Blazar Flaring Rates Measured with GLAST

We derive the minimum observing time scales to detect a blazar at a given flux level with the LAT on GLAST in the scanning and pointing modes. Based upon Phase 1 observations with EGRET, we predict the GLAST detection rate of blazar flares at different flux levels. With some uncertainty given the poor statistics of bright blazars, we predict that a blazar flare with integral flux >~ 200e-8 ph(> 100 MeV) cm^{-2} s^{-1}, which are the best candidates for Target of Opportunity pointings and extensive temporal and spectral studies, should occur every few days.Comment: 7 pages, 2 figures, in 2nd VERITAS Symposium on TeV Astrophysiscs, ed. L. Fortson and S. Swordy, in press, New Astronomy Review

Prompt and Delayed High-Energy Emission from Cosmological Gamma-Ray Bursts

In the cosmological blast-wave model for gamma ray bursts (GRBs), high energy (> 10 GeV) gamma-rays are produced either through Compton scattering of soft photons by ultrarelativistic electrons, or as a consequence of the acceleration of protons to ultrahigh energies. We describe the spectral and temporal characteristics of high energy gamma-rays produced by both mechanisms, and discuss how these processes can be distinguished through observations with low-threshold Cherenkov telescopes or GLAST. We propose that Compton scattering of starlight photons by blast wave electrons can produce delayed flares of GeV -- TeV radiation.Comment: to appear in Proceedings of VERITAS Workshop on TeV Astrophysics of Extragalactic Sources, eds. M. Catanese, J. Quinn, T. Weeke

External Shock Model for Gamma-Ray Bursts during the Prompt Phase

The hard X-ray and gamma-ray phenomenology of gamma-ray bursts (GRBs) can be explained by an external shock model where a single relativistic blast wave interacts with the surrounding medium. Besides reproducing the generic spectral behavior of GRB profiles, the external shock model provides quantitative fits to the peak flux distribution, the > 1 s t_{50} duration distribution, and the distribution of the peaks E_pk of the vF_v spectra of GRBs measured with BATSE. The apparent paradox between a relativistic beaming scenario and the empirical finding that E_pk values are preferentially measured within the triggering range of a GRB detector is resolved by this model when blast wave physics and detector triggering criteria are taken into account. Some surprising implications follow, namely that the fireball event rate is ~1 per 10^4 years per Milky Way galaxy for unbeamed sources, and proportionally more if fireball outflows are collimated. This is ~ 3 orders of magnitude larger than normally quoted. Most of the clean and dirty fireball transients are undetected due to telescope sensitivity and design limitations. Strongly variable GRB time histories with good radiative efficiencies are possible because of the strongly enhanced emissions when a blast wave interacts with density inhomogeneities located nearly along the line-of-sight to the observer. Arguments against short timescale variability in an external shock model are answered, and difficulties in an internal shock/colliding shell model are mentioned.Comment: 10 pages, 9 figures; invited review for 5th Huntsville Gamma-Ray Burst Workshop, ed. by M. Kippen, R. Mallozzi, and G. Fishma