Modelling the Multifrequency SED of AGN Candidates among the Unidentified EGRET and Fermi Gamma-Ray Sources

Of the 271 sources in the 3 EGRET catalogue, 131 were reported as unidentified, i.e. not associated with any particular class of point source in the sky. Since the largest fraction of the EGRET sources were extragalactic, a sample of 13 extragalactic unidentified sources have been selected for multi-wavelength follow-up studies. Five of the selected EGRET sources coincide with gamma-ray flux enhancements seen in the Fermi-LAT data after one year of operation. In this article, we report the multi-wavelength properties of, among others, the 5 sources detected by Fermi-LAT from our sample of high galactic latitude unidentified EGRET sources. Recent spectroscopic observations with the Southern African Large Telescope (SALT) confirmed one of the unidentified EGRET sources as a possible Seyfert 2 galaxy, or alternatively, a narrow line radio galaxy. The detected gamma-ray emission (Eγ > 30 MeV) of the 5 coinciding EGRET/Fermi-LAT sources are fitted with external Compton and Synchrotron Self Compton (SSC) models to investigate the energetics required to produce the EGRET/Fermi gamma-ray flux. In all the models the inclination angle of the jet with respect to the observer is θjet ≈ 60◦, between those of Seyfert 1 and Seyfert 2/radio galaxies. These results confirm the possibility of Seyfert and radio galaxies sources are constituting a new class of γ-ray source in the energy range Eγ > 30 MeV.


Introduction
The Energetic Gamma Ray Telescope Experiment EGRET (30 MeV -10 GeV) provided the highest gamma-ray window on board the Compton Gamma-Ray Observatory (CGRO).EGRET detected 271 gamma-ray sources above 100 MeV, 92 % of which were blazars.Of the 271 sources detected, 131 remained unidentified, i.e. could not be associated with any specific point source of gamma-ray emission (Hartman et al., 1999).The large number of unidentified EGRET sources above and below the galactic plane inspired a search for possible extra-galactic radio loud Active Galactic Nuclei (AGN) counterparts that could possibly be associated with these unidentified sources.To avoid confusion with possible galactic sources, especially molecular cloud distributions, the search for counterparts was restricted to those unidentified sources at galactic latitudes | b | > 10 • .
Of the 13 candidate sources selected, 5 have confirmed gamma-ray excesses in the Fermi-LAT catalogue containing the first year's observations (Abdo et al., 2010a).The Fermi Large Area Telescope (Fermi-LAT) is a pair conversion γ-ray telescope sensitive to photon energies between 20 MeV and 300 GeV.Launched on 11 June 2008, Fermi-LAT started to collect data in August 2008.The data are made available on a daily basis and can be accessed online at the official website of the Fermi Science Support Centre (http://fermi.gsfc.nasa.gov/cgi-bin/ssc/LAT/LATDataQuery.cgi).
The results of detections made within the first 24 months of operation were released in June 2011, in the form of the Second Fermi-LAT catalog (2FGL; see Nolan et al., 2012) 1 .
The EGRET (30 MeV-10 GeV) gamma-ray spectra of our chosen sample of sources that were observed between April 1991 -October 1995 (cycles 1, 2 3 and 4 of the mission) have been determined.The photon spectral index distribution is displayed in Fig. 2. The spectral distribution of these unidentified sources corresponds remarkably well with the gamma-ray blazar photon spectral index distribution observed by Fermi-LAT (Abdo et al., 2010a).The gamma-ray spectra of the sample of EGRET sources with Fermi-LAT counterparts were determined.The spectra are presented in Fig. 3. Noticeable is the apparent change in the spectral index between the EGRET and the Fermi-LAT gamma-ray data, which may point to a transition in the gamma-ray production process. -

Gamma-Ray Variability
The multi-wavelength emission of accretion driven sources like AGN is characterized by very high luminosity, assuming isotropic emission, and variability over several time scales (e.g.Fan, 2005).This is reconciled with the fact that the bulk of the Spectral Energy Distribution (SED) of these sources is produced in a non-homogeneous and variable jet.Gamma-ray flux variability has also been confirmed from AGN-blazars (Mattox et al., 1997).
Aperture photometry of the Fermi-LAT data over a time span of 4.6 years (from August 2008 to March 2013) has been performed to investigate possible long term variability.Photons in the energy interval 100 MeV to 200 GeV were counted in an area with a radius of 1 degree centered on the source and monthly averages were determined.Two of the sources, 2FGL J1304.3-4353 and 2FGL J1703.2-6217,showed definite signs of variability, quantified in terms of a variability index V (Nolan et al., 2012), which implies that for V > 41 there is a < 1% probability of the source being steady.Although variability was detected in 2FGL J1304.3-4353(V = 47) and 2FGL J1703.2-6217(V = 167) respectively, no periodicity could be detected (see Fig. 4).
Spectroscopic observations of other sources from our sample have been performed with the Southern African Large Telescope (SALT) (see Fig. 5), equipped with the Robert Stobie Spectrograph (RSS), during 2012, in order to determine the redshift and to identify the class of AGN.The spectrum of one of the unidentified EGRET sources, 3EG J 0159-3603, could be determined (see Fig. 6), showing distinct narrow emission lines of O ii, O iii and He ii redshifted by z = 0.35.The spectrum resembles that of a typical Seyfert 2 galaxy, or alternatively, a narrow line radio galaxy.This implies the possible association of two Seyfert galaxies with the unidentified sources 3EG J0706-3837 and 3EG J 0159-3603 respectively.The association of some of the unidentified EGRET sources with Seyfert galaxies/LINERS poses a possibility of non-aligned AGN and radio galaxies constituting a new class of gamma-ray sources.Before discussing the modeling of the SEDs presented above, a brief discussion of the gamma-ray properties of some radio galaxies are presented to illustrate that these sources possess the required energetics to produce measurable γ-ray emission in the EGRET and Fermi-LAT energy domain  The discussion presented above underlines the fact that normal, non-aligned AGN do possess the required energetics to accelerate leptons to VHE energies.The production of sub-GeV gamma-rays through a SSC process in the nuclear region, combined with the HE emission in the radio lobes and TeV emission in the inner disc region close to the black hole (BH), presents a new paradigm in particle acceleration and gamma-ray production in AGN.With this in mind, the SED of the Fermi-LAT counterparts of the unidentified EGRET sources were modelled.This is a first attempt to explain the SED of these sources within the framework of a Synchrotron self-Compton (SSC) and External Compton (EC) model.These sources are all treated as misaligned AGN, with a fixed inclination of θ jet ≈ 60 • between jet and observer.

SED Modelling
The multi-wavelength data from radio to gamma-rays have been combined to create the SED over more than 15 decades in energy (Fig. 7).From these results (see Fig. 8) it can be seen that the EGRET and Fermi-LAT data are consistent with an EC model, i.e. the IC upscattering of IR photons from the disc and UV/optical photons from the line emitting clouds (Broad Line Regions) to the EGRET and Fermi-LAT energy domain.In all the models the electron Lorentz factors were γ e ∼ few × 10 3 , rather modest in comparison with the Lorentz factors required to explain the γ-ray emission from, for example, Cen A.

Conclusions
We report the discovery of 13 flat spectrum extragalactic sources within the error boxes of some high galactic latitude, unidentified EGRET sources.Five of these EGRET sources have been detected with Fermi-LAT within the first 11 months of operation.In all cases the EGRET and Fermi-LAT gamma-ray emission could be successfully explained in terms of the IC upscattering of BEL photons, as well as IR photons from the disc, to EGRET and Fermi-LAT energies.The adopted electron energy is of the order of γ e ∼ few × 10 3 , which is rather moderate compared to the electron energies of γ e ∼ 10 6 − 10 7 , inferred from the HE and TeV emission from Cen A. These preliminary results definitely confirm that Seyfert and radio galaxies could be associated with a significant fraction of the still unidentified extragalactic EGRET and Fermi-LAT sources.

1 :
Galactic distribution of the unidentified EGRET sources (circles) and their Fermi-LAT counterparts (crosses).

Figure 7 :
Figure 7: The multifrequency SEDs for the sample of EGRET sources.
−4 eV) and infrared (IR) background photons by relativistic electrons with Lorentz factors γ e = 6 × 10 5 , in a jet with bulk flow velocities between β Γ ∼ 0.1-0.5.Another peculiar aspect of Cen A is a rather low, sub-Eddington inferred accretion rate, ṁ ∼ 10 −3 ṁEdd , resulting in a rather low bolometric luminosity L b ∼ 10 43 erg s −1(Whysong & Antonucci, 2004).The inferred equipartition magnetic field in the radio lobes is B ≈ 9 µG (Rieger, 2012b), with the equipartition magnetic field near the black hole (BH) ranging between B ∼ 10 3 − 10 4 G (Rieger, 2012b).The detection of non-aligned AGN in the HE and VHE regime poses interesting theoretical challenges regarding particle acceleration and associated gammaray emission in the jets of AGN.For example, Cen A shows that the radio lobes of radio galaxies may possess the required energetics to accelerate electrons to VHEs producing the HE gamma-rays through IC upscattering the CMB photons, even though the bulk flow Lorentz factor of the jet is fairly low.The detection of TeV gamma-rays is explained in terms of γ e ∼ 10 7 electrons up-scattering disc photons to the TeV domain(Rieger & Aharonian, 2009).The nuclear SED of Cen A, based on non-simultaneous data, shows two peaks, one around ∼ 10 13 Hz and another around 0.1 MeV (e.g.Chiaberge et al., 2001;Abdo et al., 2010c).The SED below a few GeV seems to be satisfactorily explained by a one-zone Synchrotron self-Compton (SSC) model(Chiaberge et al., 2001) but the same approach fail to account for the TeV emission observed by H.E.S.S.(Abdo et al., 2010c).

Figure 8 :
Figure 8:The modeled SEDs for the various EGRET/Fermi-LAT sources.

Table 1 :
Parameters related to SSC models for the respective sources.

Table 2 :
Parameters related to the EC models for the respective sources.