Cosmological effects on the observed flux and fluence distributions of gammaray bursts: Are the most distant bursts in general the faintest ones?
Abstract
Context. Several claims have been put forward that an essential fraction of longduration BATSE gammaray bursts should lie at redshifts larger than 5. This pointofview follows from the natural assumption that fainter objects should, on average, lie at larger redshifts. However, redshifts larger than 5 are rare for bursts observed by Swift, seemingly contradicting the BATSE estimates.
Aims: The purpose of this article is to clarify this contradiction.
Methods: We derive the cosmological relationships between the observed and emitted quantities, and we arrive at a prediction that can be tested on the ensembles of bursts with determined redshifts. This analysis is independent on the assumed cosmology, on the observational biases, as well as on any gammaray burst model. Four different samples are studied: 8 BATSE bursts with redshifts, 13 bursts with derived pseudoredshifts, 134 Swift bursts with redshifts, and 6 Fermi bursts with redshifts.
Results: The controversy can be explained by the fact that apparently fainter bursts need not, in general, lie at large redshifts. Such a behaviour is possible, when the luminosities (or emitted energies) in a sample of bursts increase more than the dimming of the observed values with redshift. In such a case dP(z)/dz > 0 can hold, where P(z) is either the peakflux or the fluence. All four different samples of the long bursts suggest that this is really the case. This also means that the hundreds of faint, longduration BATSE bursts need not lie at high redshifts, and that the observed redshift distribution of long Swift bursts might actually represent the actual distribution.
 Publication:

Astronomy and Astrophysics
 Pub Date:
 May 2011
 DOI:
 10.1051/00046361/201014918
 arXiv:
 arXiv:1101.5040
 Bibcode:
 2011A&A...529A..55M
 Keywords:

 gammaray burst: general;
 cosmology: miscellaneous;
 Astrophysics  Cosmology and Nongalactic Astrophysics
 EPrint:
 Accepted for publication in Astronomy and Astrophysics, 5 figures, 7 tables