We review the latest results on misaligned AGNs (MAGNs), i.e. radio sources with the jet pointed away from the observer, detected by the Large Area Telescope (LAT) on board the Fermi satellite. We have analyzed the spectral

properties in the γ-ray band of a sample of eleven radio sources, associated with the 3CR, 3CRR and Molonglo 4 Jy catalogues. The location of the γ-ray emission, the

radiative processes responsible for it and the emerging jet structure are discussed in the framework of unification schemes for radio-loud AGNs. The results of a dedicated study on the nuclear spectral energy distribution (SED) of the radio galaxy NGC6251, one of the MAGNs of the LAT sample, are also presented

Migliori, G.

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DOI 10.1393/ncc/i2011-10856-3Colloquia: Scineghe2010IL NUOVO CIMENTO Vol. 34 C, N. 3 Maggio-Giugno 2011Misaligned AGN with Fermi -LAT: A different perspective onrelativistic jetsG. Migliori on behalf of the Fermi-LAT CollaborationSISSA/ISAS - Via Bonomea 265, I-34136 Trieste, Italy(ricevuto il 25 Febbraio 2011; pubblicato online il 28 Aprile 2011)Summary. — We review the latest results on misaligned AGNs (MAGNs), i.e.radio sources with the jet pointed away from the observer, detected by the LargeArea Telescope (LAT) on board the Fermi satellite. We have analyzed the spectralproperties in the γ-ray band of a sample of eleven radio sources, associated with the3CR, 3CRR and Molonglo 4 Jy catalogues. The location of the γ-ray emission, theradiative processes responsible for it and the emerging jet structure are discussedin the framework of unification schemes for radio-loud AGNs. The results of adedicated study on the nuclear spectral energy distribution (SED) of the radio galaxyNGC6251, one of the MAGNs of the LAT sample, are also presented.PACS 95.85.Pw – Astronomical observations: γ-ray.PACS 98.54.Cm – Active and peculiar galaxies and related systems (including BLLacertae objects, blazars, Seyfert galaxies, Markarian galaxies, and active galacticnuclei).1. – IntroductionThe detection of GeV-TeV emission associated with radio galaxies, first with theEGRET telescope (on board the Compton Gamma-ray Observatory) and then withground-based Cherenkov observatories, was an unexpected discovery. In AGN unifiedschemes [1], an increase of the inclination angle of the jet axis with respect to the lineof sight corresponds to a de-amplification of the observed flux and a general shift ofthe emission to lower energies. In the framework of simple one-zone synchrotron self-Compton (SSC) models this should make misaligned sources unlikely γ-ray candidates.Therefore, γ-ray detected MAGNs provide a deep insight into the structure of the regionwhere the most energetic radiation is produced and a test for the unification view.Now, with Fermi -LAT we can proceed further into the study of this interesting classof sources. Its combination of unprecedented flux sensitivity and spatial resolution in theγ-ray band [2] allows us to increase the number of MAGN detections and for the firsttime to perform spatially resolved studies for a limited number of giant sources [3]. Inthe following we summarize the main results on the MAGN class for the first 15 monthsof Fermi -LAT activity presented in [4].c© Societa` Italiana di Fisica 211212 G. MIGLIORI on behalf of the FERMI-LAT COLLABORATIONTable I. – The Fermi-LAT sample of misaligned sources.Object/ Redshift Class TS(a) Γγ Flux(b) logLum(c)1FGL name Radio/Optical (> 100MeV) (0.1–10GeV)3C 78(NGC 1218)/ 0.029 FR I/G 35 1.95±0.14 4.7±1.8 42.841FGLJ0308.3+04033C 84(NGC 1275)/ 0.018 FR I/G 4802 2.13±0.02 222±8 44.001FGLJ0319.7+41303C 111/ 0.049 FR II/BLRG 34 2.54±0.19 40±8 44.001FGLJ0419.0+38113C 120 0.033 FR I/BLRG 32 2.71±0.35 29±17 43.43PKS 0625-354/ 0.055 FR I/G 97 2.06±0.16 4.8±1.1 43.71FGLJ0627.3-35303C 207 0.681 FR II/SSRQ 79 2.42±0.10 24±4 46.441FGLJ0840.8+1310PKS 0943-76 0.27 FR II/G 65 2.83±0.16 55±12 45.711FGLJ0940.2-7605M87(3C 274)/ 0.004 FR I/G 194 2.21±0.14 24±6 41.671FGLJ1230.8+1223CENA 0.0009 FR I/G 1010 2.75±0.04 214±12 41.131FGLJ1325.6-4300NGC 6251 0.024 FR I/G 143 2.52±0.12 36±8 43.301FGLJ1635.4+82283C 380 0.692 FR II/SSRG 95 2.51±0.30 31±18 46.571FGLJ1829.8+4845(a) Source statistical significance.(b) ×10−9 photon cm−2 s−1.(c) erg s−1.2. – Misaligned AGNs with Fermi-LAT: results and open issuesIn order to search for misaligned sources, the eleven-month LAT list of AGN candi-dates has been cross-correlated with the 3CR catalog [5,6], the revised 3CRR catalog [7]and the Molonglo Southern 4 Jy sample [8, 9]. There are three main reasons for thischoice: i) the low frequency (178MHz, 408MHz) selection criteria of these catalogs fa-vor radio sources primarly on the relatively steep spectrum synchrotron emission of theextended lobes, ii) they represent a good coverage of the northern and southern sky,iii) morphological radio (FRI/FRII) and optical (Radio Galaxy/Quasar) classificationare given for the majority of the sources. The list of the sources with an association isreported in table I, together with the test statistic (TS) significance and the spectral pa-rameters of the fitting technique, namely the power law spectral slope Γγ , the integratedflux above 100MeV and the K-corrected luminosity between 100MeV and 10GeV (see [4]for all the analysis details). The sample is formed by 7 FRI and 4 FRII sources, two ofthem classified as Steep Spectrum Radio Quasars (SSRQs). All the associations were al-ready reported in the first-year LAT AGN Catalog paper (1LAC, [10]) with the exceptionof 3C120, whose γ-ray detection has been reported for the first time in [4]. Dedicatedstudies have been devoted to NGC1275 [11,12], M87 [13], and Centaurus A [3,14].The misaligned AGNs of the LAT sample are generally faint and soft sources. Their100MeV–10GeV spectrum can be well described by a simple power law with an averagespectral index 〈Γγ〉 = 2.4 and the integrated flux above 100MeV is of the order of≈ 10−8 photon cm−2 s−1. A more complex spectral shape is displayed by NGC1275,which is also the only source characterized by flux and spectral variability [11, 12]. NoMISALIGNED AGN WITH FERMI-LAT: A DIFFERENT PERSPECTIVE ON RELATIVISTIC JETS 213variability in the γ-ray band has been detected for the other sources on month timescales.It is not clear if this reflects the real situation or is rather due to the low statistics, whichcould hamper the detection of flux changes of factor of 2.2.1. Broad Line Radio Galaxies. – Two of the LAT MAGNs, 3C120 and 3C111, areBroad Line Radio Galaxies (BLRG). The nuclear X-ray (2–10 keV band) flux of theseobjects is likely the combination of thermal, related to the accretion flow, and non-thermal, from the base of the jet, emission. The γ-ray information could be useful todisentangle the jet from the disk emission, as the latter is expected to rapidly drop inthis band [15].2.2. Location of the γ-ray emission in MAGNs. – The location of the region of theγ-ray emission is a crucial and delicate point. In variable sources, correlated radio andγ-ray observations seem to indicate the bright radio core as the favorite candidate of theγ-ray emission origin (but see also [16,17]).LAT observations have revealed more complexity for the class of MAGNs. High-energyemission can be certainly produced on nuclear scales. Support to this scenario comesfrom: i) the variability observed in NGC1275 [11,12]; ii) the fast (Δt ∼ 1 day) TeV flaresaccompanied by an increase of the VLBI core flux in M87 [18], which place the site of high-energy photon production at 30–60 Schwarzschild radii. Beyond this, LAT has detectedfor the first time γ-ray emission associated to the extended lobes of a radio galaxy, namelyCenA [3]. The extended γ-ray emission is explained as up-scattered radiation from thecosmic microwave background photons by the ultra-relativistic electrons in the lobes,with additional contribution at higher energies from the infrared to optical extragalacticbackground. In CenA, the nuclear and extended γ-ray luminosities [3,14] are comparable.Thus, core and lobes may be competing contributions. This factor should be carefullyconsidered in those misaligned sources whose extended structures cannot be resolved outwith the LAT point spread function.2.3. Hints on the jet structure. – In the unification scenario for radio-loud AGNs [1],radio galaxies are the misaligned counterparts of blazars: FRI and FRII radio galax-ies are the parent populations of BL Lac sources and Flat Spectrum Radio Quasars(FSRQ), respectively. In fig. 1 (left panel) Γγ is plotted against the apparent isotropicγ-ray (100MeV–10GeV band) luminosity Lγ for all the misaligned sources of the sample,along with the FSRQs and BL Lac objects of the 1LAC with known redshifts. Misalignedsources generally occupy a separate region in the Lγ-Γγ plane, with lower γ-ray luminosi-ties and softer spectral indexes than their parent populations as predicted by unificationmodels. However, when a simple one-zone synchrotron/SSC model is assumed, the SEDsof the radio galaxies require bulk Lorentz factors (Γ) smaller than the aligned sources.Furthermore, the X-ray to γ-ray luminosities of the FRI sources are lower but still largerthan expected by debeaming the radiation of a typical BL-Lac [19]. A complex jetstructure, with a velocity gradient either axial (spine-layer models [20,21]) or radial (de-celerating jet [22]) is a viable solution to reconcile observations and unified scheme. Incase of a multi-layer jet the larger FRI inclination angles would favor the observation ofexternal jet regions usually swamped by the much brighter fast component responsiblefor the blazar emission. The FRI sources of the LAT sample are ideal candidates toinvestigate the jet structure. Detailed studies of NGC1275, M87 and CenA have con-214 G. MIGLIORI on behalf of the FERMI-LAT COLLABORATIONFig. 1. – (Color online) Left panel: γ-ray spectral slopes of FRI radio galaxies (red circles),FRII radio sources (green squares), BL Lac objects (open blue circles) and FSRQs (open blacksquares), are plotted as a function of their 100MeV–10GeV γ-ray luminosity [4]. The γ-rayemitting misaligned AGNs (MAGNs) are the red and green points. Right panel: core dominance(CD) versus total flux at 178MHz of all the sources of the 3CRR sample (FRI—red circles;FRII—green squares) with a measured radio core. The MAGNs detected by Fermi (blue tri-angles in circles/squares) are characterized by large core dominances. The two FSRQs (bluetriangles in empty black circles) belonging to the 3CRR and associated to LAT sources havemuch larger CD values than the misaligned FRII sources.firmed that small beaming factors (δ(1) ≈ 2–3) are required when a simple SSC model isassumed. In the following, we present the results obtained by modeling the nuclear SEDof the radio galaxy NGC6251.2.4. The case of NGC6251 . – First, we modelled the nuclear SED of the radio galaxyNGC6251 (z = 0.0247) using a one-zone synchrotron self-Compton model. The im-portant assumption is that the γ-ray emission is all produced in the nuclear region, asassumed for BL-Lac sources. The region dimensions (R: radius of the region), bulk mo-tion (Γ) and inclination angle (θ), as also the shape of the electron energy distribution(EED, N(γ) where γ is the Lorentz factor of the electrons) responsible for the emis-sion can be partially constrained from the observations. Our best SSC model fit for thenuclear SED (in fig. 2, left panel) is obtained for the following main parameter values:R ≈ 1017 cm, Γ = 2.4, θ = 25◦, and B = 0.037 G. The EED, N(γ), is a broken power lawwith an energy break γb = 2× 104 and p1 = 2.76 and p2 = 4.04 the power law spectralindexes below and above the break, respectively. The jet is relatively slow compared tothe typical BL-Lac jet speeds [23-25]. A similar value is also reported in [26] and is inline with what has been found for the nuclear SED of CenA, NGC1275 and M87 when aone-zone SSC model is applied [11,13,14].We then tested a spine-layer model (fig. 2, right panel), where the observed high-energy emission is given by the up-scattering of the spine synchrotron photons by therelativistic electrons of the layer [21]. The spine is modelled as a typical BL-Lac region(Rspine = 1016 cm, Γspine = 15, Bspine = 1.8 G). At the given inclination angle, thedirect spine emission is severely debeamed (δspine = 0.7). The layer EED, θ and δ asalso its dimensions respect the observational constraints.(1) δ = [Γ(1−β cos(θ)]−1, where Γ = (1−β2)−1/2 is the Lorentz factor and β = v/c is the bulkvelocity of the jet. θ is the angle between the direction of the jet and the observer line of sight.MISALIGNED AGN WITH FERMI-LAT: A DIFFERENT PERSPECTIVE ON RELATIVISTIC JETS 215Fig. 2. – (Color online) Nuclear broadband SED of NGC6251 compiled using multi-epoch data.Left panel: the SED is modeled with a one-zone SSC model (solid black line: synchrotroncurve, solid red line: IC emission). Right panel: the SED is modeled with the spine-layer modelillustrated in [21]. Solid back and red lines reproduce the SSC emission of the layer. Dashedblack and red curves are the SSC model for the spine. The long-dashed blue lines is the ICemission of the spine synchrotron photons off the layer relativistic electrons.The two models reflect very different jet structures. In the SSC hypothesis, the jetis slow and heavy: the particle to magnetic field energy-density ratio is U ′e/U′B ≥ 400,with U ′e = mec2ne〈γ〉 (me: electron mass, 〈γ〉: average electron Lorentz factor) andU ′B = B2/8π. The violation of the minimum-energy assumption is rather severe andcould pose the question of how the jet is initially confined. Conversely, in the spine-layermodel the jet is on average faster and lighter, with particles and magnetic field nearto the equipartition conditions (U ′e/U′B(layer) ≈ 0.2 and U ′e/U ′B(spine) ≈ 0.3 and ≈ 1if protons are included). In this case, the magnetic field could be determinant for thejet confinement. We note however that the flux variability in the X-ray to γ-ray bandpredicted by the spine-layer model [21] is not observed.2.5. FRI and FRII sources. – In the LAT sample there is a slight dominance of FRIover FRII sources. The sample is limited but some considerations can be made. In fig. 1(right panel) the core dominance(2) (CD) of the 3CRR sources, including the 3CRRLAT sources (blue triangles in circles/squares for FRI/FRII), is plotted as a function ofthe source total flux at 178MHz. In general, the misaligned sources detected by Fermihave a large CD. When the redshift information is included we note that: i) the FRILAT sources with small CDs (i.e. observed at relatively large inclination angles) are atlow redshifts, ii) conversely, no FRII with a small CD has been so far detected by Fermi ;iii) increasing luminosity distances seem to require larger CDs. The two sources at thehighest redshift, 3C207 and 3C380 (table I), are SSRQs with small estimated inclinationangles. At this stage it is not possible to determine whether the lower number of FRIIdetections is related to the fact that they are located at redshifts higher than FRIs orreflects an intrinsic difference in the jet structure.(2) The core dominance is considered an indicator of the source inclination and defined asCD = log[Score/(Stot − Score)], where Score,tot is the core/total flux density referred to thesource rest frame [27].216 G. MIGLIORI on behalf of the FERMI-LAT COLLABORATION3. – ConclusionsThe first 15 months of LAT activity have allowed us to increase the number of mis-aligned sources detected in the GeV band. The LAT sample is dominated by sevennearby FRI radio galaxies. The γ-ray spectrum of most of the sources is a simple softpower law and no significant γ-ray variability is usually detected (except for NGC1275).The dominance of FRI sources over FRII in the sample could be explained in terms ofredshift distribution or invoking an intrisically different jet structure between the twoclasses. As the Fermi mission continues, further inputs to this and the other topicsdiscussed here are expected.∗ ∗ ∗The Fermi-LAT Collaboration acknowledges support from a number of agenciesand institutes for both development and the operation of the LAT as well as scien-tific data analysis. These include NASA and DOE in the United States, CEA/Irfu andIN2P3/CNRS in France, ASI and INFN in Italy, MEXT, KEK, and JAXA in Japan,and the K. A. Wallenberg Foundation, the Swedish Research Council and the NationalSpace Board in Sweden. Additional support from INAF in Italy and CNES in Francefor science analysis during the operations phase is also gratefully acknowledged.REFERENCES[1] Urry C. M and Padovani P., Publ. Astron. Soc. Pac., 107 (1995) 803.[2] Atwood W. B. et al., Astrophys. J., 697 (2009) 1071.[3] Abdo A. A. et al., Science, 328 (2010) 725 (Cen A lobes).[4] Abdo A. A. et al., Astrophys. J., 720 (2010) 912 (MAGN).[5] Bennett A. S., Mon. Not. R. Astron. Soc., 125 (1962) 75.[6] Spinrad H., Marr J., Aguilar L. and Djorgovski S., Publ. Astron. Soc. Pac., 97(1985) 932.[7] Laing R. A., Riley J. M. and Longair M. S., Mon. Not. R. Astron. Soc., 204 (1983)151.[8] Burgess A. M. and Hunstead R. W., Astron. J., 131 (2006) 100.[9] Burgess A. M. and Hunstead R. W., Astron. J., 131 (2006) 114.[10] Abdo A. A. et al., Astrophys. J., 715 (2010) 429 (1LAC).[11] Abdo A. A. et al., Astrophys. J., 699 (2009) 31 (NGC1275).[12] Kataoka J., Stawarz L., Cheung C. C. et al., Astrophys. J., 715 (2010) 554.[13] Abdo A. 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Misaligned AGN with Fermi-LAT: A different perspective on

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Misaligned AGN with Fermi-LAT: A different perspective on relativistic jets

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