Extended, fading emissions in multi-wavelength are observed following Gamma-ray bursts (GRBs). Recent broadband observational campaigns led by the Swift Observatory reveal rich features of these GRB afterglows. Here we review the latest observational progress and discuss the theoretical implications for understanding the central engine, composition, and geometric configuration of GRB jets, as w...

This paper summarizes recents results in γ-ray astronomy, most of which were derived with data from groundbased γ-ray detectors. Many of the contributions presented at this conference involve multiwavelength studies which combine ground-based γ-ray measurements with optical data or space-based X-ray and γ-ray measurements. Besides measurements of the diffuse emission from the Galaxy, observatio...

I consider various possibilities for making gamma-ray bursts, particularly from close binaries. In addition to the much-studied neutron star+neutron star and black hole+neutron star cases usually considered good candidates for short-duration bursts, there are also other possibilities. In particular, neutron star+massive white dwarf has several desirable features. These systems are likely to pro...

Using observations from an extensive monitoring campaign with the Hubble Space Telescope we present the detection of an intermediate time flux excess which is redder in color relative to the afterglow of GRB 011121, currently distinguished as the gamma-ray burst with the lowest known redshift. The red “bump,” which exhibits a spectral roll-over at ∼7200 Å, is well described by a redshifted Type...

It is proposed that the gamma-ray photons that characterize the prompt emission of gamma-ray bursts are produced through the Compton-drag process, which is caused by the interaction of a relativistic fireball with a very dense soft photon bath. If gamma-ray bursts are indeed associated with supernovae, then the exploding star can provide enough soft photons for radiative drag to be effective. T...

High energy gamma-ray emission has been observed above 100 MeV from at least 40 active galactic nuclei (AGN) by the EGRET instrument on the Compton Gamma Ray Observatory (CGRO), and above 300 GeV gamma rays have been detected by the Whipple observatory from two AGN. The sources detected so far are \blazars" indicating a jet origin for the gamma rays. They are seen to be variable on time scales ...

Gamma-ray burst populations

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...