Searching for galactic sources in the Swift GRB catalog

TL;DR

This study investigates the potential galactic origin of certain Swift GRBs, especially FREDs, by analyzing their anisotropy and estimating the contamination level from galactic sources, revealing a significant possible galactic contribution.

Contribution

The paper introduces a method to estimate galactic contamination in FRED GRB samples using anisotropy analysis and applies it to the Swift catalog, highlighting a notable galactic source presence.

Findings

Approximately 27% of FRED GRBs without redshift indicators may be galactic.

Contamination increases to about 34% when excluding bursts with redshift indicators and multiple peaks.

FREDs with single peaks show a higher likelihood of being galactic sources.

Abstract

Since the early 1990s Gamma Ray Bursts have been accepted to be of extra-galactic origin due to the isotropic distribution observed by BATSE and the redshifts observed via absorption line spectroscopy. Nevertheless, upon further examination at least one case turned out to be of galactic origin. This particular event presented a Fast Rise, Exponential Decay (FRED) structure which leads us to believe that other FRED sources might also be Galactic. This study was set out to estimate the most probable degree of contamination by galactic sources that certain samples of FREDs have. In order to quantify the degree of anisotropy the average dipolar and quadripolar moments of each sample of GRBs with respect to the galactic plane were calculated. This was then compared to the probability distribution of simulated samples comprised of a combination of isotropically generated sources and galactic sources. We observe that the dipolar and quadripolar moments of the selected subsamples of FREDs are found more than two standard deviations outside those of random isotropically generated samples.The most probable degree of contamination by galactic sources for the FRED GRBs of the Swift catalog detected until February 2011 that do not have a known redshift is about 21 out of 77 sources which is roughly equal to 27%. Furthermore we observe, that by removing from this sample those bursts that may have any type of indirect redshift indicator and multiple peaks gives the most probable contamination increases up to 34% (17 out of 49 sources). It is probable that a high degree of contamination by galactic sources occurs among the single peak FREDs observed by Swift.