A search for Galactic transients disguised as gamma-ray bursts

TL;DR

This study investigates whether some Swift gamma-ray bursts with FRED profiles are actually Galactic transients, finding marginal evidence for a small fraction possibly being Galactic in origin, but no significant spectral or temporal differences.

Contribution

It provides the first systematic analysis of the potential Galactic contamination in FRED GRBs, estimating the fraction of such transients among Swift detections.

Findings

Marginal (~3 sigma) excess of FREDs with unknown redshift near the Galactic plane.

Accounting for observational bias reduces the anisotropy significance to ~2 sigma.

Estimated Galactic contamination fraction is approximately 19% with an upper limit of 34%.

Abstract

A significant fraction of cosmological gamma-ray bursts (GRBs) are characterised by a fast rise and exponential decay (FRED) temporal structure. This is not a distinctive feature of this class, since it is observed in many Galactic transients and is likely descriptive of a sudden release of energy followed by a diffusion process. Possible evidence has recently been reported by Tello et al. (2012) for a Galactic contamination in the sample of FRED GRBs discovered with Swift. We searched for possible Galactic intruders disguised as FRED GRBs in the Swift catalogue up to September 2014. We selected 181 FRED GRBs (2/3 with unknown redshift) and considered different subsamples. We tested the degree of isotropy through the dipole and the quadrupole moment distributions, both with reference to the Galaxy and in a coordinate-system-independent way, as well as with the two-point angular autocovariance function. In addition, we searched for possible indicators of a Galactic origin among the spectral and temporal properties of individual GRBs. We found marginal (~3 sigma) evidence for an excess of FREDs with unknown redshift towards the Galactic plane compared with what is expected for an isotropic distribution corrected for the non-uniform sky exposure. However, when we account for the observational bias against optical follow-up observations of low-Galactic latitude GRBs, the evidence for anisotropy decreases to ~2 sigma. In addition, we found no statistical evidence for different spectral or temporal properties from the bulk of cosmological GRBs. We found marginal evidence for the presence of a disguised Galactic population among Swift GRBs with unknown redshift. The estimated fraction is f=(19 +- 11)%, with an upper limit of 34% (90% confidence).