The Accuracy of Fermi GBM Gamma-Ray Burst Localizations

Author: William Paciesas

Abstract: The twelve sodium iodide detectors of the Fermi Gamma-ray Burst (GRB) Monitor (GBM) are oriented so that the sky locations of GRBs and other cosmic transients can be derived from the detected relative counting rates. Although counting statistics ultimately limit the location accuracy for weak bursts, a substantial fraction of GBM-detected bursts are sufficiently bright that the location accuracy is limited by our incomplete knowledge of systematic effects such as scattering in the spacecraft and the Earth's atmosphere. Using a sample of more than 160 GRBs detected by GBM and localized to much better accuracy by other instruments or by the InterPlanetary Network, we quantitatively characterize the accuracy of GBM locations and provide improved estimates of the actual systematic errors.