High-energy spectral signatures in gamma-ray bursts

One of the principal results obtained by the EGRET experiment aboard theCompton Gamma-Ray Observatory (CGRO) was the detection of several Gamma-raybursts (GRBs) above 100 MeV. The broad-band spectra obtained for these burstsgave no indication of any high energy spectral attenuation that might precludedetection of bursts by ground-based Cerenkov telescopes (ACTs), thus motivatingseveral TeV observational programs. This paper explores the expectations forthe spectral properties in the TeV and sub-TeV bands for bursts, in particularhow attenuation of photons by pair creation internal to the source modifies thespectrum to produce distinctive spectral signatures. The energy of spectralbreaks and the associated spectral indices provide valuable information thatcan constrain the bulk Lorentz factor of the GRB outflow at a given time. Thesecharacteristics define palpable observational goals for ACT programs, andstrongly impact the observability of bursts in the TeV band.Comment: 5 pages, 2 figures, to appear in Proc. of Snowbird TeV Gamma-Ray Workshop ed. B. L. Dingus (AIP, New York, 2000