Improved Fermi-GBM GRB Localizations Using BALROG

The localizations of gamma-ray bursts (GRBs) detected with the Gamma-ray Burst Monitor (GBM) on board the Fermi satellite are known to be affected by significant systematic errors of 3°–15°. This is primarily due to the mismatch of the employed Band function templates and the actual GRB spectrum. This problem can be avoided by simultaneously fitting for the location and the spectrum of a GRB, as demonstrated with an advanced localization code, BALROG. Here, we analyze in a systematic way a sample of 105 bright GBM-detected GRBs for which accurate reference localizations are available from the Swift observatory. We show that the remaining systematic error can be reduced to ∼1°–2°.