HEAD Division Meeting 1999, April 1999
Session 1. Gamma Ray Bursts
Oral, Monday, April 12, 1999, 8:30am-10:04am, Colonial Room

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[1.06] ``No High Energy Emission" GRB Class Is Attributable to Brightness Bias

J.T. Bonnell (USRA \& GSFC), J.P. Norris (NASA GSFC)

The inhomogeneous brightness distribution of BATSE detected gamma-ray bursts (GRBs) is considered strong evidence for their cosmological origin. However, subclasses of GRBs have been shown to have significantly more homogeneous brightness distributions. Pendleton et al. (1997, ApJ 489, 175) have found such a result for GRBs with no detectable emission at energies greater than 300 keV. Accordingly, it has been suggested that these no high energy (NHE) emission GRBs represent an underluminous population of nearby sources. A distinct homogeneous NHE brightness distribution has also been considered as evidence for beaming of different spectral components of the prompt burst emission. We synthesize distributions of GRBs based on a sample of typical bright BATSE GRBs with intrinsic high-energy emission and adopt a single cosmological distance scale for all sources. We find that the resulting synthetic NHE bursts do indeed have a more nearly homogeneous intensity distribution when an appropriate decrease in signal to noise and redshifted spectrum are incorporated. The NHE class of GRBs is thus likely due to brightness bias.

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