A search for Galactic transients disguised as gamma-ray bursts

Context. 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 for a Galactic contamination in the sample of FRED GRBs discovered with Swift. Aims. We searched for possible Galactic intruders disguised as FRED GRBs in the Swift catalogue up to September 2014. Methods. We selected 181 FRED GRBs (2/3 with unknown redshift) and considered different subsamples. We tested the degree of isotropy through the dipole and quadrupole moment distributions, both with reference to the Galaxy and independent of any coordinate system 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. Results. We found marginal (~3σ) 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 ~2σ. In addition, we found no statistical evidence for different spectral or temporal properties from the bulk of cosmological GRBs. Conclusions. 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).