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Recent progress in pushing the sensitivity of the Imaging Atmospheric Cherenkov Technique into the 10 mCrab regime has enabled first sensitive observations of the innermost few 100 pc of the Milky Way in Very High Energy (VHE; >100 GeV)…
We present Ginga measurements of the spatial fluctuations in the diffuse X-ray background. When combined with earlier results, the new data constrain the extragalactic log N - log S relation in the 2-10 keV energy band to a form close to…
The Galactic diffuse soft gamma-ray (30-800 keV) emission has been measured from the Galactic Center by the HIREGS balloon-borne germanium spectrometer to determine the spectral characteristics and origin of the emission. The resulting…
Gamma-ray bursts are produced by the dissipation of the kinetic energy of a highly relativistic fireball, via the formation of a collisionless shock. When this happens, Ultra High Energy Cosmic Rays up to 10^20 eV are produced. I show in…
Population studies of EGRET gamma-ray sources indicate that there is a distinctive population of bright sources at low galactic latitudes. The sources have a distribution consistent with that of young galactic objects, with a concentration…
An analytic relation between the statistics of photons in pixels and the number counts of multi-photon point sources is used to constrain the distribution of gamma-ray point sources below the Fermi detection limit at energies above 1 GeV…
Ultrahigh-energy gamma rays ($E_{\gamma}>100 \, {\rm TeV}$) have been detected from a handful of astrophysical sources. Due to the Klein-Nishina suppression of inverse Compton scattering at such high energies, it has sometimes been argued…
We present a catalog of 2-10 keV ASCA Gas Imaging Spectrometer images of fields containing bright sources of GeV emission. The images cover $\sim 85%$ of the 95% confidence position contour for 28 of the 30 sources with $> 1$ GeV fluxes…
The injection of ultra-high energy cosmic rays in the intergalactic medium leads to the production of a GeV-TeV gamma-ray halo centered on the source location, through the production of a high electromagnetic component in the interactions…
We calculate the synchrotron self-Compton emission from internal shocks occurring in relativistic winds as a source of gamma-ray bursts, with allowance for self-absorption. For plausible model parameters most pulses within a Gamma-Ray Burst…
The recently discovered soft gamma-ray emission from the anomalous X-ray pulsar 1E 1841-045 has a luminosity L_g ~ 10^{36} ergs/s. This luminosity exceeds the spindown power by three orders of magnitude and must be fed by an alternative…
Cosmic ray protons generate gamma-rays, neutrinos, and secondary electrons and positrons (e+/-) through pion-producing collisions with gas atoms. Any synchrotron or Inverse Compton (IC) radiation from secondary e+/- is therefore accompanied…
One of the major goals of research for laser-plasma accelerators is the realization of compact sources of femtosecond X-rays. In particular, using the modest electron energies obtained with existing laser systems, Compton scattering a…
We present a catalog of gamma-ray sources at energies above 10 GeV based on data from the Large Area Telescope (LAT) accumulated during the first 3 years of the Fermi Gamma-ray Space Telescope mission. This catalog complements the Second…
We discuss here constraints on the particle acceleration models from the observed gamma-ray bursts spectra. The standard synchrotron shock model assumes that some fraction of available energy is given instantaneously to the electrons which…
The feasibility to generate powerful monochromatic radiation of the undulator type in the gamma region of the spectrum by means of planar channeling of ultra-relativistic electrons in a periodically bent crystal is proven. It is shown that…
We present results of a calculation of absorption of 10-500 GeV gamma-rays at high redshifts. This calculation requires the determination of the high- redshift evolution of the full spectral energy distribution of the intergalactic photon…
We present a new method for determining the sensitivity of X-ray imaging observations, which correctly accounts for the observational biases that affect the probability of detecting a source of a given X-ray flux, without the need to…
Bent crystal situated in a circulating beam can serve for efficient slow extraction or active collimation of the beams. This technique, well established at 10-1000 GeV, could be efficient also at the energies as low as 0.1-10 GeV according…
Recent progress in laser-driven plasma acceleration now enables the acceleration of electrons to several gigaelectronvolts. Taking advantage of these novel accelerators, ultra-short, compact and spatially coherent X-ray sources called…