Visible afterglow counterparts have now been detected for two GRBs (970228 and 970508) but are absent, with Lopt/Lγ ratios at least two orders of magnitude lower, for other GRBs, e.g., 970828. The causes of this variation are unknown. Any correspondence which could be discovered between the γ-ray properties of a GRB and its Lopt/Lγ would be useful, both in determining the GRB mechanisms, and in...

In this paper, we identify Cen A and M87, two nearby Fanaroff-Riley I (FRI) radio galaxies, with high-energy-peaked BL Lac-like (HBL-like) objects by investigating their spectral energy distributions (SEDs). The SED peak of Cen A at ∼150 keV, which was generally believed to be the peak of inverseCompton emission as in the case of 3C 273, is found to be actually the peak of synchrotron emission....

This chapter provides a review of γ-ray sources lying at high Galactic latitudes. Their statistical properties and variability status, as well as studies involving cross correlations with lower frequency catalogs and multiwavelength observations, are summarized. The case for active galactic nuclei is analyzed with special emphasis, since they represent the largest population of high energy sour...

R. Ruffini,1,2,3 M.G. Bernardini,1,2 C.L. Bianco,1,2,3 L. Caito,1,2 P. Chardonnet,1,3,4 C. Cherubini,1,5 M.G. Dainotti,1,2 F. Fraschetti, A. Geralico,1,2 R. Guida,1,2 B. Patricelli,1,2 M. Rotondo,1,2 J.A. Rueda Hernandez,1,2 G. Vereshchagin,1,2,3 S.-S. Xue1,3 1 ICRANet and ICRA, Piazzale della Repubblica 10, I-65122 Pescara, Italy. 2 Dip. di Fisica, Università di Roma “La Sapienza”, Piazzale Al...

Roughly 60 blazars have been identiied with > 100 MeV EGRET gamma-ray sources, and exhibit inferred isotropic luminosities as large as 3 10 49 ergs s ?1. The two most remarkable characteristics about the EGRET observations are that the-ray luminosity often dominates the bolometric power, and that variability on timescales at least as short as a day is seen from several sources in their aring st...

Pulsars are among the most exciting gamma ray sources in the Universe and can serve as unique sites for the study of emission processes in extreme physical environments. The Gamma ray Large Area Space Telescope (GLAST) will increase dramatically our knowledge of gamma ray pulsars physics. In particular the Large Area Telescope (LAT), the main GLAST instrument, will provide more detailed observa...

High-energy gamma rays are a valuable tool for studying particle acceleration and radiation in the magnetospheres of energetic pulsars. The six or more pulsars seen by CGRO/EGRET show that: the light curves usually have double-peak structures (suggesting a broad cone of emission); gamma rays are frequently the dominant component of the radiated power; and all the spectra show evidence of a high...

We estimate the fraction of the total energy in a Gamma-Ray Burst (GRB) that is radiated in photons during the main burst. Random internal collisions among different shells limit the efficiency for converting bulk kinetic energy to photons. About 6% of the energy of explosion is converted to radiation for long duration bursts (lasting 10s or more); the efficiency is significantly smaller for sh...

Gamma ray sources with high flux and spectral densities are the main requirements for new nuclear physics experiments to be performed in several worldwide laboratories with dedicated facilities. The paper is focalized on a proposal of experiment on γ photons production using Compton collisions between the DAΦNE electron beam and a high average power laser pulse, amplified in a Fabry-Pérot optic...