Synoptic studies of seventeen blazars detected in very high-energy gamma-rays
aa r X i v : . [ a s t r o - ph ] J a n Mon. Not. R. Astron. Soc. , 000–000 (2008) Printed 1 November 2018 (MN L A TEX style file v2.2)
Synoptic studies of seventeen blazars detected in very high-energygamma-rays
R. M. Wagner ⋆ Max-Planck-Institut f¨ur Physik, F¨ohringer Ring 6, D-80805 M¨unchen, Germany
Accepted 2007 December 12. Received 2007 December 10; in original form 2007 September 13
ABSTRACT
Since 2002, the number of detected blazars at gamma-ray energies above 100 GeV has morethan doubled. I study 17 blazars currently known to emit E >
100 GeV gamma rays. Theirintrinsic energy spectra are reconstructed by removing extragalactic background light atten-uation e ff ects. Luminosity and spectral slope in the E >
100 GeV region are then comparedand correlated among each other, with X-ray, optical and radio data, and with the estimatedblack hole (BH) masses of the respective host galaxies.According to expectations from synchrotron self-Compton emission models, a correlationon the 3.6- σ significance level between gamma-ray and X-ray fluxes is found, while corre-lations between gamma-ray and optical / radio fluxes are less pronounced. Further, a generalhardening of the blazar spectra in the E >
100 GeV region with increasing gamma-ray lumi-nosity is observed, both for the full 17-source sample and for those sources which have beendetected at distinct flux levels. This goes in line with a correlation of the gamma-ray luminos-ity and the synchrotron peak frequency, which is also seen. Tests for possible selection e ff ectsreveal a hardening of the spectra with increasing redshift. The blazar gamma-ray emissionmight depend on the mass of the central BH. The blazars under study do, however, show nocorrelation of the BH masses with the spectral index and the luminosity in the E >
100 GeVregion.I also consider temporal properties of the X-ray and E >
100 GeV gamma-ray flux. Nogeneral trends are found, except for the observation that the blazars with the most massive BHsdo not show particularly high duty cycles. These blazars include Mkn 501 and PKS 2155-304,for which recently very fast flares have been reported. In general, VHE flare time-scales arenot found to scale with the BH mass.As a specific application of the luminosity study, a constraint for the still undeterminedredshift of the blazar PG 1553 +
113 is discussed.
Key words: galaxies: active – BL Lacertae objects: individual (1ES 0229 + + + + + + + All but one of the detected extragalactic very high energy (VHE,defined by E >
100 GeV) gamma ( γ ) ray sources so far are blazars.Within the unified scheme (e.g. Urry & Padovani 1995) of activegalactic nuclei (AGN), blazars comprise the rare and extreme sub-classes of BL Lac objects and flat spectrum radio quasars (FSRQs).These are characterised by high apparent luminosities, short vari-ability time-scales, and apparent superluminal motion of jet com-ponents. These observations can be explained by highly relativis-tic, beamed plasma outflows (jets) closely aligned to the observer’sline of sight (Blandford & K¨onigl 1979) powered by central super- ⋆ E-mail: [email protected] massive black holes accreting at sub-Eddington rates (Lynden-Bell1969; Rees 1978b). The prime scientific interest in VHE γ -rayemitting blazars (in the following, ‘VHE blazars’) is twofold: (1)To understand the particle acceleration and γ -ray production mech-anisms, assumed to take place in the jets and to be linked to the cen-tral supermassive black hole (BH). Knowledge of the VHE emis-sion process will also contribute to the further understanding of theaccretion processes in AGN, jet formation processes, and the jetstructure. (2) To use the VHE γ -rays as a probe of the extragalac-tic background light (EBL; e.g. Hauser & Dwek 2001; Kashlinsky2005) spectrum in the wavelength range between about 0 . µ m. Determining the EBL spectrum in this wavelength rangemay allow to constrain the star formation rate (convolved with theinitial mass function) in the early Universe. In order to assess both c (cid:13) R. M. Wagner o b=-180 o b=+180 o l=90 o l=-90 M k n M k n ES + ES + PKS - H + M B L La c PKS - ES + P G + M k n
180 1 ES - H - PKS - ES +
200 1 ES - ES +
496 3 C ( z = . ) R ed s h i ft z Figure 1.
Currently known VHE γ -ray blazars along with the identified (66 objects) and tentatively identified AGN (27 objects) in the 3rd EGRET catalogueof γ -ray sources (solid grey dots: identified AGN; open grey dots: tentatively identified AGN). EGRET data from Hartman et al. (1999). The sources are shownin a galactic coordinate system. issues, it is essential to have a large sample of VHE γ -ray blazars athand. Ideally it should encompass a wide range in redshift for EBLstudies and at the same time include groups of sources at similardistances in order to probe and compare properties of the individ-ual sources without possible systematic uncertainties caused by theEBL de-absorption.The preconditions for such studies have much improved re-cently: Before 2004, only a few nearby extragalactic sources hadbeen established as VHE γ -ray emitters (e.g. Mori 2003) and pro-vided hardly enough data to perform comparative studies. Around2004, the third generation of imaging air Cerenkov telescopes(IACTs, e.g. MAGIC, Baixeras et al. 2004; Cortina et al. 2005 andH.E.S.S., Hinton 2004), the most successful tools so far to exploreVHE γ -rays, started to deliver scientific results. To date, the VHEblazar sample with available spectral information in the VHE re-gion comprises 17 BL Lac objects, among them one LBL object,BL Lacertae. Furthermore, now the redshifts of the known VHEblazars reach up to z = .
212 – or even to z = . γ -rays,3C 279 (Teshima et al. 2007). For 3C 279, however, no VHE spec-trum has been published yet, therefore the VHE luminosity andspectral slope of 3C 279 are not yet available for this study. M87,a FR I radio galaxy also detected in VHE γ -rays, is not included inthe study, as its VHE γ -ray production mechanism may di ff er fromthat in blazars (Aharonian et al. 2006c). Fig. 1 shows the sky posi-tions of the known VHE blazars in galactic coordinates along withthe AGN identified in the 3rd EGRET catalogue.The electromagnetic continuum spectra of blazars extend overmany orders of magnitude from radio frequencies to sometimesmulti-TeV energies and are dominated by non-thermal emissionthat consists in a ν F ν representation of two pronounced peaks.The low-energy peak, located between the IR and hard X-rays, isthought to arise from synchrotron emission of ultrarelativistic elec-trons, accelerated by shocks moving along the jets at relativisticbulk speed. Depending on the location of the low-energy peak,BL Lac objects are often referred to as high-frequency peaked(HBL; in the UV to X-ray domain) or low-frequency peaked (LBL;in the near-IR to optical) BL Lac objects (Fossati et al. 1998), al-though the transition is smooth rather than dichotomic. The ori-gin of the high-energy peak at MeV to TeV energies is still de-bated. It is commonly explained by inverse Compton upscatter-ing of low-energy photons by electrons. The seed photons may originate from synchrotron radiation produced by the same elec-tron population (synchrotron-self Compton (SSC) models; e.g.Maraschi, Ghisellini & Celotti 1992; Coppi 1992) or belong toambient thermal photon fields (external inverse Compton mod-els; e.g. Melia & K¨onigl 1989; Sikora, Begelman & Rees 1994;Dermer & Schlickeiser 1994). In hadronic models, which can alsoexplain the observed features, interactions of a highly relativistic jetoutflow with ambient matter (Dar & Laor 1997; Bednarek 1993),proton-induced cascades (Mannheim 1993), synchrotron radiationby protons (Aharonian 2000; M¨ucke & Protheroe 2001), or curva-ture radiation, are responsible for the high energy photons. TheAGN identified in the EGRET data are predominantly powerful FS-RQs and quasars with SEDs peaking at rather low frequencies, andthus only few of these (Mkn 421, PKS 2155-304, BL Lacertae and3C 279) were also detected in the VHE range.Knowing the variability time-scales of the VHE γ -ray emis-sion and the form of the two-bump spectral energy distribution(SED) enables the derivation of all input parameters of one-zoneSSC models (Tavecchio et al. 1998), which describe the observedemission in BL Lac objects reasonably well. Strictly simultaneousand temporally-resolved measurements of the SED, however, areonly rarely possible and more often than not are also severely re-stricted by the (temporal) instrumental resolution. Generally, de-tailed spectral studies particularly in low-emission states are ratherdemanding. In addition, the determination of the location of thehigh-energy peak ν ICpeak generally would require complementarysatellite detector coverage of the SED between some hundred MeVand ≈
50 GeV. This region of the SED, however, is di ffi cult to ac-cess due to the low fluxes expected (up to ≈
100 MeV) from extra-galactic sources in between the two bumps and due to insu ffi cientinstrumental sensitivity for energies exceeding some GeVs.In this paper, for the first time studies of the VHE emissionproperties of the complete set of all currently known VHE blazars(cf. Tab. 1) are performed. First, the detected VHE blazars arebrought into context with the AGN searches conducted by IACTsso far and the expected γ -ray attenuation by the EBL in Sect. 2. Af-ter a study of the black hole mass distribution of the VHE blazarsin Sect. 3, I infer intrinsic emission properties in the VHE γ -rayregime in Sect. 4. Because measurements of the location and shapeof the high-energy bump are elusive at present for almost all VHEblazars, the observed γ -ray luminosity and spectral slope in theVHE region are used as auxiliary observables to characterise the c (cid:13) , 000–000 ynoptic VHE blazar studies Table 1.
Extragalactic VHE γ -ray sources, listed in chronological order oftheir discovery.Source Type Redshift z Discovery referenceMkn 421 HBL 0.030 Punch et al. (1992)Mkn 501 HBL 0.034 Quinn et al. (1996)1ES 2344 +
514 HBL 0.044 Catanese et al. (1998)1ES 1959 +
650 HBL 0.047 Nishiyama et al. (1999)PKS 2155-304 HBL 0.116 Chadwick et al. (1999)H 1426 +
428 HBL 0.129 Horan et al. (2002)M87 a FR I 0.0044 Aharonian et al. (2004)PKS 2005-489 HBL 0.071 Aharonian et al. (2005b)1ES 1218 +
304 HBL 0.182 Albert et al. (2006b)H 2356-309 HBL 0.165 Aharonian et al. (2006a)1ES 1101-232 HBL 0.186 Aharonian et al. (2006a)PG 1553 +
113 HBL b Aharonian et al. (2006b),Albert et al. (2007a)Mkn 180 HBL 0.045 Albert et al. (2006c)PKS 0548-322 HBL 0.069 Superina et al. (2007)BL Lacertae LBL 0.069 Albert et al. (2007d)1ES 1011 +
496 HBL 0.212 Albert et al. (2007e)1ES 0229 +
200 HBL 0.139 Raue et al. (2007)1ES 0347-121 HBL 0.188 Aharonian et al. (2007c)3C 279 c FSRQ 0.536 Teshima et al. (2007)The upper part of the table shows the confirmed sources prior to the adventof new generation instruments like MAGIC and H.E.S.S., while the lowerpanel summarises the sources discovered after 2002. HBL: High-frequencypeaked BL Lac object, LBL: Low-frequency peaked BL Lac object, FSRQ:Flat spectrum radio quasar, FR: Fanaro ff –Riley galaxy. a M87 is not in-cluded in the present study. b This redshift is currently under discussion, cf.Sect. 5.6. c No spectrum of 3C 279 has been published yet.
VHE γ -ray emission. The main part of the paper (Sect. 5.1–5.5)is devoted to the search for correlations of these observables withthe X-ray emission properties, the optical and the radio luminos-ity, and with the black hole mass estimations. In Sect. 5.6 the VHEblazar luminosity distribution is used to address the specific prob-lem of the unknown redshift of PG 1553 +
113 by deriving an upperredshift for this VHE blazar. Finally, Sect. 5.7 turns to the studyof X-ray and VHE γ -ray timing properties. Sect. 6 summarises themain conclusions of the studies. γ -RAY HORIZON When travelling cosmological distances, VHE γ -rays interact withthe low-energy photons of the EBL (see, e.g. Nikishov 1962;Gould & Schr´eder 1966; Hauser & Dwek 2001; Kashlinsky 2005).The predominant reaction γ VHE + γ EBL → e + e − modifies source-intrinsic γ -ray energy spectra. The cross-section of this processpeaks strongly at E CM = . × m e c , therefore a given VHE photonenergy probes a narrow range of the EBL spectrum. The part of theEBL to which VHE γ -rays are sensitive comprises the (redshifted)relic emission of galaxies and star-forming systems and the lightabsorbed and re-emitted by dust. The EBL attenuation results in amaximum distance over which photons with a particular energy cansurvive: The Fazio-Stecker relation (FSR, Fazio & Stecker 1970;Stecker, de Jager & Salamon 1992) describes the distance at whichthe optical depth for a VHE photon of a given energy reaches unity(attenuation by a factor e − ). Thus the FSR defines the cosmological γ -ray horizon . Fig. 2a shows the instrumental low-energy thresh-olds from searches for VHE γ -ray emission from AGN. Along with these, the FSR for di ff erent EBL models as given by Kneiske et al.(2004) is plotted. The models di ff er in the IR density, dust prop-erties and the star formation rate in the early Universe. The FSRdivides the plot into a region from which no γ -rays can reach theEarth and into another region, in which positive detections are to beexpected or a too weak source-intrinsic emission made detectionsfail (due to insu ffi cient instrumental sensitivity). Obviously, witha decreasing instrumental energy threshold, the visible Universe‘opens up’, providing access to a larger source population. CurrentEBL models result in a steepening of the intrinsic spectra from ≈
200 GeV on (power-law spectra are softened, but their shape is ap-proximately retained), while for lower energies the e ff ects are min-imal. When an optical depth of one is reached, a quasi-exponentialcuto ff in the observed spectra occurs. Fig. 2b shows the energyranges over which blazars in VHE γ radiation have been detected.Up to now, only for the strong, close-by blazars Mkn 421 andMkn 501 indications of the expected exponential high energy cut-o ff have been observed thanks to high γ statistics (Aharonian et al.2001a; Krennrich et al. 2001; Albert et al. 2007c). The observedspectra of all other blazars can be accurately described by power-laws or broken power-laws. While most of the nearby VHE blazarscannot constrain the current EBL models, some of the sources at z > . ff s have been observed so far atthe high-energy ends of their γ -ray spectra. It is well established that all galaxies with a massive bulge compo-nent host supermassive black holes in their centres (Richstone et al.1998; Bender & Kormendy 2003). There are a couple of indirectmethods to infer the masses of the central BHs: One is to estimate M • using the correlation between M • and the central velocity dis-persion σ of the host galaxy ( M • − σ relation, Ferrarese & Merritt2000; Gebhardt et al. 2000) found from stellar and gas kinemat-ics and maser emission. I estimated the black hole masses ofVHE γ -ray emitting blazars using the M • − σ relation given byTremaine et al. (2002). This approach assumes that AGN hostgalaxies are similar to non-active galaxies. The velocity dispersionswere collected from the literature or are inferred from the funda-mental plane (Djorgovski & Davis 1987), a relation between σ , thee ff ective galaxy radius R e , and the corresponding surface bright-ness h µ e i , which is valid for elliptical galaxies, in particular alsofor AGN and radio galaxies (Bettoni et al. 2001). Whenever morethan one σ value is given in the literature, individual masses werederived for each of the σ values and averaged. The σ values andthe resulting black hole masses for the VHE γ -ray blazars studiedhere are given in Tab. 2.The determination of M • su ff ers from rather large sys-tematic uncertainties due to the di ff erent methods used to de-rive σ . The relation between M • and bulge luminosity L B (Kormendy & Richstone 1995) has generally a larger scatter thanthe M • − σ relation and was therefore only used for PKS 2155-304, because for this blazar no σ or h µ e i measurement is available.I used the R -band luminosity given by Falomo (1996) to calcu-late log( M • / M ⊙ ) = . ± .
44 using eq. 12 in Graham (2007).Aharonian et al. (2007b) give an estimate of M • = (1 . . . × M ⊙ . For a comparative study using values inferred by di ff er-ent methods is not advisable due to possible di ff erent systematics.Therefore, the M • value used for PKS 2155-304 should be takenwith care in the following. The BH mass of 3C 279 was determined c (cid:13) , 000–000 R. M. Wagner
Redshift z -3 -2 -1
10 1 E n e r g y [ G e V ] EBL models
Low-IRWarm-dustLow star formation rateBest-fit
Upper limits
HEGRAH.E.S.S.WhippleVERITASCANGAROOMAGIC (a)
Redshift z -3 -2 -1
10 1 E n e r g y [ G e V ] EBL models
Low-IRWarm-dustLow star formation rateBest-fit (b)
Optical Depth < 1 Optical Depth > 1
Figure 2. (a) Lower energy thresholds from searches for VHE γ -ray emission from AGN. Dots: blazars, crosses: other AGN types (starburst galaxies, radiogalaxies, etc.). The curves represent Fazio–Stecker relations (flux attenuation by a factor e − ) for di ff erent EBL models given by Kneiske et al. (2004). Datafrom searches by HEGRA (Aharonian et al. 2004), H.E.S.S. (Aharonian et al. 2005c; Benbow & B¨uhler 2007), Whipple (Kerrick et al. 1995; Horan et al.2004), VERITAS (Cogan 2007; Krawczynski 2007), CANGAROO (Nishijima 2002) and MAGIC (Albert et al. 2008). (b) Detected VHE γ -ray sources (seeTab. 3 for references). Shown are the energy ranges of the measured γ -ray spectra; high-energy cuto ff s were found only for Mkn 421 and Mkn 501, for all othersources arrows represent spectra possibly continuing to higher energies. The dashed lines represent PG 1553 +
113 at the lower and upper limit for its redshift, z > .
09 (Sbarufatti et al. 2006) and z < .
74 (Aharonian et al. 2006b; Albert et al. 2007a).
Table 2.
Measured velocity dispersions and resulting estimated black hole masses for the VHE blazars.Object σ [km s − ] log( M • / M ⊙ ) σ [km s − ] log( M • / M ⊙ ) σ [km s − ] log( M • / M ⊙ ) log( M • / M ⊙ )Reference 1 Reference 2 Reference 3 averagedMkn 421 219 ±
11 8 . ± .
18 236 ±
10 8 . ± .
15 324 ±
18 8 . ± .
12 8.56Mkn 501 372 ±
18 9 . ± .
11 291 ±
13 8 . ± .
11 . . . . . . 9.001ES 2344 +
514 294 ±
24 8 . ± .
15 . . . . . . 389 ±
20 9 . ± .
12 9.04Mkn 180 209 ±
11 8 . ± .
19 244 ±
10 8 . ± .
14 251 ±
16 8 . ± .
15 8.401ES 1959 +
650 . . . . . . 195 ±
15 8 . ± .
23 219 ±
15 8 . ± .
19 8.18BL Lacertae . . . . . . . . . . . . 245 ±
16 8 . ± .
15 8.48PKS 0548-322 202 ±
24 8 . ± .
24 . . . . . . . . . . . . 8.14PKS 2005-489 . . . . . . . . . . . . 257 ±
16 8 . ± .
14 . . .H 1426 +
428 . . . . . . . . . . . . 269 ±
16 8 . ± .
13 . . .H 2356-309 . . . . . . . . . . . . 195 ±
14 8 . ± .
23 . . .1ES 0229 +
200 . . . . . . . . . . . . 363 ±
19 9 . ± .
11 . . .1ES 1218 +
304 . . . . . . . . . . . . 191 ±
14 8 . ± .
24 . . .1ES 0347-121 . . . . . . . . . . . . 214 ±
15 8 . ± .
19 . . .1ES 1011 +
496 . . . . . . . . . . . . 219 ±
15 8 . ± .
19 . . .References: (1) Barth, Ho & Sargent (2003); (2) Falomo, Kotilainen & Treves (2002); (3) Wu, Liu & Zhang (2002). The velocitydispersions σ were translated into estimated BH masses using the M • − σ relation from Tremaine et al. (2002). The σ values takenfrom Wu et al. (2002) were indirectly determined using the fundamental plane of radio galaxies (Bettoni et al. 2001). BH massesgiven in units of the solar mass, M ⊙ . If more than one M • value is given, the average M • is used. Due to the possible di ff erentsystematic errors of the individual data sets, the largest error was assumed as error of the average M • . using the virial BH mass estimate of McLure & Dunlop (2002) andis given as log( M • / M ⊙ ) = .
912 by Gu et al. (2001). For two ofthe blazars under study, PG 1553 +
113 and 1ES 1101-232, no M • estimations exist yet.Recent estimations of the BH masses for 452 AGN find themdistributed over a large range of (10 − × ) M ⊙ with no evidencefor dependencies on the radio loudness of the objects (Woo & Urry2002a,b). A recent study of the BH mass distribution of 66 BL Lacobjects (Woo et al. 2005) reports an M • range of (10 − × ) M ⊙ and could also not find a correlation of M • with radio or X-rayluminosity. (In Sect. 5.5 the distribution of VHE blazars in lu-minosity and M • is discussed). As Fig. 3 shows, there is no de-pendence of the BH masses of the VHE blazars on their redshift,but they are rather flatly distributed in their BH masses between(10 − . )M ⊙ . Although AGN harbour BHs with M • > M ⊙ ,up to now only blazars with rather massive BHs, M • & M ⊙ ,have been discovered in VHE γ -rays, raising the question whether a physics reason is responsible for the non-detection of blazars withless massive BHs in the mass range (10 − ) M ⊙ . There ex-ist studies that find radio-loud AGN, and therefore also blazars,to be associated with BHs with M • & M ⊙ (Laor 2000) or atleast on average with more massive BHs than radio-quiet AGN(Metcalf & Magliocchetti 2006). The latter authors also report athreshold BH mass for the onset of radio activity, with very littledependence of the radio output on the BH mass once above thethreshold mass. Whether such a mass threshold is also at workfor the VHE emission, remains subject for further studies at thispoint. The BH masses of the VHE blazars are compared to those of375 AGN collected by Woo & Urry (2002a) in Fig. 4. The confine-ment of Seyfert galaxy measurements to low redshifts presumablyis due to a selection e ff ect: These are spiral galaxies and thereforeexpected to harbour comparatively low-mass BHs. Distant Seyfertgalaxies ( z & .
0) might just not be luminous enough to obtain M • c (cid:13) , 000–000 ynoptic VHE blazar studies ) Sun /M BH log(M R e d s h i ft z Source
Mkn 421Mkn 5012344+514Mkn 1801959+650BL Lac0548-3222005-4892155-3041426+4280229+2002356-3091218+3040347-1211011+4963C 279
Figure 3.
Redshift vs. M • distribution for the known VHE γ -ray emitting AGN. The superimposed histogram shows the BH mass distribution of the 375 AGNgiven in Fig. 4 (linear vertical scale independent of z ). ) Sun /M BH log(M R e d s h i ft z AGN class emitters g VHE BL LacRadio-loud quasarRadio-quiet quasarRadio galaxiesSeyfert 1Seyfert 2
Figure 4.
The redshift vs. M • distribution for 375 AGN collected by Woo & Urry (2002a) and the known VHE γ -ray emitting AGN. measurements. Conversely, quasars are too rare as to be found insmall volumes and thus at small distances. γ -RAY EMISSION PARAMETERS VHE γ -ray observations enable us to look deep into the emissionregions of blazar jets and thus convey information on the respon-sible particle acceleration and cooling processes. Here I study pri-marily the di ff erential energy spectra in the VHE domain, whichare summarised in Tab. 3. For the sources Mkn 421, Mkn 501,PKS 2155-304, 1ES 1959 + +
514 observations ofclearly distinct flux states exist. Accordingly, for each of thosesources two spectra, one ‘low-state’ and one ‘high-state’ spectrum,are considered. Low-state spectra are characterised by the absenceof high ( & . ) flux levels and short-term variabil-ity (probably beyond instrumental sensitivity though), while flarespectra were obtained during outbursts of the respective sources( viz. the 1995 December 20 flare of 1ES 2344 + The Crab nebula exhibits a strong, constant VHE γ -ray flux and is there-fore often considered a standard candle in VHE γ -ray astronomy of Mkn 501, the 2002 flare of 1ES 1959 +
650 and the 2006 July 28flare of PKS 2155-304). At present for none of these sources, evenfor those with only one flux state, can a true baseline flux state beclaimed, although low-flux states have been observed (Albert et al.2006a, 2007b). Long-term monitoring campaigns are currently per-formed to address this issue (Goebel et al. 2007; Steele et al. 2007;Punch 2007).The measured spectra su ff er γγ absorption on photons of theEBL as shown in Sect. 2. The intrinsic source spectra are recon-structed employing (Mazin 2003) the EBL ‘low-IR’ model given inKneiske et al. (2004), which assumes the least possible infrared starformation rate as allowed by galaxy counts and which is in reason-able agreement with other models (Primack, Bullock & Somerville2005; Stecker, Malkan & Scully 2006). Note that due to the factthat the VHE γ -rays are attenuated exponentially with the opticaldepth, an accurate knowledge of the EBL is crucial for the individ-ual interpretation of the intrinsic VHE γ -ray spectra.In the following, I will use two observables to characterise theVHE γ -ray emission: The K -corrected (Hogg et al. 2002) luminos-ity at 500 GeV, ν γ L γ = π d · (500 GeV) F (500 GeV / (1 + z )) / (1 + z )with the luminosity distance d L and the intrinsic photon index Γ in the region around 500 GeV, which is determined by fitting c (cid:13) , 000–000 R. M. Wagner
Table 3.
Measured VHE blazar spectra, reconstructed intrinsic spectral indices and luminosities.Object Measured Energy Spectrum d F / d E Reference Intrinsic ν γ L γ [TeV − cm − s − ] Slope Γ [erg s − sr − ]Mkn 421 (12 . ± . − ( E / . − . ± . Aharonian et al. (1999b) 2 . ± .
58 (9 . ± . × Mkn 501 (8 . ± . − ( E / . − . ± . Aharonian et al. (2001b) 2 . ± .
84 (6 . ± . × +
514 (1 . ± . − ( E / . − . ± . Albert et al. (2007b) 2 . ± .
21 (2 . ± . × Mkn 180 (4 . ± . − ( E / . − . ± . Albert et al. (2006c) 3 . ± .
50 (2 . ± . × +
650 (3 . ± . − ( E / . − . ± . Albert et al. (2006a) 2 . ± .
29 (5 . ± . × BL Lacertae (1 . ± . − ( E / . − . ± . Albert et al. (2007d) 3 . ± .
25 (2 . ± . × PKS 0548-322 (1 . ± . − ( E / . − . ± . Superina et al. (2007) 2 . ± .
28 (9 . ± . × PKS 2005-489 (1 . ± . − ( E / . − . ± . Aharonian et al. (2005b) 3 . ± .
27 (2 . ± . × PKS 2155-304 (1 . ± . − ( E / . − . ± . for E <
700 GeV, Aharonian et al. (2005a) 2 . ± .
64 (6 . ± . × (2 . + . − . )10 − (0 . ± . (3 . + . − . − . + . − . ) × ( E / . − . + . − . for E >
700 GeVH 1426 +
428 (2 . ± . − ( E / .
43 TeV) − . ± . Horan & Finley (2001), 1 . ± .
23 (7 . ± . × Aharonian et al. (2002)1ES 0229 +
200 (2 . ± . − ( E / . − . ± . Raue et al. (2007) 1 . ± .
30 (6 . ± . × H 2356-309 (3 . ± . − ( E / . − . ± . Aharonian et al. (2006a) 1 . ± .
37 (3 . ± . × +
304 (8 . ± . − ( E / .
25 TeV) − . ± . Albert et al. (2006b) 1 . ± .
40 (1 . ± . × . ± . − ( E / . − . ± . Aharonian et al. (2006a) 1 . ± .
37 (3 . ± . × . ± . − ( E / . − . ± . Aharonian et al. (2007c) 1 . ± .
14 (3 . ± . × +
496 (2 . ± . − ( E / . − . ± . Albert et al. (2007e) 2 . ± .
29 (15 . ± . × PG 1553 + a (1 . ± . − ( E / . − . ± . Albert et al. (2007a) 3 . ± .
68 (6 . ± . × PG 1553 + b (1 . ± . − ( E / . − . ± . Albert et al. (2007a) 2 . ± .
46 (3 . ± . × Mkn 421 c (23 . ± . − ( E / . − . ± . Krennrich et al. (2002) 2 . ± .
30 (1 . ± . × Mkn 501 c (2 . ± . − ( E / . − . ± . Aharonian et al. (1999a) 1 . ± .
41 (1 . ± . × + c (5 . ± . − ( E / . − . ± . Schroedter et al. (2005) 2 . ± .
31 (6 . ± . × + c (1 . ± . − ( E / . − . ± . Daniel et al. (2005) 2 . ± .
29 (2 . ± . × PKS 2155-304 c (2 . ± . − ( E / . − . ± . for E <
340 GeV, Aharonian et al. (2007b) 2 . ± .
40 (2 . ± . × (2 . ± . − (0 . ± . (3 . ± . − (2 . ± . × ( E / . − . ± . for E >
340 GeV Γ denotes the reconstructed (intrinsic) VHE spectral power-law index at 500 GeV and ν γ L γ represents the source luminosity at 500 GeV. Both values werecalculated from the measured spectra assuming a Kneiske et al. ‘low-IR’ EBL density. a at an assumed z = . b at an assumed z = . c spectrum measuredduring a flare state of the respective blazar. the intrinsic spectra with pure power-laws of the form d F / d E = f · ( E / E ) − Γ . These two parameters act as proxies for the peakposition and the spectral shape on the falling edge of the high-energy bump, which cannot, as explained before, easily be deter-mined from the existing VHE data. For extraction of the luminosityand of the spectral slope, the region around 500 GeV was chosenbecause all blazars under study have measured spectra in this en-ergy region. All calculations and fits have been performed in en-ergy ranges where γ -ray spectra for the respective blazars have ac-tually been observed, so that no extrapolations in energy regionsnot covered by the data were required. All in all, extragalacticsource observations included in this paper cover the energy range85 GeV E
11 TeV. For the determination of the luminosity dis-tances the cosmological parameters given in Spergel et al. (2007)were used: Ω m h = . + . − . ; Ω b h = . + . − . with the Hub-ble constant H = · h km s − Mpc − = + − km s − Mpc − .Tab. 3 also shows the resulting source luminosities in the VHEregion and the spectral slope of the reconstructed source-intrinsicspectra. While the luminosities range from ≈ erg s − sr − to ≈ × erg s − sr − ( ≈ erg s − sr − to ≈ × erg s − sr − for blazars in outburst), the photon indices of the reconstructed in-trinsic spectra vary between Γ = . − .
3, except for 1ES 1101-232, which probably has an intrinsic spectrum peaking far beyond E = Γ = Γ < . ff ects are still smallerthan currently modelled (e.g. Aharonian et al. 2006a): a lower EBLlevel would soften the intrinsic spectra inferred here, i.e. increasethe value of Γ .Before instruments like MAGIC and H.E.S.S. became op-erational, the average observed photon index in BL Lac objectswas Γ ≈ .
3. This raised the expectation that AGN would ingeneral exhibit rather hard spectra in the VHE range, which alsowould be compatible with the average EGRET blazar spectrum(at MeV-GeV energies), found to have a slope of Γ EGRET = . Γ increasedas a new population of objects with intrinsically rather soft spectra(Mkn 180, PKS 2005-489) has been tapped, and at the same timedistant, hard-spectrum sources were found.For PG 1553 +
113 with its unknown redshift (see Sect. 5.6 fordetails), two possible distances ( z = . z = .
3) were assumedin this paper. The resulting ‘intrinsic spectra’, however, are onlygiven for illustrative purposes and are not used for any conclusionsthroughout this study unless stated otherwise. c (cid:13) , 000–000 ynoptic VHE blazar studies ] -1 s -1 [erg sr X L X n ] - s - [ e r g s r g L gn Source
Mkn 421Mkn 5012344+514Mkn 1801959+650BL Lac0548-3222005-4892155-3041426+4280229+2002356-3091218+3041101-2320347-1211011+4961553+113
Figure 5.
VHE γ -ray luminosity ν γ L γ vs. X-ray luminosity at 1 keV, ν X L X for 17 VHE blazars. The two data points for PG 1553 +
113 (open crosses) are forassumed redshifts of z = . ν γ L γ = . × erg sr − s − ) and z = . ν γ L γ = . × erg sr − s − ), respectively, and are not used in the fit and fordetermining the correlation coe ffi cient (see text). It should be noted that VHE data points have an additional systematic error of typically 35 per cent. Thesystematic error of the X-ray luminosities is unfortunately unknown. γ -rayluminosity In SSC models, the X-ray and the VHE emission are closely con-nected, owing to their common origin. While in some blazars clearevidence for a corresponding correlation has been observed (Mkn421, Krawczynski et al. 2001; Bła˙zejowski et al. 2005; Albert et al.2007c), the connection is only weak for other ones (Mkn 501,Albert et al. 2007f) or even non-existing during some flare states(the 1ES 1959 + orphan flare case, Daniel et al. 2005). Fig. 5shows ν γ L γ versus the X-ray luminosity at 1 keV ( ν X L X ; fromCostamante & Ghisellini 2002). Note that high thermal contribu-tions at 1 keV are unlikely and would imply a very high amountof gas and pressure. Unfortunately, the X-ray and VHE data havenot been taken simultaneously. While VHE measurements duringoutbursts were not used in the Figure, variations in the X-ray do-main for the blazars under study are, according to the compilationof X-ray fluxes in Donato et al. (2001), not larger than a factor 4.6(most extreme object: Mkn 501) with an average of a factor 1.4and a variance of 2.2. According to expectations, a trend towardsa correlation is visible. When including all data points except forthose representing PG 1553 + ffi cient of r = . + . − . , which is within 3.6 standard deviations di ff erent fromzero. A linear fit to the data yields a slope of m = . ± . χ = . / + + d L .Fig. 6 shows the corresponding correlations: the data in the VHE –optical plane feature a larger scatter than that in the VHE – X-raydata, while in the VHE – radio plane no clear trend is seen.VHE blazars might populate only restricted ranges in X-ray,optical, or radio luminosity distributions. To test this, I use X-ray,optical and radio data from the full set of 246 sources considered byCostamante & Ghisellini (2002). I rejected the sources for whichno redshift is known and converted the remaining 183 fluxes intoluminosities. These blazars are compared to the VHE blazars inFig. 7. In none of the three distributions can substantial deviationsof the VHE blazars from the overall set of blazars be found. In SSC models the VHE peak, identified with the inverse Comp-ton (IC) peak with a maximum at ν ICpeak , resembles the form (e.g.Fossati et al. 1998) of the synchrotron peak at ν Sypeak , displacedby the squared Lorentz factor ν ICpeak /ν Sypeak ∼ γ (Tavecchio et al.1998). Nieppola, Tornikoski & Valtaoja (2006) collected (non-simultaneous) multiwavelength data for a large ( > c (cid:13)000
113 (open crosses) are forassumed redshifts of z = . ν γ L γ = . × erg sr − s − ) and z = . ν γ L γ = . × erg sr − s − ), respectively, and are not used in the fit and fordetermining the correlation coe ffi cient (see text). It should be noted that VHE data points have an additional systematic error of typically 35 per cent. Thesystematic error of the X-ray luminosities is unfortunately unknown. γ -rayluminosity In SSC models, the X-ray and the VHE emission are closely con-nected, owing to their common origin. While in some blazars clearevidence for a corresponding correlation has been observed (Mkn421, Krawczynski et al. 2001; Bła˙zejowski et al. 2005; Albert et al.2007c), the connection is only weak for other ones (Mkn 501,Albert et al. 2007f) or even non-existing during some flare states(the 1ES 1959 + orphan flare case, Daniel et al. 2005). Fig. 5shows ν γ L γ versus the X-ray luminosity at 1 keV ( ν X L X ; fromCostamante & Ghisellini 2002). Note that high thermal contribu-tions at 1 keV are unlikely and would imply a very high amountof gas and pressure. Unfortunately, the X-ray and VHE data havenot been taken simultaneously. While VHE measurements duringoutbursts were not used in the Figure, variations in the X-ray do-main for the blazars under study are, according to the compilationof X-ray fluxes in Donato et al. (2001), not larger than a factor 4.6(most extreme object: Mkn 501) with an average of a factor 1.4and a variance of 2.2. According to expectations, a trend towardsa correlation is visible. When including all data points except forthose representing PG 1553 + ffi cient of r = . + . − . , which is within 3.6 standard deviations di ff erent fromzero. A linear fit to the data yields a slope of m = . ± . χ = . / + + d L .Fig. 6 shows the corresponding correlations: the data in the VHE –optical plane feature a larger scatter than that in the VHE – X-raydata, while in the VHE – radio plane no clear trend is seen.VHE blazars might populate only restricted ranges in X-ray,optical, or radio luminosity distributions. To test this, I use X-ray,optical and radio data from the full set of 246 sources considered byCostamante & Ghisellini (2002). I rejected the sources for whichno redshift is known and converted the remaining 183 fluxes intoluminosities. These blazars are compared to the VHE blazars inFig. 7. In none of the three distributions can substantial deviationsof the VHE blazars from the overall set of blazars be found. In SSC models the VHE peak, identified with the inverse Comp-ton (IC) peak with a maximum at ν ICpeak , resembles the form (e.g.Fossati et al. 1998) of the synchrotron peak at ν Sypeak , displacedby the squared Lorentz factor ν ICpeak /ν Sypeak ∼ γ (Tavecchio et al.1998). Nieppola, Tornikoski & Valtaoja (2006) collected (non-simultaneous) multiwavelength data for a large ( > c (cid:13)000 , 000–000 R. M. Wagner ] -1 s -1 [erg sr O L O n ] - s - [ e r g s r g L gn Source
Mkn 421Mkn 5012344+514Mkn 1801959+650BL Lac0548-3222005-4892155-3041426+4280229+2002356-3091218+3041101-2320347-1211011+4961553+113 (a)
VHE vs. optical luminosity ] -1 s -1 [erg sr R L R n ] - s - [ e r g s r g L gn Source
Mkn 421Mkn 5012344+514Mkn 1801959+650BL Lac0548-3222005-4892155-3041426+4280229+2002356-3091218+3041101-2320347-1211011+4961553+113 (b)
VHE vs. radio luminosity
Figure 6. (a) Correlation of VHE luminosity ν γ L γ and optical luminosity ν O L O . (b) Correlation of VHE luminosity ν γ L γ and radio luminosity ν R L R . The twodata points for PG 1553 +
113 (open crosses) are for assumed redshifts of z = . ν γ L γ = . × erg sr − s − ) and z = . ν γ L γ = . × erg sr − s − ),respectively. ] -1 s -1 [erg sr X L X n log 41 42 43 44 45 46 47 s ou r ces C&G sampleVHE sample (a) ] -1 s -1 [erg sr O L O n log 42 43 44 45 46 47 48 s ou r ces Optical luminosity
C&G sampleVHE sample (b) ] -1 s -1 [erg sr R L R n log 39 40 41 42 43 44 45 s ou r ces Radio luminosity
C&G sampleVHE sample (c)
Figure 7.
Histograms of (a) 1 keV X-ray, (b) 5500 Å optical and (c) 5 GHz radio luminosity of the VHE blazars and all those blazars considered byCostamante & Ghisellini (2002) with determined redshifts (183 out of the total set of 246 blazars). ν peak . I test for correlations between ν Sypeak and the VHE luminosityand spectral slope (Fig. 8). According to expectations from SSCmodels, I find a correlation of the photon index Γ with ν Sypeak . Thecorrelation coe ffi cient is r = − . + . − . and a linear fit to the dataas shown in Fig. 8a yields a χ = . /
8. Fig. 8b shows the corre-sponding data points in the ν γ L γ − ν Sypeak plane, in which no correla-tion is apparent. Finally, in Fig. 9, I check whether the LBL–HBLtransition of the VHE blazars is connected with the BH masses oftheir host galaxies. Ten of the VHE blazars, for which both ν Sypeak and M • measurements are available, show no trend towards a cor-relation. γ -ray luminosity Fig. 10a relates the intrinsic photon indices Γ to the VHE γ -ray lu-minosities ν γ L γ . For all objects under study (excluding flare statesand the PG 1553 +
113 data), the Γ − ν γ L γ correlation reads as Γ = Γ + m · log ( ν γ L γ ) with Γ = . ± . m = − . ± . χ = . /
14. Theline at
Γ = Γ asa function of ν γ L γ , I notice that the distribution sharpens towards Γ =
2. This might reflect that the highest luminosity occurs at
Γ =
2. Thus the spread of the data reflects the spread of the shapesof the high energy peaks. The general behaviour above Γ ≈ γ -ray luminosity, the harder the spectrum –can within SSC models be described with a moving IC peak to-wards higher energies with increasing luminosity. The five blazars with observed spectra at quiescent and flare states partially be-have in a similar manner (Fig. 10b): Mkn 421 (Krawczynski et al.2001; Bła˙zejowski et al. 2005), Mkn 501 (Albert et al. 2007f) and1ES 2344 +
514 (Albert et al. 2007b) also show a spectral harden-ing during high-flux states, flares and outbursts. Fig. 11a showsthe corresponding luminosity di ff erences ∆ ( ν γ L γ ) and slope di ff er-ences ∆Γ . Mkn 501 and 1ES 2344 +
514 show a similar change inspectral slope and a dynamical range of ∆ ( ν γ L γ ) ≈
20. The lu-minosity increase of Mkn 421 observed up to now is much lowerwith ∆ ( ν γ L γ ) ≈
10. While the spectral slope of Mkn 421 alsohardens with increasing luminosity, 1ES 1959 +
650 and PKS 2155-304 show within errors no variation in their spectral slopes duringflares, while their luminosities increase rather drastically by a fac-tor of 40 and 50, respectively. When plotting the luminosity di ff er-ence versus the BH masses (Fig. 11b), a trend, although broken by1ES 1959 + ff ects: redshift dependencies? Possible correlations of Γ and ν γ L γ with the redshift z are not ex-pected, but may identify selection e ff ects in the data set and / oran inaccurate EBL model. Conspicuously, very hard ( Γ ≪ . z & .
1) blazars (Fig. 12a), a trend also observed byStecker, Baring & Summerlin (2007), who used a smaller set ofVHE blazars. At the same time, none of the measured spectra ofnearby sources shows Γ much smaller than 2.0, although for theseblazars no strong EBL modifications apply and the measured spec- c (cid:13) , 000–000 ynoptic VHE blazar studies [Hz] peakSy n G I n t r i n s i c pho t on i nd ex Source
Mkn 421Mkn 5012344+514Mkn 1801959+650BL Lac1426+4280229+2001218+3041011+4961553+113
LBL IBL HBL XBL [Hz] peakSy n ] - s - [ e r g s r g L gn Source
Mkn 421Mkn 5012344+514Mkn 1801959+650BL Lac1426+4280229+2001218+3041011+4961553+113 (b)
LBL IBL HBL XBL
Figure 8. (a) Photon index Γ vs. synchrotron peak frequency ν Sypeak and (b) VHE luminosity vs. synchrotron peak frequency ν Sypeak for eleven VHE blazars.The synchrotron peak frequencies and the LBL-IBL-HBL classification are taken from Nieppola et al. (2006). The two data points for PG 1553 +
113 (opencrosses) are for assumed redshifts of z = . Γ = . ± . ν γ L γ = . × erg sr − s − ) and z = . Γ = . ± . ν γ L γ = . × erg sr − s − ),respectively, and are not used in the fit and for determining the correlation coe ffi cient in the left Figure (a). [Hz] peakSy n ) S un / M BH l og ( M Source
Mkn 421Mkn 5012344+514Mkn 1801959+650BL Lac1426+4280229+2001218+3041011+496
Figure 9.
Estimated BH mass vs. synchrotron peak frequency ν Sypeak for those VHE blazars with both quantities known. tra should not di ff er substantially from the intrinsic ones in the en-ergy region studied here. Why are only blazars with rather hardintrinsic spectra visible at large distances ( z > . z is an overcorrection of the EBL attenuation e ff ects.Fig. 12b shows the corresponding distribution of VHE lumi-nosities ν γ L γ as a function of z . Two curves indicate the sensitivitylimit of current IACTs (e.g. Aharonian et al. 2006b), a significantdetection of 1 per cent of the flux of a Crab nebula-like source in25 hours, and the sensitivity of previous (before 2002) IACTs of ≈
10 per cent of this flux. Interestingly, most of the sources foundat z > .
05 seem to be rather low-luminosity sources in the sensethat their luminosity is not much higher than the current instrumen-tal sensitivity allows for. This means that not only the substantiallylower energy thresholds of the current IACTs ( .
100 GeV as com-pared to ≈
300 GeV until 2004), but also their increased sensitivityenabled some of the new blazar discoveries. Exceptions to thesedetections close to the current sensitivity limit are (trivially) the sixVHE blazars discovered before 2002, 1ES 1011 +
496 (but discov-ered during an optical flare, while its low-flux state seems to lie be-low the current instrumental reach, Albert et al. 2007e, 2008), and1ES 1218 + +
428 has not yet been detectedafter 2002 (e.g. Krawczynski 2007; Albert et al. 2008), which alsocurrently places it below sensitivity limits. γ -ray emission parameterswith the black hole mass The γ -ray production is thought to take place at shock fronts insidethe AGN jets at very close (sub-parsec) distances from the centralBH (Jester et al. 2006; Uchiyama et al. 2006). While the jet produc-tion and collimation mechanism is still elusive, accepted modelsare generally based on magnetohydrodynamic (Blandford & Payne1982; Kudoh et al. 1999) or electromagnetic jet models. In the lat-ter, a Poynting flux dominated flow is launched from a Kerr BH(Blandford & Znajek 1977) or from the accretion disc (Blandford1976). The conversion from Poynting dominance into particle dom-inance is not yet understood. The properties of the blazar γ -rayemission are expected to be connected to the properties of the cen-tral BH, like M • and the BH spin, since scaling laws govern BHphysics (McHardy et al. 2006), in particular length and time-scalesof flows (Mirabel & Rodr´ıguez 1999; Mirabel 2004), e.g. the or-bital period of the last stable BH orbit. Currently, only M • canbe reliably estimated; the BH spin remains inaccessible by large.Moreover, the environment in which the BH is embedded might beequally important; one of its properties, the accretion rate, is indi-rectly accessible through the (radio) jet power (Liu et al. 2006), orfrom multiwavelength modelling (Maraschi & Tavecchio 2003). Aprevious study of the connection of spectral properties and M • forfive VHE blazars (Krawczynski et al. 2004) did not find any corre-lations with the BH mass.For 15 VHE blazars with known BH masses, I neither find c (cid:13) , 000–000 R. M. Wagner
Redshift z0.03 0.04 0.1 0.2 0.3 G I n t r i n s i c pho t on i nd ex Source
Mkn 421Mkn 5012344+514Mkn 1801959+650BL Lac0548-3222005-4892155-3041426+4280229+2002356-3091218+3041101-2320347-1211011+4961553+113 (a)
Photon index vs. redshift
Redshift z0.03 0.04 0.1 0.2 0.3 ] - s - [ e r g s r g L gn Source
Mkn 421Mkn 5012344+514Mkn 1801959+650BL Lac0548-3222005-4892155-3041426+4280229+2002356-3091218+3041101-2320347-1211011+4961553+113 (b)
Luminosity vs. redshift
Figure 12.
Correlations of redshift with (a) the intrinsic photon index and (b) VHE γ -ray luminosity. For Mkn 421, Mkn 501, 1ES 2344 + +
650 flare flux levels are also included in the plots; the corresponding data points are marked by additional grey circles. The two data points forPG 1553 +
113 (open crosses) are for assumed redshifts of z = . z = .
3, respectively. The dotted curve in the right Figure (b) marks the sensitivity (e.g.Aharonian et al. 2006b) limit of current IACTs (selection e ff ect); the dashed curve indicates the sensitivity of previous (before 2002) IACTs. ] -1 s -1 [erg sr g L g n G I n t r i n s i c pho t on i nd ex Source
Mkn 421Mkn 5012344+514Mkn 1801959+650BL Lac0548-3222005-4892155-3041426+4280229+2002356-3091218+3041101-2320347-1211011+4961553+113 ) g L g n ( log (cid:215) + m G = G – = 38.45 G – m = -0.82 = 40.18/14 red2 c (a) ] -1 s -1 [erg sr g L g n G I n t r i n s i c pho t on i nd ex Source
Mkn 421Mkn 5012344+5141959+6502155-304 (b)
Figure 10. (a) Intrinsic photon index vs. luminosity. Additional flare statesof sources are marked by grey circles. The results of a linear fit of the form
Γ = Γ + m log ( ν γ L γ ) are given in the figure. The two data points forPG 1553 +
113 (open crosses), not included in this fit, are for assumed red-shifts of z = . z = . (b) As before, but only for the five blazars for which low and highVHE γ flux states have been observed. a correlation between M • and the spectral slope Γ (Fig. 13a), norbetween M • and the VHE γ -ray luminosity (Fig. 13b). While theVHE blazar set disfavours a dependency of the VHE γ -ray emissionproperties studied here on M • , the uncertainties of the M • determi-nation are still rather large and might conceal otherwise interestingphysics. In particular, indirectly inferred σ values from the funda- ) g L g n ( D l o w G - f l a r e G -1.5-1-0.500.5 Mkn 421 Mkn 501 2344+5141959+6502155-304 (a) ) g L g n ( D ) S un / M BH l og ( M (b) Figure 11. (a) Evolution of intrinsic spectral index Γ and source lumi-nosity from low to high VHE γ flux states: luminosity ratio ∆ ( ν γ L γ ) = ( ν γ L γ ) flare / ( ν γ L γ ) low versus the di ff erence of intrinsic photon indices. (b)luminosity ratio ∆ ( ν γ L γ ) = ( ν γ L γ ) flare / ( ν γ L γ ) low versus M • . mental plane or bulge luminosity measurements have rather largeuncertainties. To improve on the M • uncertainties, it would be de-sirable to obtain direct measurements of σ for all VHE blazars.Moreover, VHE emission properties may also depend sensitivelyon the BH spin, the accretion rate, or on properties of the accel-eration region in the jet. Also results on timing properties (seeSect. 5.7) support such claims. + The redshift determination for blazars is challenging, as these AGNgenerally exhibit only weak spectral lines. In several attempts, noemission or absorption lines could be found in the optical / IR spec- c (cid:13) , 000–000 ynoptic VHE blazar studies ) Sun /M BH log(M7.8 8 8.2 8.4 8.6 8.8 9 9.2 9.4 9.6 9.8 G I n t r i n s i c pho t on i nd ex Source
Mkn 421Mkn 5012344+514Mkn 1801959+650BL Lac0548-3222005-4892155-3041426+4280229+2002356-3091218+3040347-1211011+496 (a) BH Photon index vs. M ) Sun /M BH log(M7.8 8 8.2 8.4 8.6 8.8 9 9.2 9.4 9.6 9.8 ] - s - [ e r g s r g L gn Source
Mkn 421Mkn 5012344+514Mkn 1801959+650BL Lac0548-3222005-4892155-3041426+4280229+2002356-3091218+3040347-1211011+496 (b) BH Luminosity vs. M
Figure 13.
Correlations of black hole mass with (a) the intrinsic photon index and (b) VHE γ -ray luminosity. For Mkn 421, Mkn 501, 1ES 2344 +
514 and1ES 1959 +
650 flare flux levels are also included in the plots; the corresponding data points are marked by additional grey circles. trum of PG 1553 + z = .
36 (Miller & Green 1983) wasfound to be based on a misidentified emission line and could not bereproduced (Falomo & Treves 1990; Falomo, Scarpa & Bersanelli1994). VLT optical spectroscopy (Sbarufatti et al. 2006) yields alower limit of z > .
09, while the analysis of Hubble Space Tele-scope images leads to the prediction of a redshift in the range of z = . − . γ -rayspectrum find upper limits of z < .
74 and z < .
42, respectively.With increasing distance, the luminosity of PG 1553 +
113 hasto increase stronger than quadratic due to EBL γγ absorption asto sustain the measured VHE flux. Fig. 14 shows the source lu-minosity (1) when only considering the distance e ff ect and (2)when also taking into account the EBL absorption. Due to theexponential behaviour of the EBL attenuation, the latter e ff ect isby far dominant. I assume here that PG 1553 +
113 is an ‘o ff theshelf’ blazar, i.e. with no extraordinarily high luminosity ν γ L γ .This assumption is di ffi cult to quantify; given the overall dynam-ical range of the (non-flare) blazar luminosities in this study of ≈
75, I consider the case in which the luminosity of PG 1553 + +
113 has in two yearsof observations not shown any apparent flaring behaviour; withina factor of three, the measured flux was constant (Aharonian et al.2006b; Albert et al. 2007a; Benbow et al. 2007). Therefore I con-sider only sources in non-flare states, of which 1ES 1218 +
304 with ν γ L γ = . × erg s − sr − is the most luminous one. A 30-timeshigher luminosity then implies a limit of z < .
45, while an ex-treme luminosity of ν γ L γ = . × erg sr − s − yields a limitof z < .
64. These limits do not only depend on a good knowl-edge of the EBL over a wide range in redshift, but also on theassumed maximum VHE blazar luminosity that strongly dependson the Doppler factor δ . In any case, either a strikingly high lu-minosity or a very high δ is needed to explain the observationsshould PG 1553 +
113 be more distant than z & .
35. Note that δ .
20 su ffi ces for most of the blazars modelled up to now dur-ing non-outburst times, as also for flare observations (e.g. Mkn 421,Maraschi et al. 1999). SSC modelling for PG 1553 +
113 resulted in δ =
21 (Costamante & Ghisellini 2002; Albert et al. 2007a).The di ffi culties in finding emission and absorption lines mightindicate a very close alignment of the jet axis of PG 1553 + +
113 is a rather distant source and the blazar populationsat large distances show significantly di ff erent properties than theclose-by objects at z < .
2, and that such very extreme objects areso rare that a su ffi ciently large volume had to be probed to find oneof them. Following a method described in Krawczynski et al. (2004), I de-termine the time fraction (‘duty cycle’) for which the (2 −
10) keVX-ray flux exceeds the average flux by 50 per cent. In this paper,for a blazar to be regarded ‘on duty’ this deviation is additionallyrequired to be significant on the 3 σ level. 2-10 keV X-ray lightcurves are obtained from the All-Sky Monitor detector on boardthe Rossi X-ray Timing Explorer ( RXTE ) and are available from1996 January 5 on. Fig. 15 shows the corresponding light curvesalong with the resulting duty cycles. Objects which are classified asextreme BL Lacs (Mkn 501, 1ES 2344 + + M • / M ⊙ ) > .
8) do not have duty cycles in excess of 17per cent. This goes in line with a speculation of an anticorrelationbetween the X-ray flare duty cycle and M • seen in the five VHEblazars studied in Krawczynski et al. (2004). The distribution of theX-ray duty cycle as a function of luminosity is found to be ratherflat (Fig. 16b), supporting the claim that variability on all scales isa defining property of blazars. Note, however, that Mkn 501 and available at http: // xte.mit.edu / c (cid:13) , 000–000 R. M. Wagner
Redshift z ]) - s - [ e r g s r g L gn l og ( distance + EBL lowdistance only sensitivity limit Source
Mkn 421Mkn 5012344+514Mkn 1801959+650BL Lac0548-3222005-4892155-3041426+4280229+2002356-3091218+3041101-2320347-1211011+4961553+113
Figure 14.
Luminosity evolution for PG 1553 +
113 assumed at di ff erent distances. Together with the luminosities of the other known VHE γ -ray emittingblazars, the luminosity of PG 1553 +
113 as a function of its assumed distance is shown. The solid curve includes both the distance and the EBL attenuatione ff ect, the latter calculated using the ‘low-IR’ model given in Kneiske et al. (2004). The dashed curve illustrates how weak the e ff ect only by increasing thedistance is. The dotted curve indicates the current IACT sensitivity limit neglecting EBL attenuation e ff ects. Blazars in flaring state, marked with additionalgray circles, were ignored for determining the redshift limit (see text). PKS 2155-304, the objects which show the fastest variability inVHE γ -rays, are among the objects with a rather low duty cycle.Blazars are characterised by a highly variable emission. Inparticular the VHE γ -ray emission is often found to be more vari-able than the emission at other wavelenghts. Still, comprehensivedata on the temporal behaviour of VHE blazar emission has of-ten been collected only for blazars in outburst, and thus, overshort time spans. Although VHE light curves ranging over tenyears’ worth of measurements were collected occasionally (e.g.Albert et al. 2007b,f; Tluczykont et al. 2007), the sampling is onlysparse, and continuous, unbiased long-term monitoring campaignshave not been started until 2005 (Goebel et al. 2007; Steele et al.2007; Punch 2007).Variability time-scales τ are intimately linked to the exten-sion of the region R from which the observed emission originatesby the causality condition R . δ c τ/ (1 + z ). Cui (2004) has sug-gested that the flare hierarchy seen in Mkn 501 long-term dataimplies a scale-invariant nature of the flare process and that theremight not be any fundamental di ff erence among long, intermediateand rapid flares. Although not thoroughly understood, the flares inblazars might be related to internal shocks in the jet (Rees 1978a;Spada et al. 2001) or to major ejection events of new componentsof relativistic plasma into the jet (B¨ottcher, Mause & Schlickeiser1997; Mastichiadis & Kirk 1997). Di ff erent flare time-scales thusmay be caused by a hierarchy of inhomogeneities in the jet, ener-gised so as to produce flares. As a timing property of the VHE γ -rayemission, the minimum flux doubling times for the VHE blazarswere collected from literature and plotted versus M • (Fig. 17a) andthe VHE luminosity (Fig. 17b). Because of the very limited database of VHE variability measurements, which also is biased to-wards high-flux states and outbursts, time-scales are mostly givenas upper limits, which disables strong conclusions.The observed VHE flux doubling times do not scale with theBH mass (Fig. 17a), which may simply mean that (1) the flaringmechanism is working in a much smaller region than the BH ra-dius / least stable orbit and more importantly (2) the BH and its prop-erties as such do not influence the flaring process substantially, andthe embedding environment of the BH and the jet environment playmore dominant roles (e.g., the accretion power of the system). Theextremely short doubling times of τ < T = . τ < δ in the γ -ray production regions as to avoid self-absorption. In contrast to the expected scaling behaviour of the flowproperties around BH with their masses, the three AGN that hostrather massive BHs, PKS 2155-304, Mkn 501 and Mkn 421 werefound to exhibit the shortest variability time-scales. Until minute-scale flaring was found in PKS 2155-304, one could have arguedthat short time variability could only be measured for high fluxesdue to the proximity of the respective sources: Mkn 421 and Mkn501 are the closest blazars at z < . z = . ff ect: Minute-scale flares were foundonly during exceptional high-flux states of the respective sourcesso far.In Fig. 18 I translate the BH masses into the correspondinggravitational radii r g = GM / c ( = . R . δ c τ/ (1 + z ), assuming δ =
10. For blazarsin which fast variability has been observed, the extreme compact-ness of the emission region is apparent, being clearly comparableto or smaller than the Schwarzschild radius of the central BH. Evenif the flares were driven by extremely large Doppler factors, say δ ≈ Before the new generation of IACT became operational, only sixfirmly detected extragalactic VHE γ -ray sources were known (e.g.,Mori 2003); not for all of them di ff erential energy spectra hadbeen inferred. To date, there are 17 BL Lac objects and the FSRQ3C 279 known to emit VHE γ -rays. This substantially enlarged setof blazars called for a synoptic study. I collected and derived in-trinsic properties of the VHE γ -ray emission (luminosity, spectralhardness, temporal properties) and further included X-ray, opticaland radio emission properties. As an accessible property of the BHs c (cid:13) , 000–000 ynoptic VHE blazar studies Time [MJD-50000]0 1000 2000 3000 4000 ] - C oun t s [ s BL Lacertae (LBL) -1 – Average flux: 0.15 Duty cycle: 18.0% s >3 Time [MJD-50000]0 1000 2000 3000 4000 ] - C oun t s [ s Mkn 421 (HBL) -1 – Average flux: 0.86 Duty cycle: 27.3% s >3 Time [MJD-50000]0 1000 2000 3000 4000 ] - C oun t s [ s Mkn 501 (XBL) -1 – Average flux: 0.49 Duty cycle: 12.7% s >3 Time [MJD-50000]0 1000 2000 3000 4000 ] - C oun t s [ s -1 – Average flux: 0.17 Duty cycle: 16.7% s >3 Time [MJD-50000]0 1000 2000 3000 4000 ] - C oun t s [ s Mkn 180 (HBL) -1 – Average flux: 0.12 Duty cycle: 18.7% s >3 Time [MJD-50000]0 1000 2000 3000 4000 ] - C oun t s [ s -1 – Average flux: 0.30 Duty cycle: 18.7% s >3 Time [MJD-50000]0 1000 2000 3000 4000 ] - C oun t s [ s PKS 0548-322 (XBL) -1 – Average flux: 0.17 Duty cycle: 16.7% s >3 Time [MJD-50000]0 1000 2000 3000 4000 ] - C oun t s [ s PKS 2005-489 (HBL) -1 – Average flux: 0.28 Duty cycle: 21.3% s >3 Time [MJD-50000]0 1000 2000 3000 4000 ] - C oun t s [ s PKS 2155-304 (HBL) -1 – Average flux: 0.29 Duty cycle: 14.7% s >3 Time [MJD-50000]0 1000 2000 3000 4000 ] - C oun t s [ s
1H 1426+428 (XBL) -1 – Average flux: 0.19 Duty cycle: 16.0% s >3 Time [MJD-50000]0 1000 2000 3000 4000 ] - C oun t s [ s -1 – Average flux: 0.09 Duty cycle: 14.7% s >3 Time [MJD-50000]0 1000 2000 3000 4000 ] - C oun t s [ s -1 – Average flux: 0.21 Duty cycle: 17.3% s >3 Time [MJD-50000]0 1000 2000 3000 4000 ] - C oun t s [ s -1 – Average flux: 0.18 Duty cycle: 16.0% s >3 Time [MJD-50000]0 1000 2000 3000 4000 ] - C oun t s [ s -1 – Average flux: 0.12 Duty cycle: 14.0% s >3 Time [MJD-50000]0 1000 2000 3000 4000 ] - C oun t s [ s -1 – Average flux: 0.12 Duty cycle: 22.7% s >3 Time [MJD-50000]0 1000 2000 3000 4000 ] - C oun t s [ s PG 1553+113 (HBL) -1 – Average flux: 0.22 Duty cycle: 15.3% s >3 Time [MJD-50000]0 1000 2000 3000 4000 ] - C oun t s [ s
3C 279 (FSRQ) -1 – Average flux: 0.07 Duty cycle: 8.0% s >3 Figure 15.
X-ray (2-10 keV) light curves of VHE γ -ray emitting blazars. The flare duty cycle, i.e. the fraction of time in which the respective object significantlyexceeds its average X-ray flux by 50 per cent is shown. For convenience the ranges of the vertical axes are fixed for all plots. FSRQ: Flat spectrum radio quasar,LBL: low-frequency peaked BL Lac object; HBL: high-frequency peaked BL Lac object, synchrotron peak in the UV / X-ray range; XBL: extreme BL Lacobject.c (cid:13) , 000–000 R. M. Wagner ) Sun /M BH log(M D u t y cyc l e Source
Mkn 421Mkn 5012344+514Mkn 1801959+650BL Lac0548-3222005-4892155-3041426+4280229+2002356-3090347-1211011+4963C 279 (a) BH Duty cycle vs. M ] -1 s -1 [erg sr g L g n D u t y cyc l e Source
Mkn 421Mkn 501Mkn 1801959+650BL Lac0548-3222005-4892155-3041426+4280229+2002356-3091101-2320347-1211011+4961553+113 (b)
Duty cycle vs. luminosity
Figure 16.
Correlations of the X-ray duty cycle with (a) the BH mass and (b) the VHE luminosity. ) Sun /M BH log(M [ d ] t -3 -2 -1 Source
Mkn 421Mkn 5012344+5141959+6502005-4892155-3043C 279 (a) BH VHE time scale vs. M ] -1 s -1 [erg sr g L g n [ d ] t -3 -2 -1 Source
Mkn 421Mkn 5012344+5141959+6502005-4892155-3041101-2321553+113 (b)
VHE time scale vs. luminosity
Figure 17.
Correlations of the VHE γ flux doubling time τ with (a) the BH mass and (b) the VHE luminosity. Upper limits on τ are taken from Costamante et al.(2007) and from the references given in Tab. 3. Additional data points represent the recently found fast doubling times for Mkn 501 (Albert et al. 2007f) andPKS 2155-304 (Aharonian et al. 2007b). Further for Mkn 421 (Gaidos et al. 1996), 1ES 2344 +
514 and 1ES 1959 +
650 doubling times during flare states arealso included, which are marked by additional grey circles. The two data points for PG 1553 +
113 represent assumed redshifts of z = . z = . [m] g Gravitational black hole radius r · · [ m ] d S i z e o f e m i tt i ng r e g i on R / Source
Mkn 421Mkn 5012344+5141959+6502005-4892155-3041101-2323C 279 ( S c h w a r z s c h i l d r a d i u s ) g = r d R / ( L a s t s t a b l e o r b i t ) g = r d R / g = . r d R / g = . r d R / Figure 18.
Gravitational radius r g vs. upper limits on the size of the VHE γ -ray emission region R determined by the causality condition. δ = · δ denotesthe Doppler factor. The lines correspond to emission region sizes of the last stable orbit radius (dotted line), of the Schwarzschild radius (dashed line), and of10 per cent and 1 per cent of the Schwarzschild radius, respectively (dot-dashed lines). c (cid:13) , 000–000 ynoptic VHE blazar studies in the centres of the blazar host galaxies, the BH mass was alsoused. The studies yield the following results.(i) So far, only blazars with M • & M ⊙ show VHE γ emission.Whether this experimental finding constitutes a selection e ff ect orwill reveal interesting physics, remains to be seen.(ii) The VHE luminosity of the blazars under study and the corre-sponding X-ray luminosity show a hint of a correlation, as expectedfrom leptonic acceleration / SSC models.(iii) A correlation between the spectral slope in the VHE regionand the peak location of the synchrotron peak is found. Such a cor-relation is also expected from SSC models.(iv) There were no correlations found between the γ emissionproperties and the M • of the galaxies that host the blazars. Also,no correlation of M • could be observed with the flare duty cyclesand the flare time-scales. Thus, VHE γ -ray emission properties maynot dominantly depend on M • . Other possibly interesting BH pa-rameters are not yet within instrumental reach.(v) There is an indication that the VHE γ luminosity is correlatedwith the spectral hardness. This correlation can be formulated as adecrease of ∆Γ ≈ .
82 per decade of luminosity.(vi) This behaviour is also found for some individual blazars thatwere observed in di ff erent emission states: There are indicationsthat in some variable sources the observed spectra become harderwith increasing luminosity, while in others no hardening is found.Investigations of the temporal properties of the X-ray emissionshow that the blazars with the most massive BHs have rather lowduty cycles. The temporal behaviour of the VHE γ -ray emission isnot well studied for most of the VHE blazars, but the present datado not seem to support a scaling of the flux doubling time-scaleswith the BH masses.Obviously we are still dealing with a low number of sourcesand certainly with an incomplete source sample, which leaves re-gions in the parameter space empty. Nevertheless, the rather im-pressive number of 17 VHE blazars has permitted first comparativestudies. It also allows conclusions concerning EBL models: A hintat a marginal correlation between the intrinsic spectral hardness andthe source distance is likely due to an EBL over-prediction. This re-sult was also recently quantified by Aharonian et al. (2006a) usingthe two distant blazars 1ES 1101-232 and H 2356-309. In clarify-ing the situation, more distant sources particularly with intrinsi-cally hard spectra (as e.g. 1ES 0229 + +
113 with its yet undetermined distance might also turnout to be a good candidate for such analyses once a measurementof its redshift succeeds. In the meantime, the observed luminositydistribution of the studied VHE γ blazars was used to constrain theunknown distance of PG 1553 +
113 by assuming that the propertiesof this blazar are not too di ff erent from the most extreme objectsin the blazar sample. Conversely, a large distance of PG 1553 + ff ects and intrinsic absorp-tion e ff ects from the measured spectra. Already currently, the VHEblazar sample contains groups of objects at very similar distances, e.g. 1ES 1218 +
304 and 1ES 1101-232 ( ∆ z = . + +
650 ( ∆ z = . ACKNOWLEDGMENTS
Luigi Costamante kindly provided the full object list fromCostamante & Ghisellini (2002). I would like to thank EckartLorenz, Nina Nowak, Włodek Bednarek and Hinrich Meyer forfruitful discussions on this study and the
RXTE team for provid-ing the all-sky monitor X-ray data. The financial support by MaxPlanck Society is gratefully acknowledged. This research has madeuse of NASA’s Astrophysics Data System.
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