Abstract
Of the 271 sources in the 3rd EGRET catalogue, 131 were reported as unidentied, 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 unidentied 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 unidentied EGRET sources. Recent spectroscopic observations with the Southern African Large Telescope (SALT) conrmed one of the unidentied EGRET sources as a possible Seyfert 2 galaxy, or alternatively, a narrow line radio galaxy. The detected gamma-ray emission (E<sub>γ</sub> &gt; 30 MeV) of the 5 coinciding EGRET/Fermi-LAT sources are tted 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 jet 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 gamma-ray source in the energy range E<sub>γ</sub> &gt; 30 MeV.
Highlights
The Energetic Gamma Ray Telescope Experiment EGRET (30 MeV - 10 GeV) provided the highest gamma-ray window on board the Compton GammaRay Observatory (CGRO)
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
Source Selection Criteria: The candidate counterparts should be inside the error box associated with the EGRET detection (Hartman et al, 1999), confirmed as extragalactic in the NASA Extragalactic Database (NED), possess a radio brightness above 100
Summary
The Energetic Gamma Ray Telescope Experiment EGRET (30 MeV - 10 GeV) provided the highest gamma-ray window on board the Compton GammaRay 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). 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 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 spectral distribution of these unidentified sources corresponds remarkably well with the gamma-ray blazar photon spectral index distribution observed by FermiLAT (Abdo et al, 2010a). The gamma-ray spectra of the sample of EGRET sources with Fermi-LAT counterparts were determined. 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
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