Potential synchrotron-compton high energy blazars among unidentified egret gamma-ray sources

[摘要] English: The synchrotron self-Compton (SSC), a process where synchrotron emitting electrons upscatter the same photons to high energies, remains one of the dominant mechanisms for ')'-ray production inside homogeneous jets, explaining the observed broadband emission in blazars. Therefore, the synchrotron-Compton blazars can provide information regarding production mechanisms of Very High Energy (VHE) photons and possibly the acceleration mechanisms of particles to high energies. Synchrotron-Cornpton blazars have been observed by ')'-ray telescopes, both ground- and space-based, for example the Eneryet-ic Gamnw-Ray Experiment Telescope (EGRET) on board the Compton Gamma-Ray Observatory (CGRO) and the Large Area Telescope (LAT) on board the Fermi Observatory and are believed to be radio-loud Active Galactic Nuclei (AGNs) with their jets oriented at relatively small angles with respect. to the line of sight. The main purpose of this study was to search for possible synchrotron-Compton blazars among the southern high galactic latitude sources among the unidentified EGRET sources with the aim to understand their nature. The identification of possible point sources, associated with the EGRET ')'-ray sources, is complicated by the large EGRET gamma-ray error boxes, that is between 0.5-1.5 degrees, which can harbour several potential sources, especially at low galactic latitudes. The initial phase of this study constituted the tedious search for extra-galactic flat spectrum radio counterparts in the online and published catalogues inside the EGRET error boxes of high galactic sources, with Ibl > 10°. The strategy was to select sources with lad < 0.7 for further multi-wavelength studies. To enable t.he utilization of the 26-m radio telescope of the Hortebeesthoek Radio Astronomical Observatory (HartRAO), only sources with the declination range -70° < Dec < +45° were considered. A further consideration was to single out only those fiat spectrum sources with a fiux density above 200 rnJy at 12.4 GHz, the latter are those sources with rising nonthermal spectra towards high energies. Based upon the above-mentioned criteria, a selection of thirteen blazar-like candidates was made for further investigation. In the second phase of this study, multi-wavelength photometric and spectroscopie observations of the selected sources were carried out in order to reconstruct their Spectral Energy Distributions (SEDs) which are the signature of the main emission mechanisms (that is the synchrotron and the inverse Compton processes) occurring in the jets. The main result of this investigation has been the determination of the redshifts of two radio sources selected initially as radio counterparts of two unidentified EGRET sources, 3EG J0821-5814 and 3EG J0706- 3837. The Ca H & K line depression, occurring around 4000 A (in the rest frame of the source), was used to diagnose the domination of the nonthermal emission ill the sources which helped disentangle Flat Spectrum Radio Quasars (FSRQs) from normal radio galaxies. In the third phase, the homogeneous synchrotron self-Compton model was used to constrain the physical parameters of the emitting plasma in the sources. Results show that the model can be successful, provided that there are sufficient data to pin down the observed parameters, such as the position of the peaks in emission as well the spectral indices in different parts of the SED. However, it has been pointed out that the SSC model could not provide a good fit for the synchrotron low-energy radio emission and it has been suggested subsequently to consider an inhomogeneous jet model. In the MeV to GeV energy range, the contribution of inverse Compton scattering from external photons has been considered to explain the SED satisfactorily.

[发布机构] University of the Free State