Studies of intrinsic properties of gamma ray bursts detected by the HETE-II satellite
Abstract
Analysis of HETE-II data is discussed with the aim of understanding the intrinsic properties of gamma-ray 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 Wide-field X-ray Monitor (WXM) on HETE-II. A closely related photon-by-photon statistical bootstrap analysis is then described that can be used to compute the non-Gaussian 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 gamma-ray burst catalogs and is widely believed to be log-normally distributed. It is shown that intrinsically, GRBs may in fact have a much narrower distribution, and the wide log-normal 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 flux-limited 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 redshift-corrected 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 gamma-ray bursts are derived using the non-parametric methods of Lynden-Bell 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 log-normal errors in the redshifts are estimated using Monte-Carlo 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. 14-0551, Cambridge, MA 02139-4307. Ph. 617-253-5668; Fax 617-253-1690.)- Publication:
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Ph.D. Thesis
- Pub Date:
- 2007
- Bibcode:
- 2007PhDT.........7C
- Keywords:
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- Gamma-ray bursts;
- HETE-II satellite;
- Redshift