Abstract
The Fermi Gamma-Ray Burst Monitor (GBM) has been detecting 240 Gamma-Ray Bursts (GRBs) per year since 2008, 40-45 of them per year short GRBs. GBM is an all-sky transient monitor of the hard X-ray sky operating between 8 keV and 40 MeV (Meegan et al. 2009). GBM localizes sources by triangulating the most likely source position based on observed count rates in detectors with different orientations to the sky. GRB locations are disseminated using GRB Coordinate Network (GCN) notices. We report here an analysis of over 300 GBM localizations for which more accurate positions were known. Systematic uncertainties of about 2-4° affect about 90% of GBM localizations (68% confidence level), with larger systematic effects for the remaining 10%. These systematic components are added in quadrature to the statistical uncertainties of 1-∼10° and provided as probability maps to the follow-up community an hour or less after the GRB trigger. The intermediate Palomar Transient Factory has used these maps to detect three GRB afterglows using the GBM positional information.
