245 research outputs found
Observations of GRB 990123 by the Compton Gamma-Ray Observatory
GRB 990123 was the first burst from which simultaneous optical, X-ray and
gamma-ray emission was detected; its afterglow has been followed by an
extensive set of radio, optical and X-ray observations. We have studied the
gamma-ray burst itself as observed by the CGRO detectors. We find that
gamma-ray fluxes are not correlated with the simultaneous optical observations,
and the gamma-ray spectra cannot be extrapolated simply to the optical fluxes.
The burst is well fit by the standard four-parameter GRB function, with the
exception that excess emission compared to this function is observed below ~15
keV during some time intervals. The burst is characterized by the typical
hard-to-soft and hardness-intensity correlation spectral evolution patterns.
The energy of the peak of the nu f_nu spectrum, E_p, reaches an unusually high
value during the first intensity spike, 1470 +/- 110 keV, and then falls to
\~300 keV during the tail of the burst. The high-energy spectrum above ~MeV is
consistent with a power law with a photon index of about -3. By fluence, GRB
990123 is brighter than all but 0.4% of the GRBs observed with BATSE, clearly
placing it on the -3/2 power-law portion of the intensity distribution.
However, the redshift measured for the afterglow is inconsistent with the
Euclidean interpretation of the -3/2 power-law. Using the redshift value of >=
1.61 and assuming isotropic emission, the gamma-ray fluence exceeds 10E54 ergs.Comment: Submitted to The Astrophysical Journal. 16 pages including 4 figure
LOTIS Upper Limits and the Prompt OT from GRB 990123
GRB 990123 established the existence of prompt optical emission from
gamma-ray bursts (GRBs). The Livermore Optical Transient Imaging System (LOTIS)
has been conducting a fully automated search for this kind of simultaneous low
energy emission from GRBs since October 1996. Although LOTIS has obtained
simultaneous, or near simultaneous, coverage of the error boxes obtained with
BATSE, IPN, XTE, and BeppoSAX for several GRBs, image analysis resulted in only
upper limits. The unique gamma-ray properties of GRB 990123, such as very large
fluence (top 0.4%) and hard spectrum, complicate comparisons with more typical
bursts. We scale and compare gamma-ray properties, and in some cases afterglow
properties, from the best LOTIS events to those of GRB 990123 in an attempt to
determine whether the prompt optical emission of this event is representative
of all GRBs. Furthermore, using LOTIS upper limits in conjunction with the
relativistic blast wave model, we weakly constrain the GRB and afterglow
parameters such as density of the circumburster medium and bulk Lorentz factor
of the ejecta.Comment: 5 pages, 2 figures, To appear in Proceedings of the 5th Huntsville
Gamma-Ray Burst Symposiu
Evidence for an Early High-Energy Afterglow Observed with BATSE from GRB980923
In this Letter, we present the first evidence in the BATSE data for a prompt
high-energy (25-300 keV) afterglow component from a gamma-ray burst (GRB),
GRB980923. The event consists of rapid variabilty lasting ~40 s followed by a
smooth power law emission tail lasting ~400 s. An abrupt change in spectral
shape is found when the tail becomes noticeable. Our analysis reveals that the
spectral evolution in the tail of the burst mimics that of a cooling
synchrotron spectrum, similar to the spectral evolution of the low-energy
afterglows for GRBs. This evidence for a separate emission component is
consistent with the internal-external shock scenario in the relativistic
fireball picture. In particular, it illustrates that the external shocks can be
generated during the gamma-ray emission phase, as in the case of GRB990123.Comment: 4 pages, 4 figures, accepted for publication in Astrophysical Journal
Letter
The Fourth BATSE Gamma-Ray Burst Catalog (Revised)
The Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray
Observatory (CGRO) has triggered on 1637 cosmic gamma-ray bursts between 1991
April 19 and 1996 August 29. These events constitute the Fourth BATSE burst
catalog. The current version (4Br) has been revised from the version first
circulated on CD-ROM in September 1997 (4B) to include improved locations for a
subset of bursts that have been reprocssed using additional data. A significant
difference from previous BATSE catalogs is the inclusion of bursts from periods
when the trigger energy range differed from the nominal 50-300 keV. We present
tables of the burst occurrence times, locations, peak fluxes, fluences, and
durations. In general, results from previous BATSE catalogs are confirmed here
with greater statistical significance.Comment: 45 pages, 12 Postscript figures, accepted for publication in Ap. J.
Supp
Observation of X-ray lines from a Gamma-Ray Burst (GRB991216): Evidence of Moving Ejecta from the Progenitor
We report on the discovery of two emission features observed in the X-ray
spectrum of the afterglow of the gamma-ray burst (GRB) of 16 Dec. 1999 by the
Chandra X-Ray Observatory. These features are identified with the Ly
line and the narrow recombination continuum by hydrogenic ions of iron at a
redshift , providing an unambiguous measurement of the distance
of a GRB. Line width and intensity imply that the progenitor of the GRB was a
massive star system that ejected, before the GRB event, \approx 0.01 \Ms of
iron at a velocity , probably by a supernova explosion.Comment: 11 pages,2 fig.s, link to the published paper in Science, 290, 955
(2000) through http://www.ias.rm.cnr.it/grb/gb991216.htm
Rest-frame properties of 32 gamma-ray bursts observed by the Fermi Gamma-Ray Burst Monitor
Aims: In this paper we study the main spectral and temporal properties of
gamma-ray bursts (GRBs) observed by Fermi/GBM. We investigate these key
properties of GRBs in the rest-frame of the progenitor and test for possible
intra-parameter correlations to better understand the intrinsic nature of these
events. Methods: Our sample comprises 32 GRBs with measured redshift that were
observed by GBM until August 2010. 28 of them belong to the long-duration
population and 4 events were classified as short/hard bursts. For all of these
events we derive, where possible, the intrinsic peak energy in the spectrum (\eprest), the duration in the rest-frame, defined as the
time in which 90% of the burst fluence was observed (\tninetyrest) and the
isotropic equivalent bolometric energy (\eiso). Results: The distribution of
\eprest has mean and median values of 1.1 MeV and 750 keV, respectively. A
log-normal fit to the sample of long bursts peaks at ~800 keV. No high-\ep
population is found but the distribution is biased against low \ep values. We
find the lowest possible \ep that GBM can recover to be ~ 15 keV. The
\tninetyrest distribution of long GRBs peaks at ~10 s. The distribution of
\eiso has mean and median values of erg and erg, respectively. We confirm the tight correlation between \eprest
and \eiso (Amati relation) and the one between \eprest and the 1-s peak
luminosity () (Yonetoku relation). Additionally, we observe a parameter
reconstruction effect, i.e. the low-energy power law index gets softer
when \ep is located at the lower end of the detector energy range. Moreover, we
do not find any significant cosmic evolution of neither \eprest nor
\tninetyrest.Comment: accepted by A&
Gammaâray burst studies by COMPTEL during its first year of operation
During the first year of Compton GRO operations, more than 20 cosmic gammaâray burstâdetected by the BATSE instrument â occurred inside the 1 sr field of view of the imaging gammaâray telescope COMPTEL. Using COMPTELâs primary mode of operation (the telescope mode) direct images (with âŒ1° GRB location accuracy) and event spectra (0.7 MeV â 30 MeV) with spectral resolution better than 10% FWHM have been obtained. In its secondary mode of burst operations, COMPTEL has recorded time resolved spectra (0.1 MeV â 10 MeV) from its large NaI detectors. This paper summarises the results on cosmic GRB sources obtained by COMPTEL during its first year of operation
Probing the low-luminosity GRB population with new generation satellite detectors
We compare the detection rates and redshift distributions of low-luminosity
(LL) GRBs localized by Swift with those expected to be observed by the new
generation satellite detectors on GLAST (now Fermi) and, in future, EXIST.
Although the GLAST burst telescope will be less sensitive than Swift's in the
15--150 keV band, its large field-of-view implies that it will double Swift's
detection rate of LL bursts. We show that Swift, GLAST and EXIST should detect
about 1, 2 & 30 LL GRBs, respectively, over a 5-year operational period. The
burst telescope on EXIST should detect LL GRBs at a rate of more than an order
of magnitude greater than that of Swift's BAT. We show that the detection
horizon for LL GRBs will be extended from for Swift to in the EXIST era. Also, the contribution of LL bursts to the observed GRB
redshift distribution will contribute to an identifiable feature in the
distribution at .Comment: 6 pages, 4 figures, accepted by MNRA
The first COMPTEL Source Catalogue
The imaging Compton telescope COMPTEL aboard NASA's Compton Gamma-Ray
Observatory has opened the MeV gamma-ray band as a new window to astronomy.
COMPTEL provided the first complete all-sky survey in the energy range 0.75 to
30 MeV. The catalogue, presented here, is largely restricted to published
results. It contains firm as well as marginal detections of continuum and line
emitting sources and presents upper limits for various types of objects. The
numbers of the most significant detections are 32 for steady sources and 31 for
gamma-ray bursters. Among the continuum sources, detected so far, are spin-down
pulsars, stellar black-hole candidates, supernova remnants, interstellar
clouds, nuclei of active galaxies, gamma-ray bursters, and the Sun during solar
flares. Line detections have been made in the light of the 1.809 MeV 26Al line,
the 1.157 MeV 44Ti line, the 847 and 1238 keV 56Co lines, and the neutron
capture line at 2.223 MeV. For the identification of galactic sources, a
modelling of the diffuse galactic emission is essential. Such a modelling at
this time does not yet exist at the required degree of accuracy. Therefore, a
second COMPTEL source catalogue will be produced after a detailed and accurate
modelling of the diffuse interstellar emission has become possible.Comment: 50 pages including 4 figures; accepted for publication in A&A
Supplement
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