Studies of intrinsic properties of gamma ray bursts detected by the HETEII satellite
Abstract
Analysis of HETEII data is discussed with the aim of understanding the intrinsic properties of gammaray bursts (GRBs). A technique is developed that allows the simultaneous estimation of source and background counts during a burst with coded aperture instruments such as the Widefield Xray Monitor (WXM) on HETEII. A closely related photonbyphoton statistical bootstrap analysis is then described that can be used to compute the nonGaussian error distribution of GRB temporal statistics. This is applied to the T 90 and T 0.45 duration measures. The distribution of T 90 has been extensively studied since the availability of the BATSE gammaray burst catalogs and is widely believed to be lognormally distributed. It is shown that intrinsically, GRBs may in fact have a much narrower distribution, and the wide lognormal may primarily be due to measurement artifacts.
Computation of the T 0.45 parameter enables the inference of redshifts through the recently discovered L iso  E pk  T 0.45 relation. This in turn allows the compilation of a fluxlimited sample of bursts with redshifts that is free of the observational selection effects inherent in spectroscopic catalogs. This analysis is performed for 31 WXM bursts and redshiftcorrected distributions of T 90 and T 0.45 are computed. It is shown for the first time that the distribution of T 0.45 can be modeled by an exponential distribution.
The redshifts calculated through the L iso  E pk  T 0.45 relation are also used to calculate the implied isotropic luminosities. The normalized luminosity function and redshift distribution of gammaray bursts are derived using the nonparametric methods of LyndenBell and Efron & Petrosian. The results imply strong evidence for luminosity evolution with redshift and are consistent with prior studies based on BATSE bursts. Concordance cosmology (O L = 0.7, O M = 0.3, H 0 =70 km s 1 Mpc 1 ) is assumed throughout the analysis. Effects of lognormal errors in the redshifts are estimated using MonteCarlo methods. Results indicate that a fraction close to 10% of GRBs are to be expected at high redshifts (> 5) in consonance with theoretical predictions of high redshift Swift detections. (Copies available exclusively from MIT Libraries, Rm. 140551, Cambridge, MA 021394307. Ph. 6172535668; Fax 6172531690.)
 Publication:

Ph.D. Thesis
 Pub Date:
 2007
 Bibcode:
 2007PhDT.........7C
 Keywords:

 Gammaray bursts;
 HETEII satellite;
 Redshift