Radio Galaxies with the Cherenkov Telescope Array
R. Angioni, P. Grandi, E. Torresi, C. Vignali, J. Knödlseder
aa r X i v : . [ a s t r o - ph . H E ] F e b Radio Galaxies with the Cherenkov Telescope Array
R. Angioni a,b,c,1 , P. Grandi a, ∗ , E. Torresi a,b , C. Vignali b,d , J. Kn¨odlseder e a INAF-IASFBO, Via Gobetti 101, I-40126 Bologna, Italy b Dipartimento di Fisica e Astronomia, Universit`a degli Studi di Bologna, viale Berti Pichat6/2, I-40127 Bologna, Italy c Max-Planck-Institut f¨ur Radioastronomie, Auf dem H¨ugel 69, D-53121 Bonn, Germany d INAF–Osservatorio Astronomico di Bologna, Via Ranzani 1, I-40127 Bologna, Italy e Institut de Recherche en Astrophysique et Plan´etologie, 9 avenue Colonel-Roche, F-31028Toulouse, Cedex 4, France
Abstract
Misaligned AGN (MAGNs), i.e., radio-loud AGNs with the jet not pointingdirectly towards us, represent a new class of GeV emitters revealed by the
Fermi -LAT space telescope. Although they comprise only a small fractionof the high-energy sources, MAGNs are extremely interesting objects offeringa different perspective to study high-energy processes with respect to blazars.The aim of this work is to evaluate the impact of the new-generation CherenkovTelescope Array (CTA) on the MAGN class and propose possible observationalstrategies to optimize their detection.
Keywords:
Galaxies: active; Galaxies: nuclei; Galaxies: jets;Gamma rays: galaxies
1. Introduction
Radio-Loud Active Galactic Nuclei (RL AGNs) constitute the majority ofthe extragalactic sources observed at Very High Energies (VHE,
E >
100 GeV;see, for example, the TeVCat online catalog ). RL AGNs are characterizedby the presence of well-collimated jets of relativistic plasma ejected from thenuclear region [1]. The triggering and launching mechanism of these jets, andtherefore the physical conditions and parameters under which an AGN becomesradio-loud, are one of the most debated and studied topics in this field [2].Jets host a population of relativistic particles which emit non-thermal radia-tion across the whole electromagnetic spectrum. The Spectral Energy Distribu-tion (SED) associated with this emission has a characteristic shape comprisingtwo peaks: one at low energy (radio to optical/soft X-rays) and one at highenergy (hard X-rays to TeV) [3] . The low-energy peak is confidently associatedwith synchrotron emission from a population of relativistic electrons in the jet, ∗ Corresponding author
Email address: [email protected] (P. Grandi) Member of the International Max Planck Research School (IMPRS) for Astronomy andAstrophysics at the Universities of Bonn and Cologne URL: http://tevcat.uchicago.edu/
Preprint submitted to Elsevier September 11, 2018 hile the high-energy emission process is not well understood. The most popu-lar model invokes Inverse Compton (IC) up-scattering of low-energy seed pho-tons by the same relativistic electrons responsible for the synchrotron radiation.The seed photons can be provided by the electron synchrotron emission itself, inwhich case the process is referred to as Synchrotron Self-Compton (SSC) [4], orby an external photon field (such as the accretion disk, the Broad Line Region,the Narrow Line Region, the torus, or the CMB radiation), referred to as Exter-nal Compton (EC) process [5, 6]. These models are collectively called leptonic models since all the observed emission is ascribed to relativistic electrons. In re-cent years, it has become clear that a leptonic modeling of the SED of RL AGNsis not capable of reproducing all the observed properties. Therefore, alternativemodels have been developed which consider, alongside the relativistic leptons, apopulation of relativistic protons. This component can give the main contribu-tion to the observed emission, particularly at the highest energies, reproducingthe flat VHE spectra that are observed in some sources, while the electronIC component provides the hard X-ray/soft γ -ray emission. The main emissionprocesses involved are proton synchrotron emission, photo-pion production, andcascade emission from secondary particles produced by the interactions of pro-tons with ambient photons or themselves. These models are called hadronic or lepto-hadronic models; see, e.g., [7], [8] for a review on leptonic and hadronicmodels.If the jet of a radio-loud AGN is closely aligned with the observer’s lineof sight (l.o.s), its radiation is strongly beamed and amplified via relativisticDoppler effects. In this case, the AGN is called a blazar, and the non-thermal jetemission typically dominates over other contributions to the observed SED. Theblazar class includes Flat Spectrum Radio Quasars (FSRQs) and BL Lacertae(BL Lacs). FSRQs are powerful sources displaying strong broad optical emissionlines, while BL Lacs are less luminous than FSRQs and lack emission lines (downto equivalent widths of a few ˚A) in their optical spectra.In the framework of the orientation-based unified model [9, 10] radio galaxies(and Steep Spectrum Radio Quasars, SSRQs) are the radio-loud AGNs with thejet pointed away from the observer and are therefore referred to as MisalignedAGNs. Radio galaxies can be divided into two classes: FR I at low radiopowers, which present decelerating jets and edge-darkened diffuse lobes, andFR II at high radio powers, with large-scale relativistic jets and edge-brightenedlobes [11]. The transition between these two classes happens at a luminosity of ∼ W Hz − sr − at 178 MHz. FR I and FR II radio galaxies are consideredthe parent population of BL Lacs and FSRQs, respectively ([12, 13, 14, 15]).Recently, the new class of “FR 0” radio galaxies has been proposed (e.g., [16]and references therein). These sources share similar nuclear and host propertieswith FR Is, but lack extended radio emission. Though their behavior is stillpoorly understood, they appear to represent the majority of the population oflocal RL AGNs.Because of the larger inclination angle between the jet and our l.o.s. withrespect to blazars, the observed non-thermal emission from radio galaxies is notsignificantly Doppler-boosted, therefore these sources have a less jet-dominatedSED. Hence, radio galaxies provide a view of AGN jets which is less biasedby relativistic effects, allowing us to observe both the jets and the accretionprocess and potentially establish a connection between the two. This representsthe first, fundamental step to start a proper investigation of the origin of radio-2oudness in general [17, 18, 19]. Moreover, radio galaxies allow us to investigatethe presence of a transverse jet structure, which has been proposed to explainthe observed SED of FR Is. Structured (multi-zone) jets are indeed promisingin describing the high-energy radiation of radio galaxies. SSC models, whenapplied to RGs, require (modest) beaming factors ( δ ∼ − implying, forlarge inclination angles, small Lorentz factors (Γ ∼ −
3) [20, 21, 22, 23, 24].Slow jets are in conflicts with the idea that RGs are the parent population ofblazars. Very high jet velocities (Γ ∼
10 or more) are indeed found in AGNwith jet pointed directly to the observer [25].If the condition of a homogeneously emitting region as assumed in a SSCone-zone model is relaxed, an efficient (radiative) feedback between differentregions can explain the observed IC peak of radio galaxies (without violatingthe unified models [24]). This can be explained by a decelerating flow [26] anda spine-layer jet [26, 27, 28, 29]. In one case, the presence of regions at differentvelocities along the relativistic flow is assumed; in the other case, the flow issupposed to be fast in the inner part (the region observed in blazars) and slowin the external envelope. Interestingly enough, a limb-brightened structure ofthe jet has been discovered in Mrk 501 [30], in M 87 [31, 32] and in NGC 1275[29], thus supporting the latter scenario.The weaker Doppler boosting of MAGN emission is particularly evident inthe γ -ray band, where blazars represent the great majority of the observedAGNs. For example, in the latest Fermi -LAT AGN catalog, the 3LAC [33],MAGNs with a solid identification constitute only ∼ Fermi -LAT catalog of hard spectrum sources (2FHL [34]), which coversthe energy range between 50 and 2000 GeV, includes 271 sources associatedwith AGNs, of which 6 are radio galaxies (2% of the entire sample). All the2FHL radio galaxies have been detected by Cherenkov Telescopes with the onlyexception of 3C 264 that, however, has no detection above 170 GeV in the
Fermi -LAT band. The TeV MAGNs are well known local FR I radio galaxies(see the TeVCat online catalog), i.e. NGC 1275 [35], M 87 [36], and Centau-rus A [37], plus the transitional FR I-BL Lac source IC 310 [38]. Recently, thedetection of the FR I radio galaxy PKS 0625 −
35 was announced by the H.E.S.S.collaboration [39].Although TeV observations of MAGNs have played an important role ininvestigating the open questions in high-energy studies of jets, the limited num-ber of detected sources, along with the low signal-to-noise spectra produced bythe current Imaging Atmospheric Cherenkov Telescopes (IACT), i.e. MAGIC,H.E.S.S. and VERITAS, do not allow us to draw general conclusions.The new-generation facility for VHE astronomy, the Cherenkov TelescopeArray (CTA) [40], is expected to achieve order-of-magnitude improvements insensitivity and energy range with respect to previous facilities which operatein the same energy domain. These capabilities will be achieved through thedeployment of a large number of Cherenkov telescopes of different sizes at twosites (Northern and Southern hemisphere) for full sky coverage. In the finalconfiguration, the Northern array will include 4 Large Size Telescopes (LST,23m diameter) and 15 Medium Size Telescopes (MST, 12m diameter). The δ = [Γ(1 − βcos ( θ ))] − , being θ the inclination angle and Γ = (1 − β ) − / the Lorentzfactor and β the bulk jet velocities in unit of the speed of light. −
200 GeV), intermediate (0 . −
10 TeV), and high-energy ranges (up to 300 TeV),respectively. Because of the different array configurations, the Southern arraywill have a better sensitivity, especially at energies > Fermi .In Section 2 we describe the sample of radio galaxies observed in the GeVband, which is the starting point for our study. In Section 3 we describe theperformed simulations and the results. In Section 4 we present more generalsimulations that allow us to estimate the chances of a CTA detection for any
Fermi -LAT AGN, given its spectral parameters in the 1-100 GeV band. InSection 5 we draw the conclusions and discuss the optimal observing strategyto detect more MAGNs, in light of our results.
2. The
Fermi -LAT radio galaxy sample
The third
Fermi -LAT AGN catalog (3LAC) [33], based on 4 years of observa-tions, provides the most up-to-date list of identified AGNs emitting in the GeVband. In our sample, we include all the radio galaxies with a solid counterpartreported in the 3LAC. We also add the radio galaxy 3C 120, firmly establishedas a γ -ray source by [41, 42] and the FR 0 radio galaxy Tol 1326 − − z < .
15 to avoid anysignificant γγ absorption by the Extragalactic Background Light (EBL). Thisexcludes all the SSRQs in the 3LAC catalog from our MAGN sample, leaving17 radio galaxies.Table 1 reports the γ -ray properties of the 17 Fermi -LAT radio galaxies.They are weak sources with 1–100 GeV fluxes of the order of ∼ − − − pho-tons cm − s − , and have power-law spectral indices (Γ Fermi ) in the range 1.8–2.8.In only two cases, NGC 6251 and NGC 1275, the spectra are curved and bet-ter reproduced by a logparabola model, rather than a power-law [33, 44]. Inthe logparabola model the deviation from the power law is modeled by the β parameter, also listed in Table 1.In Table 2, TeV spectral slopes and fluxes ( >
100 GeV) of the five sourcesdetected also by Cherenkov Telescopes up to 2 −
10 TeV are listed together withtheir 2FHL properties. In spite of the partial overlap of the energy bands, the
Fermi -LAT and IACT results are consistent within the large uncertainties.Finally, we note that our
Fermi -LAT sample is obviously biased towardssources with a high-energy (approximately in the MeV-GeV band) SED peak.It is possible that we are missing MAGN peaking at very high energies. Theycould be below the
Fermi -LAT sensitivity threshold and emerge in the CTAband. IC 310 is an example, although less extreme. It is barely detected with avery low flux by
Fermi -LAT but firmly detected with a flat spectrum at VHE[48, 46] with no sign of a falling trend up to 10 TeV. F ( E ) = kE [Γ Fermi − β (log( E )] able 1: Fermi -LAT radio galaxies3FGL Name Object Class z Model a Γ Fermi F −
100 GeV β phot cm − s − ± ± × − ± ± × − ± ± × − (6.5 ± × − − b FRI 0.0058 PL 2.2 ± ± × − ± ± × − ± ± × −
3C 120 c FRI 0.033 PL 2.7 ± ± × − − ± ± × − − −
35 FRI 0.055 PL 1.87 ± ± × − ± ± × − ± ± × − ± ± × − − ± ± × − − −
379 FR0 0.0284 PL 2.8 ± . ± × − − ± ± × − ± ± × − ± ± × − ± . a – PL - F ( E ) = kE − Γ F ermi ; LogPar - F ( E ) = kE − [Γ F ermi + β log( E )] . b – γ -ray emission associated to radio lobes [45] . c – Spectral parameters from [42] .Table 2: Sub-sample of MAGNs with TeV detection. All the data are fitted with a powerlaw model. For variable sources, minimum and maximum fluxes (and relative spectral slopes)measured by the Cherenkov telescopes are reported.Object a Γ F b Γ TeV F b TeV
TeV >
50 GeV >
100 GeV c refIC 310 d ± . ±
09 1.81-1.85 6-43 [46]NGC 1275 3.0 ± . ±
13 4.1 ± . stat ± . sys ± stat ± sys [35]PKS 0625 −
35 1.9 ± . ±
11 2.8 ± . e [39]M 87 d ± . ±
10 2.2-2.6 30-60 [47]Cen A 2.6 ± . ± ± . stat ± . sys e [37] a – The 2FHL catalog reports the detection of 3C 264. It was not included in Table becauseits Fermi -LAT spectrum does not extend above 171 GeV [34]. b – In unit of 10 − photon cm − s − . c – Fluxes provided by different Cherenkov telescopes can cover different energy bands. F TeV was extrapolated down to 100 GeV, when necessary. d – Variable source. e – Flux uncertainty of the order of ∼
3. CTA simulations of
Fermi -LAT radio galaxies
The connection between the
Fermi -LAT and IACT data is complex. Firstof all, data in these two bands are usually not contemporaneous. This is rel-evant because MAGNs can vary both in the LAT band [49] and in the TeVband [50, 51, 47, 52, 46] and the variability is not necessarily correlated. More-over, while an extrapolation of the
Fermi -LAT spectrum fits well the non-simultaneous MAGIC TeV data points of M 87 in a low state [53], a curvaturein the overall spectrum of NGC 1275 is clearly established by two simultane-ous MAGIC -
Fermi -LAT campaigns [49]. In addition, the MeV-GeV andthe TeV emissions might be produced by distinct components as in the case ofCentaurus A, where a second spectral component seems to emerge between the5 ermi -LAT and H.E.S.S. spectra [54, 55]. This spectral component could bethe signature of efficient pulsar-like electron acceleration mechanisms occurringin the black-hole magnetosphere [56] or could mark the presence of an hadronicprocess [57]; alternatively, it could be related to a population of millisecondpulsars. Another, even more exciting, possibility is that such a component isproduced by heavy dark matter (DM) particles clustered around the black hole[58]. The discovery of the VHE hardening of the Centaurus A spectrum opensnew interesting scenarios that CTA will be able to explore.Finally we note that a GeV contribution from the radio lobes has been re-vealed in two nearby sources: CenA [59] and Fornax A [45]. The poor LATspatial resolution ( ∼ . ◦ at 10 GeV and larger at lower energies) does not al-low us to disentangle different emission regions in other MAGN. However GeVflux variability observed in 3C 120 [42, 60], 3C 111 [61], NGC 1275 [62, 63],M 87 [47] and IC 310 [46] suggest that most of the high and very high energyphotons are dissipated in compact region.Keeping in mind all the caveats reported above, we decided to simulate theGeV-TeV spectra of radio galaxies assuming the compact jet core as the mainsource of GeV photons. Studies of extended emitting regions require furtherCTA simulations that are beyond the scope of this first explorative work on theCTA performances.We considered different spectral shapes. At first, a simple extrapolation ofthe Fermi -LAT power-law (PL) was assumed, then an exponential cutoff wasincluded. We considered three possible cutoffs at decreasing energies E c =1 TeV, E c =500 GeV, and E c =100 GeV to take into account different spectral steepen-ings. As a spectral curvature is already present in the Fermi -LAT spectrum ofNGC 6251, we extrapolated the logparabola model of the 3FGL catalog (ratherthan a power-law) and, in addition, we tested a power-law with a cutoff at100 GeV as another possible parameterization of the high-energy steepness.Actually, it is usually difficult to distinguish between a logparabola model anda cutoff power-law with the current data, as also shown by the Fermi-MAGICstudy of NGC 1275 [49]. Obviously, our approach reflects the lack of informa-tion on the SED of MAGNs in the TeV domain. It is mainly driven by the
Fermi -LAT and IACT observations of M 87 and NGC 1275. It is also evidentthat we are adopting the less favorable scenarios, not considering any flatteningof the very high-energy spectrum, as observed in Centaurus A.In Fig. 1 the adopted models are shown along with the current differen-tial sensitivity curves for 50 hours of observation for the Northern (blue line)and the Southern CTA array (red line). As an example, we plot the simulatedmodels for two different power-law spectral slopes, i.e Γ
Fermi =2.1 (left panel) and Γ
Fermi =2.7 (right panel) and same input flux F −
100 GeV = 10 − ph cm − s − . It is clear from Fig. 1 that, apart from the cutoff energy position ( E ), thespectral index is a fundamental parameter: flat Fermi -LAT sources are moresuitable CTA targets. This will be further confirmed by the specific simulationsof the MAGN sample discussed later in detail. Finally, we note that, as ex-pected, the best performance is provided by the Southern Array which consists F ( E ) = k ( E GeV ) − Γ Fermi e ( − E/E c )
6f a larger number of telescopes.The spectra of the radio galaxies in Table 1 were simulated in the energyrange 0.02–100 TeV using the software ctools v1.0 [64], developed for the sci-entific analysis of CTA data. Also Fornax A, that is part of the sample, wasconsidered, although the γ − ray emission has been recently associated with theradio lobes [45]. As our simulations are based on point-like sources, the resultsshould be taken with caution. We did not consider the sources with TeV ob-servations, with the exception of PKS 0625 −
35 for which only a recent claim ofdetection has been reported [39].For each source, an event list corresponding to a particular model was pro-duced considering a 5 ◦ circular region of interest (ROI) centered on the point-like target. The most recent Instrument Response Functions (IRF, version from2015 − − provided by the CTA, supplying information on effective area,point spread function, energy dispersion and instrumental background of theCTA configurations in the two hemispheres were assumed for an observationtime of 50 hours.A standard unbinned likelihood analysis was then performed to test thesignificance of the source. We considered the Test Statistic that is defined as TS= 2[logL s -logL ] where L s is the maximum likelihood value for a model with oursource at a specified location and L is the maximum likelihood value for a modelwithout the source [65]. Only the normalization and the spectral index wereallowed to vary. With the square root of the TS corresponding approximatelyto the detection significance, we consider thresholds of TS >
100 (correspondingto approximately 10 σ ) and TS >
25 ( ∼ σ ) to assess the detectability of MAGNsby CTA. Figure 1: Example of simulated CTA models (black curves, legend on the right panel), com-pared with differential sensitivity curves from CTA North (blue line) and CTA South (redline), for two different input spectra. The relevance of the power-law slope to a possible CTAdetection is evident, as well as the effects of different exponential cutoff energies. http://cta.irap.omp.eu/ctools . Results The results of the simulations are summarized in Table 3. We expect that7 out of 12 MAGNs (8 out 13 if Fornax A is also considered) will be detectedat Very High Energies if the simulated CTA spectrum is a direct extrapolationof the
Fermi -LAT power-law. The number of likely CTA candidates obviouslydecreases with the steepening of the spectrum in the TeV band. However, halfof the sources are still above the CTA sensitivity threshold (and three at asignificance level larger than ∼ σ ) if the spectral cutoff occurs at energies ≥
500 GeV.PKS 0625 −
35 is the only MAGN with a TS value larger than 100 foreach tested model, and indeed a TeV detection has been recently reported bythe H.E.S.S. collaboration [39]. In Figure 2, the CTA simulated spectra ofPKS 0625 −
35 are shown together with the
Fermi -LAT and H.E.S.S. data. TheH.E.S.S. data (falling between two CTA models) suggest a high-energy cutoffbetween 0.5 and 1 TeV. In order to quantify the CTA ability to discriminateamong different models, the simulation of PKS 0625 −
35 with E c ut = 500 GeVwas fitted with both a power law and a cutoff power law. All the parameters(spectral slope, normalization, and E c ut ) were freely adjusted by the fit. A like-lihood ratio test TScurve=2[log L(Power Law+cutoff) - log L(Power Law)] wasthen calculated. As TScurve is distributed as χ with 1 degree of freedom [66],we can assume that a curved spectrum is better than a power law when TScurveis larger than 16 (corresponding to ∼ σ significance for the curvature). TheTScurve value of ∼
400 obtained for PKS 0625 −
35 shows that a spectral bend-ing is statistically preferred to a power law and attests that the CTA Southernarray will not only be able to observe this source but also to discriminate amongdifferent spectral shapes. Finally, we note that PKS 0625 −
35 could be detectedin only 5 hours with a TS >
600 for E cut ≥
500 GeV.Faint ( F −
100 GeV < − phot cm − s − ) and steep (Γ Fermi ≥ .
5) MAGNshave TS values below the threshold of 25 (see Table 3), independently of theadopted input model (see also Section 2). We note, however, that radio galaxiesare variable sources. For example, 3C 120 underwent several flares, reaching insome cases fluxes of F >
100 MeV ∼ − − − ph cm − s − [60], and 3C 111 wasdetected in a very high state in more than one observation, exceeding the fluxof 10 − ph cm − s − . If the jet perturbations responsible for the Fermi -LATflares also cause bursts at TeV energies, 3C 120 and 3C 111 could be detectedby the CTA (assuming power-law spectra) during flaring episodes.Finally, we observe that a direct extrapolation of the logparabola model forNGC 6251 gives a non-detection with the CTA. However, a less abrupt decreaseof the VHE emission, as described by a power-law with a cutoff at 100 GeV,provides a marginal detection of the source (TS ∼ Figure 2: Comparison between observed and simulated data for PKS 0625-35.
Fermi -LATdata (pink points) and H.E.S.S. best-fit spectrum (green point and lines) are plotted alongwith our simulated spectra (black points and curves), corresponding to three different inputmodels, i.e. a power-law with a cutoff energy at 100 GeV, 500 GeV and 1 TeV.
Fermi -LATdata are from the 3FGL [33] and the 2FHL [34] catalogs. H.E.S.S. data are from [39].
5. Extension of the MAGN results
In order to generalize the results obtained for the
Fermi -LAT radio galaxies,we simulated a grid of possible CTA observations with the Northern and South-ern arrays, separately with the aim of producing a diagnostic plot to verify thedetectability of any AGN in the TeV band, provided that its spectral slope andflux in the
Fermi -LAT band are known. The simulation of the event files andthen the likelihood analysis are the same as described in the previous section.We explored γ -ray targets with fluxes (1,2,4,6, 8) × (10 − , − , − , − ) phcm − s − between 1 and 100 GeV (the flux ranges covered by the AGN in the3LAC catalog) and power-law spectral slope values ranging from 1.8 to 4.0 withan incremental step of ∆Γ = 0 .
1. We did not consider Γ
Fermi < . − ph cm − s − , can easily reach a TS larger than 100. Bothspectral slopes and normalizations were allowed to vary during the likelihoodanalysis. 9 able 3: Results of the simulated CTA observations for our radio galaxy sample. TS a is thestatistical significance of the source for the different spectral models tested here. Source k b PL TS PL TS TS . TS .
3C 78 47 3113 256 96 ...Fornax A c
20 1671 100 25 ...B2 0331+39 24 1049 65 ... ...3C 111 1.6 ... ... ... ...3C 120 1.9 ... ... ... ...Pictor A 3.3 ... ... ... ...PKS 0625 −
35 300 154480 16987 7294 2873C 189 13 321 ... ... ...3C 264 33 2129 128 44 ...Tol 1326 −
379 0.7 ... ... ... ...Cen B 48 4066 520 212 ...3C 303 32 3010 134 55 ...NGC 6251 d a Resulting TS from simulations assuming a simple extrapolation of the
Fermi -LATpower-law, and an exponential cutoff at 1 TeV, 0.5 TeV and 0.1 TeV, respectively.(...) indicates TS < b Normalization in units of 10 − photons cm − s − MeV − at 300 GeV. c Fornax A γ -ray emission is probably produced in the extended radio lobes. See textfor details. d Only a power-law with cutoff at 100 GeV was simulated for NGC 6251, as a spectralbending is already present in the
Fermi -LAT band.
Figure 3 summarizes the results: the input parameters, i.e. the 1–100 GeVflux and the power-law spectral slope, are reported on the x-axis and y-axis, re-spectively. The red (Southern hemisphere) and the blue (Northern hemisphere)curves connect the points of the grid for which a TS = 100 is obtained for 50hours of observation. The targets with a significance larger than ∼
10 occupythe left part of the plot, those below the CTA sensitivity the right one. Asa check of the reliability of our simulations, we also plot the previous studiedMAGNs (red and blue circles) and the radio galaxies already detected by theCherenkov telescopes (green points). They fall in the ”correct” regions of theplot (compare with Table 3). Cen A falls exactly in the strip delimiting thedetection from the non-detection regions, although an H.E.S.S spectrum is al-ready available for this source [37]. This is clearly related to the hardening ofthe spectrum [54] that our conservative simulation does not take into account.We predict that hard sources with Γ
Fermi ≤ . − phot cm − s − (as already antici-pated in Section 2, see also Figure 2), while AGNs with moderately steep slopes(Γ Fermi ∼ − phot cm − s − to overcome the sensitivity threshold of the Cherenkov arrays. Sources withvery steep spectra (Γ Fermi > .
8) need 1–100 GeV fluxes as high as 10 − –10 − phot cm − s − to emerge in the TeV sky.10 igure 3: Diagnostic space for sources with known photon index and flux in the 1 −
100 GeVband. The curves define the regions where a source can be detected by CTA assuming as athreshold TS=100. The blue line refers to the Northern array, while the red one to the South-ern array. The data points represent the
Fermi -LAT radio galaxy sample, also distinguishedby hemisphere. Filled points represent statistically significant detections, empty points rep-resent sources with TS < Fermi -LAT spectra cannot be extrapolated with a power-law into the CTA band. The radiogalaxy Centaurus A falls exactly in the strip delimiting the detection from the non-detectionregions. . Conclusions The aim of this work is to evaluate the impact of the next-generation CherenkovTelescope Array on TeV studies of Misaligned Active Galactic Nuclei. Up tonow, this class includes only 17 radio galaxies detected at GeV energies by
Fermi -LAT (out of more than 3000 objects detected in total) and 5 sourcesdetected in the TeV band by Cherenkov telescopes (out of about 60 radio-loudAGNs in the TeVCat online catalog).In this study, we investigated the CTA detection prospects for MAGNs bysimulating CTA observations for a sample of candidate sources observed by
Fermi -LAT . The main results of our work can be summarized as follows. • We predict 8 new MAGNs at the ∼ σ significance level, under theassumption of a straight extrapolation of the Fermi -LAT power-law inthe CTA energy range. • Assuming a power-law with an exponential cutoff as a more realistic case,we still predict 5 (6) new detections at a significance higher than ∼ σ (5 σ ), for a high-energy cutoff of 500 GeV. • Additionally, our simulated data show that CTA will be able to providehigher quality spectra with respect to current Cherenkov facilities. Weshow this for the case of the newly TeV detected source PKS 0625 − Fermi -LAT AGN will be detectable with the CTA, givenits flux and spectral slope in the 1–100 GeV energy range and assuming a simpleextrapolation of the LAT power law spectrum into the TeV band.We note that our estimates leave room for additional detections, since ra-dio galaxies are variable sources, both in the
Fermi -LAT band and at VeryHigh Energies. Additionally, a harder-when-brighter spectral behavior has beenobserved in several
Fermi -LAT AGNs, including radio galaxies [60]. This canfurther enhance even more the possibility of detecting a larger number of sourcesduring flaring states, thanks to the harder γ -ray spectrum.It should be also taken into account that our sample is, by definition, biasedtowards sources with a high-energy SED peak in the Fermi -LAT band. Thisleaves out high-energy-peaked sources such as (or more extreme than) IC 310,which are faint in the
Fermi -LAT band but should be detectable in the VHEregime.Finally, we point out that radio galaxies could have a more complex SEDthan expected. For example, there is evidence for a second spectral componentin Cen A that hardens the spectrum above 2 GeV (the slope changes from 2.7a 2.1) [54]. If this were a common feature in radio galaxies, MAGNs could bemore easily revealed by CTA, opening at the same time new extremely appeal-ing scenarios.Our results indicate that long exposures are necessary, however, to studyMAGNs that are in general steep and faint sources. An extragalactic surveydoes not appear suitable for the exploration of this class of objects. The CTAKey Science Projects, for example, include an extragalactic survey of 1/4th of12he sky in about 1000 hours [67]. The area covered is of the order of ∼ deg .Given that the largest field-of-view for the CTA will be ∼ ◦ (for the SSTs),this implies an effective observing time per pointing of the order of a few hours.An efficient strategy for radio galaxies seems to require long targeted campaignswith effective observing times of the order of 50 hours. This could be achievedmore easily, for example, by operating the CTA in subarrays. Because of therelatively steep spectra of the sources, most of the emission will fall in the en-ergy band covered by the LSTs and MSTs. Therefore, it could be possible toobserve this small sample of sources with these telescopes as a subarray, whilethe SSTs gain exposure on an extreme source of γ -rays at the highest energies.In conclusion, we predict that the CTA will have a significant impact in ourunderstanding of MAGNs at TeV energies, with the likely detection of addi-tional sources. This would be an additional step towards an understanding ofthis class of AGNs at TeV energies that relies less on the properties of a singlesource, and more on the common behavior. Moreover, obtaining better qualitydata on already detected sources would be crucial in order to distinguish be-tween different emission models to explain their VHE emission. Acknowledgements
This paper has gone through internal review by the CTA Consortium. Wewould like to thank the two reviewers of the CTA Speaker’s and PublicationOffice (SAPO) for their useful comments on the manuscript, and the anony-mous referee for a helpful and constructive report. We thank Valentina Fiorettifor precious help in the installation of the software package ctools . This re-search made use of ctools , a community-developed analysis package for ImagingAir Cherenkov Telescope data. ctools is based on GammaLib, a community-developed toolbox for the high-level analysis of astronomical gamma-ray data.
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