Localization of Gamma-Ray Bursts using the Fermi Gamma-Ray Burst Monitor

TL;DR

This paper evaluates the localization accuracy of the Fermi GBM for gamma-ray bursts, revealing significant systematic uncertainties and positional dependencies that impact the precision of GRB localizations.

Contribution

It provides a detailed analysis of systematic uncertainties in GBM GRB localizations and models their dependence on spacecraft coordinates.

Findings

Systematic uncertainties are larger than statistical errors.

The distribution of uncertainties is well modeled by a 3.7° Gaussian with a tail.

Localization accuracy depends on the GRB position relative to the spacecraft.

Abstract

The Fermi Gamma-ray Burst Monitor (GBM) has detected over 1400 Gamma-Ray Bursts (GRBs) since it began science operations in July, 2008. We use a subset of over 300 GRBs localized by instruments such as Swift, the Fermi Large Area Telescope, INTEGRAL, and MAXI, or through triangulations from the InterPlanetary Network (IPN), to analyze the accuracy of GBM GRB localizations. We find that the reported statistical uncertainties on GBM localizations, which can be as small as 1 degree, underestimate the distance of the GBM positions to the true GRB locations and we attribute this to systematic uncertainties. The distribution of systematic uncertainties is well represented (68% confidence level) by a 3.7 degree Gaussian with a non-Gaussian tail that contains about 10% of GBM-detected GRBs and extends to approximately 14 degrees. A more complex model suggests that there is a dependence of the systematic uncertainty on the position of the GRB in spacecraft coordinates, with GRBs in the quadrants on the Y-axis better localized than those on the X-axis.