Related papers: Optical spectroscopy of BL Lac objects: TeV candid…
The very high energy (VHE) gamma ray spectral index of high energy peaked blazars correlates strongly with its corresponding redshift whereas no such correlation is observed in the X-ray or the GeV bands. We attribute this correlation to a…
Hard-TeV BL Lacs are a new type of blazars characterized by a hard intrinsic TeV spectrum, locating the peak of their gamma-ray emission in the spectral energy distribution (SED) above 2-10 TeV. Such high energies are problematic for the…
The Extragalactic Background Light (EBL) includes photons with wavelengths from ultraviolet to infrared, which are effective at attenuating gamma rays with energy above ~10 GeV during propagation from sources at cosmological distances. This…
We report on the detection of very-high-energy (VHE; $E > 100$ GeV) $\gamma$-ray emission from the BL Lac objects KUV 00311-1938 and PKS 1440-389 with the High Energy Stereoscopic System (H.E.S.S.). H.E.S.S. observations were accompanied or…
The recent Cherenkov telescope observations and detections of the BL Lac objects Mkn 421, Mkn 501, 1ES 2344+514, PKS 2155--304 and possibly 1ES 1959+658 have shown that there exists a subclass of BL Lac objects emitting a substantial…
Recently, a new method to constrain the distances of blazars with unknown redshift using combined observations in the GeV and TeV regimes has been developed. The underlying assumption is that the Very High Energy (VHE, E > 100 GeV) spectrum…
In this paper, we present results of TeV $\gamma$--ray observations of the high synchrotron peaked BL Lac object 1ES 1218+304 (z=0.182) with the $TACTIC$ (TeV Atmospheric Cherenkov Telescope with Imaging Camera). The observations are…
Nearly one-third of the sources in the $Fermi$-LAT catalogs lack a lower energy counterpart, hence being referred as unidentified/unassociated gamma-ray sources (UGSs). In order to firmly classify them, dedicated multifrequency follow-up…
We present the results of spectroscopic observations of the optical counterparts of 47 southern radio sources from the candidate International Celestial Reference Catalogue (ICRC), as part of a Very Long Baseline Interferometry (VLBI)…
In this work, we use two different methods to determine the opacity of the TeV gamma-rays caused by the extragalactic background light (EBL) via e-e+ production due to photon-photon interaction. The first method, Model-Dependent Approach,…
We report on a detailed investigation of the gamma-ray emission from 18 broad line radio galaxies (BLRGs) based on two years of Fermi Large Area Telescope (LAT) data. We confirm the previously reported detections of 3C 120 and 3C 111 in the…
We present optical candidates for 75 X-ray sources in a $\sim 1$ deg$^2$ overlapping region with the medium deep ROSAT survey. These candidates are selected using the multi-color CCD imaging observations made for the T329 field of the…
Recently, a new method to constrain the distance of blazars with unknown redshift using combined observations in the GeV and TeV regimes has been developed, with the underlying assumption that the Very High Energy (VHE) spectrum corrected…
We report on the results of a search for $\gamma$-ray pair halos with a stacking analysis of low-redshift blazars using data from the Fermi Large Area Telescope. For this analysis we used a number of a-priori selection criteria, including…
The bright gamma-ray quasar 4C +55.17 is a distant source ($z = 0.896$) with a hard spectrum at GeV energies as observed by the Large Area Telescope (LAT) on board the {{\it Fermi}} satellite. This source is identified as a good source…
We have discovered six galaxies with spectroscopically confirmed redshifts of 4.8<z<5.8 in a single 44 square arcminute field imaged deeply in R, I and z-bands. All the spectra show an emission-line in the region around 7000-8400 angstroms…
Gamma rays with energy above 10 GeV interact with optical-UV photons resulting in pair production. Therefore, a large sample of high redshift sources of these gamma rays can be used to probe the extragalactic background starlight (EBL) by…
We considered the fourth catalog of gamma-ray point sources produced by the Fermi Large Area Telescope (LAT) and selected only jetted active galactic nuclei (AGN) or sources with no specific classification, but with a low-frequency…
Blazars and in particular the subclass of high synchrotron peaked Active Galactic Nuclei are among the main targets for the present generation of Imaging Atmospheric Cherenkov Telescopes (IACTs) and will remain of great importance for very…
Blazars have traditionally been separated into two broad categories based upon their optical emission characteristics; BL Lacs, with faint or no emission lines, and flat spectrum radio quasars (FSRQs) with prominent, broad emission lines.…