214 research outputs found
Spectral analysis of 35 GRBs/XRFs observed with HETE-2/FREGATE
We present a spectral analysis of 35 GRBs detected with the HETE-2 gamma-ray
detectors (the FREGATE instrument) in the energy range 7-400 keV. The GRB
sample analyzed is made of GRBs localized with the Wide Field X-ray Monitor
onboard HETE-2 or with the GRB Interplanetary Network. We derive the spectral
parameters of the time-integrated spectra, and present the distribution of the
low-energy photon index, alpha, and of the peak energy, e_peak . We then
discuss the existence and nature of the recently discovered X-Ray Flashes and
their relationship with classical GRBs.Comment: 14 pages, 43 figures, accepted for publication in Astronomy and
Astrophysic
HETE Observations of the Gamma-Ray Burst GRB030329: Evidence for an Underlying Soft X-ray Component
An exceptionally intense gamma-ray burst, GRB030329, was detected and
localized by the instruments on board the High Energy Transient Explorer
satellite (HETE) at 11:37:14 UT on 29 March 2003. The burst consisted of two
\~10s pulses of roughly equal brightness and an X-ray tail lasting >100s. The
energy fluence in the 30-400 keV energy band was 1.08e-4 erg/cm2, making
GRB030329 one of the brightest GRBs ever detected. Communication of a 2 arcmin
error box 73 minutes after the burst allowed the rapid detection of a
counterpart in the optical, X-ray, radio and the ensuing discovery of a
supernova with most unusual characteristics. Analyses of the burst lightcurves
reveal the presence of a distinct, bright, soft X-ray component underlying the
main GRB: the 2-10 keV fluence of this component is ~7e-6 erg/cm2. The main
pulses of GRB030329 were preceded by two soft, faint, non-thermal bumps. We
present details of the HETE observations of GRB030329.Comment: 22 pages, 5 figures, to be published in ApJ 617, no. 2 (10 December
2004). Referee comments have been incorporated; results of improved spectral
analysis are include
HETE-2 Observation of two gamma-ray bursts at z > 3
GRB 020124 and GRB 030323 constitute half the sample of gamma-ray bursts with
a measured redshift greater than 3. This paper presents the temporal and
spectral properties of these two gamma-ray bursts detected and localized with
HETE-2. While they have nearly identical redshifts (z=3.20 for GRB 020124, and
z=3.37 for GRB 030323), these two GRBs span about an order of magnitude in
fluence, thus sampling distinct regions of the GRB luminosity function. The
properties of these two bursts are compared with those of the bulk of the GRB
population detected by HETE-2. We also discuss the energetics of GRB 020124 and
GRB 030323 and show that they are compatible with the Epeak - Eiso relation
discovered by Amati et al. (2002). Finally, we compute the maximum redshifts at
which these bursts could have been detected by HETE-2 and we address various
issues connected with the detection and localization of high-z GRBs.Comment: 19 pages, 4 figures, submitted to Ap
Spectrum and Variability of Mrk501 as observed by the CAT Imaging Telescope
The CAT Imaging Telescope has observed the BL Lac object Markarian 501
between March and August 1997. We report here on the variability over this time
including several large flares. We present also preliminary spectra for all
these data, for the low emission state, and for the largest flare.Comment: 4 pages, 4 figures, Late
Detection of Vhe Gamma-Rays from MRK 501 with the Cat Imaging Telescope
The CAT imaging telescope on the site on the former solar plant Themis has
been observing gamma-rays from Mrk501 above 220 GeV in March and April 1997.
This source is shown to be highly variable and the light curve is presented.
The detected gamma-ray rate for the most intense flare is in excess of 10 per
minute.Comment: 5 pages, 4 figures, Tex, contribution to 25th ICRC Durba
Observation of the Crab Nebula Gamma-Ray Emission Above 220 Gev by the Cat Cherenkov Imaging Telescope
The CAT imaging telescope, recently built on the site of the former solar
plant Themis (French Pyrenees), observed gamma-rays from the Crab nebula from
October 1996 to March 1997. This steady source, often considered as the
standard candle of very-high-energy gamma-ray astronomy, is used as a test-beam
to probe the performances of the new telescope, particularly its energy
threshold (220 GeV at 20 degrees zenith angle) and the stability of its
response. Due to the fine-grain camera, an accurate analysis of the
longitudinal profiles of shower images is performed, yielding the source
position in two dimensions for each individual shower.Comment: 5 pages, 3 figures, Tex, contribution to 25th ICRC Durba
Global Characteristics of X-Ray Flashes and X-Ray-Rich GRBs Observed by HETE-2
We describe and discuss the global properties of 45 gamma-ray bursts (GRBs)
observed by HETE-2 during the first three years of its mission, focusing on the
properties of X-Ray Flashes (XRFs) and X-ray-rich GRBs (XRRs). We find that the
numbers of XRFs, XRRs, and GRBs are comparable. We find that the durations and
the sky distributions of XRFs and XRRs are similar to those of GRBs. We also
find that the spectral properties of XRFs and XRRs are similar to those of
GRBs, except that the values of the peak energy of the
burst spectrum in , the peak energy flux \Fp, and the energy
fluence of XRFs are much smaller -- and those of XRRs are smaller -- than
those of GRBs. Finally, we find that the distributions of all three kinds of
bursts form a continuum in the [(2-30 keV),(30-400) keV]-plane, the
[(2-400 keV), ]-plane, and the [(50-300 keV),
]-plane. These results provide strong evidence that all three
kinds of bursts arise from the same phenomenon.Comment: 33 pages, 15 figures, submitted to Ap
X-ray Flashes or soft Gamma-ray Bursts? The case of the likely distant XRF 040912
In this work, we present a multi-wavelength study of XRF 040912, aimed at
measuring its distance scale and the intrinsic burst properties. We performed a
detailed spectral and temporal analysis of both the prompt and the afterglow
emission and we estimated the distance scale of the likely host galaxy. We then
used the currently available sample of XRFs with known distance to discuss the
connection between XRFs and classical Gamma-ray Bursts (GRBs). We found that
the prompt emission properties unambiguously identify this burst as an XRF,
with an observed peak energy of E_p=17+/-13 keV and a burst fluence ratio
S(2-30keV)/S(30-400keV)>1. A non-fading optical source with R~24 mag and with
an apparently extended morphology is spatially consistent with the X-ray
afterglow, likely the host galaxy. XRF 040912 is a very dark burst since no
afterglow optical counterpart is detected down to R>25 mag (3 sigma limiting
magnitude) at 13.6 hours after the burst. The host galaxy spectrum detected
from 3800A to 10000A, shows a single emission line at 9552A. The lack of any
other strong emission lines blue-ward of the detected one and the absence of
the Ly alpha cut-off down to 3800A are consistent with the hypothesis of the
[OII] line at redshift z=1.563+/-0.001. The intrinsic spectral properties rank
this XRF among the soft GRBs in the E_peak-E_iso diagram. Similar results were
obtained for most XRFs at known redshift. Only XRF 060218 and XRF 020903
represent a good example of instrinsic XRF(i-XRF) and are possibly associated
with a different progenitor population. This scenario may calls for a new
definition of XRFs.Comment: 10 pages, 7 figures, accepted for publication in Astronomy &
Astrophysic
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