  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).
