4 research outputs found
Spectral analysis of 50 GRBs detected by HETE-2
FREGATE, the gamma-ray detector of HETE-2 is entirely dedicated to the study
of GRBs. Its main characteristic is its broad energy range, from 7 keV to 400
keV. This energy range can be further extended down to 2 keV using the data
from the WXM, the X-ray detector of HETE-2. Such a large energy range allows to
study in details the prompt emission of GRBs, determining with a high precision
their spectral parameters. Moreover, because this energy range extends at low
energies, the sample of GRBs detected by both FREGATE and WXM contains a
significant fraction of X-Ray Rich GRBs and X-Ray Flashes.
We present here the distributions of the spectral parameters mesured for the
time integrated spectra of 50 GRBs. We put emphasis on the distribution of the
low energy spectral index alpha. Because FREGATE and WXM detected all classes
of GRBs, we also discuss the connection between GRBs, X-Ray Rich GRBs and X-Ray
Flashes.Comment: 4 pages aipTEX, contribution to the 2003 GRB Conference, held at
Santa Fe,N
Evidence From HETE-2 For GRB Evolution With Redshift
After taking into account threshold effects, we find that the
isotropic-equivalent energies E_iso and luminosities L_iso of gamma-ray bursts
(GRBs) are correlated with redshift at the 5% and 0.9% signficance levels,
respectively. Our results are based on 10 BeppoSAX GRBs and 11 HETE-2 GRBs with
known redshifts. Our results suggest that the isotropic-equivalent energies and
luminosities of GRBs increase with redshift. They strengthen earlier clues to
this effect from analyses of the BATSE catalog of GRBs, using the variability
of burst time histories as an estimator of burst luminosities (and therefore
redshifts), and from an analysis of BeppoSAX bursts only. If the
isotropic-equivalent energies and luminosities of GRBs really do increase with
redshift, it suggests that GRB jets at high redshifts may be narrower and thus
the cores of GRB progenitor stars at high redshifts may be rotating more
rapidly. It also suggests that GRBs at very high redshifts may be more luminous
-- and therefore easier to detect -- than has been thought, which would make
GRBs a more powerful probe of cosmology and the early universe than has been
thought.Comment: 5 pages, 3 figures, to appear in proc. 2003 GRB Conference, Santa Fe,
N
Observation and implications of the Epeak - Eiso correlation in Gamma-Ray Bursts
The availability of a few dozen GRB redshifts now allows studies of the
intrinsic properties of these high energy transients. Amati et al. recently
discovered a correlation between Epeak, the intrinsic peak energy of the spectrum, and Eiso, the isotropic equivalent energy radiated by the
source. Lamb et al. have shown that HETE-2 data confirm and extend this
correlation. We discuss here one of the consequences of this correlation: the
existence of a 'spectral standard candle', which can be used to construct a
simple redshift indicator for GRBs.Comment: Proceedings of the GRB 2003 Conference in SantaFe, 5 pages, 4 figure