Searching for Galactic sources in the Swift GRB catalog
Statistical analyses of the angular distributions of FREDs⋆
1 Instituto de Astrofísica de Andalucía (IAA-CSIC), Spain
2 Universidad de Jaén, Spain
3 Nikolaev National University, Ukraine
4 Eötvös University, Budapest, Hungary
5 Bolyai Military University, Budapest, Hungary
Received: 9 October 2012
Accepted: 5 November 2012
Context. Since the early 1990s, gamma ray bursts (GRB) have been accepted to be of extra-Galactic origin because of the isotropic distribution observed by BATSE and the redshifts observed via absorption line spectroscopy. Nevertheless, upon closer examination at least one case turned out to be of Galactic origin. This particular event presented a fast rise and exponential decay (FRED) structure, which leads us to believe that other FRED sources might also be Galactic.
Aims. This study was set out to estimate the most probable degree of contamination by Galactic sources that certain samples of FREDs have.
Methods. 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 comprising a combination of isotropically generated sources and Galactic sources.
Results. 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 represents roughly 27%. Furthermore, we observe that by removing from this sample those bursts that have any type of indirect redshift indicator and multiple peaks, the most probable contamination increases to 34% (17 out of 49 sources).
Conclusions. It is probable that a high degree of contamination by Galactic sources occurs among the single-peak FREDs observed by Swift. Accordingly, we encourage additional studies on these types of events to determine the nature of what could be an exotic type of Galactic source.
Key words: gamma-ray burst: general / stars: magnetars / methods: statistical
Appendix A is available in electronic form at http://www.aanda.org
© ESO, 2012