (Sun et al., 2016). Moreover, the gamma-ray spectra originating from different regions within the lobes have been shown to differ considerably. Such variations are difficult to explain within either a simple leptonic or hadronic emission scenario. In summary, exciting and increasingly challenging new results have arisen in the maturing discipline of VHE AGN astrophysics. The achievement of these results have come about both through the discovery of bright new objects which are demonstrating new unexpected phenomenon, as well as through observations of more familiar objects which have proven to be more complex and rich than perhaps previously realised. These developments collectively ensure that the field of VHE astrophysics continues to both broaden and deepen our understanding of AGN, fundamentally providing key insights into how these effective particle accelerators operate.

Acknowledgement AT acknowledges a Schroedinger fellowship at DIAS.

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Please cite this article as: A.M. Taylor, Active galactic nuclei horizons from the gamma-ray perspective, New Astronomy Reviews (2017), http://dx.doi.org/10.1016/j.newar.2017.06.001