Gamma-ray bursts (GRBs) and following afterglows are considered to be produced by dissipation of kinetic energy of a relativistic fireball and radiation process is widely believed as synchrotron radiation or inverse Compton scattering of electrons. We argue that the transfer of kinetic energy of ejecta into electrons may be inefficient process and hence the total energy released by a GRB event ...

SPI on INTEGRAL has provided spectra and a map of the sky in the emission from annihilations of positrons in the interstellar medium of our Galaxy. From high-resolution spectra we learned that a warm, partially-ionized medium is the site where the observed gamma-rays originate. The gamma-ray emission map shows a major puzzle for broader astrophysics topics, as it is dominated by a bright and ex...

It is believed that the observed diffuse gamma ray emission from the galactic plane is the result of interactions between cosmic rays and the interstellar gas. Such emission can be amplified if cosmic rays penetrate into dense molecular clouds. The propagation of cosmic rays inside a molecular cloud has been studied assuming an arbitrary energy and space dependent diffusion coefficient. If the ...

Structure formation in the intergalactic medium (IGM) produces large-scale, collisionless shock waves, where electrons can be accelerated to highly relativistic energies. Such electrons can Compton scatter cosmic microwave background photons up to γ-ray energies. We study the radiation emitted in this process using a hydrodynamic cosmological simulation of a ΛCDM universe. The resulting radiati...

We calculate the ux of cosmic gamma rays expected from the annihilation of neutralinos in the Galactic halo. Our calculation of the annihilation cross section to two photons improves the existing calculations by inclusion of exact one-loop diagrams for the amplitudes involving Higgs boson and chargino states as well as those involving fermion and sfermion states. A survey of supersymmetric para...

We estimate the contribution of millisecond pulsars to the diffuse gamma-ray background of the Galaxy, and show that a significant fraction of the Galactic background may originate from them. A small number of the unidentified COS-B point sources may, in fact, be millisecond pulsars. It is argued that several hundred millisecond pulsars may be detectable as point sources by the GRO satellite.

High energy γ-rays from individual giant molecular clouds contain unique information about the hidden sites of acceleration of galactic cosmic rays, and provide a feasible method for study of propagation of cosmic rays in the Galactic Disk on scales ≤ 100 pc. I discuss the spectral features of π-decay γ-radiation from clouds/targets located in proximity of relatively young proton accelerators, ...

Blazar emission of gamma rays and cosmic ray production of gamma rays in gas-rich clusters have been proposed recently as alternative sources of the high energy extragalactic diffuse gamma ray background radiation. We show that these sources also produce a very different high energy extragalactic diffuse neutrino background radiation. This neutrino background may be detected by the new generati...

I give a brief review of high energy gamma-ray signatures of dark matter. The decay of massiveX-particles and subsequent hadronization have been suggested as the origin of the highest energy cosmic rays. Propagation over cosmological distances to Earth (as would be the case in some topological defect origin models for the Xparticles) results in potentially observable gamma-ray fluxes at GeV ene...