Here is reviewed our current understanding of Galactic and extragalactic diffuse γ-ray emission. The spectrum of the extragalactic γ-ray background above 30 MeV can be well described by a power law with photon index α=2.1. In the 2-10 MeV range a preliminary analysis of COMPTEL data indicates a lower intensity than previously found, with no evidence for an MeV bump. Most of the models of a trul...

BACKGROUND The molecular mechanisms of DNA repair following chronic medium-dose-rate (MDR) γ-ray-induced damage remain largely unknown. METHODOLOGY/PRINCIPAL FINDINGS We used a cell function imager to quantitatively measure the fluorescence intensity of γ-H2A.X foci in MDR (0.015 Gy/h and 0.06 Gy/h) or high-dose-rate (HDR) (54 Gy/h) γ-ray irradiated embryonic fibroblasts derived from DNA-depe...

If γ−ray bursts originate in dense stellar forming regions, the interstellar material can imprint detectable absorption features on the observed X–ray spectrum. Such features can be detected by existing and planned X–ray satellites, as long as the X–ray afterglow is observed after a few minutes from the burst. Detection of these X–ray features will make possible the determination of the redshif...

The details of Čerenkov light produced by a γ− ray or a cosmic ray incident at the top of the atmosphere is best studied through systematic simulations of the extensive air showers. Recently such studies have become all the more important in view of the various techniques resulting from such studies, to distinguish γ− ray initiated showers from those generated by much more abundant hadronic com...

Recent nonthermal X-ray and γ-ray observations, attributed to electron emission processes, for the first time give an experimental confirmation that electrons are accelerated on SNR shocks up to the energy ∼ 10 eV. We have no direct observational confirmations about proton acceleration by SNR. Different models of γ-emission from SNRs predict different emission mechanisms as dominating. Only π o...

We determined the basic parameters, such as the central black hole mass (M), the boosting factor (or Doppler factor) (δ), the propagation angle (Φ) and the distance along the axis to the site of γ-ray production (d) for 23 γ-ray-loud blazars using their available variability timescales. In this method, the absorption effect depends on the γ-ray energy, emission size and property of the accretio...

In its first three months of operations, the Fermi Gamma-ray Observatory has detected approximately one quarter of the radio-flux limited MOJAVE sample of bright flat-spectrum AGN at energies above 100 MeV. We have investigated the apparent parsec-scale jet speeds of 26 MOJAVE AGN measured by the VLBA that are in the LAT Bright AGN Sample (LBAS). We find that the γ-ray bright quasars have faste...

The isotropic but radially non-uniform distribution of γ-ray bursts (GRBs) observed with the Burst and Transient Source Experiment (BATSE) on board the Compton Gamma Ray Observatory (CGRO) suggests that the bursts may be at cosmological distances. At such distances, the expansion of the Universe should have two main effects on the statistics of γ-ray bursts: first, it should stretch the tempora...

The High Energy Stereoscopic System (H.E.S.S.) experiment, a ground-based γ-ray Čerenkov telescope array located in Namibia, has now detected many extragalactic objects, which redshifts range from z=0.00183 up to z=0.2, possibly more. With the increasing performances of Čerenkov telescopes, it now becomes possible to probe these objects at small timescales in γ-ray, allowing the study of region...