Mathematics – Logic
Scientific paper
May 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993aas...182.3301t&link_type=abstract
American Astronomical Society, 182nd AAS Meeting, #33.01; Bulletin of the American Astronomical Society, Vol. 25, p.845
Mathematics
Logic
Scientific paper
The distribution of gamma-ray burst sources detected with BATSE is iso\-tro\-pic yet non-uniform. There is a deficit of weak events which is commonly expressed by a \vvm less than the 0.5 expected for a uniform Eu\-clid\-ean distribution. It has been argued (e.g., Mao and Paczynski 1992) that this may be a signature of a Cosmological expansion in a FRW universe with a uniform, co-moving distribution of sources. However, a comparison of the \vvm and burst detection rates of several experiments presents an apparent paradox: e.g., Konus detected the bursts more frequently than SMM, implying a better sensitivity and larger sampling volume, yet its measured \vvm was ~ 0.45 compared to SMM's value of ~ 0.40, suggesting instead a lesser deviation from Eu\-clid\-ean space. We point out that this apparent conflict stems from the assumption that all bursts have a standard spectrum with a single power law. But realistic spectra often exhibit broken power laws, with a break energy 100;keV ~ lessepsilon_b ~ less 3 MeV. \ Thus, the flux measured by a particular instrument depends on both the redshift of the source and the energy window of the detector. In other words, for a given energy window, the flux of a more distant burst will be further decreased below the value expected from the Cosmological model due to the red-shifting of epsilon_b across the window. We show that the inclusion of an intrinsic epsilon_b ( ~ 500 keV) and a consideration of the different detector energy responses and sensitivities can self-consistently account for the rates and \vvm of the Konus, PVO, SMM, and BATSE datasets. Because of the dependence of the observed characteristics on the energy window, a subset of the BATSE catalog, based solely on the burst fluence in the higher energy (i.e., 4{th}) channel, provides an additional ``data'' point in the \vvm versus rate plane. In particular, Konus's higher rate for a larger value of \vvm is due to its better sensitivity, but smaller and lower energy window than SMM. \ In the context of this improved model, BATSE is apparently sampling a source distribution out to a redshift zmax ~ 1.3, whereas Konus was limited to zmax ~ 0.5, SMM to ~ 0.3, and PVO to ~ 0.2.
Melia Fulvio
Tamblyn Peter
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