30 research outputs found
Implications of the Visible and X-Ray Counterparts to GRB970228
The gamma-ray burst source GRB970228 has been observed after a delay of 8--12
hours in X-rays and after one day in visible and near infrared light. This
marks the first detection of emission at lower frequencies following the
gamma-ray observation of a GRB and the first detection of any visible
counterpart to a GRB. We consider possible delayed visible and X-ray emission
mechanisms, and conclude that the intrinsic gamma-ray activity continued at a
much reduced intensity for at least a day. There are hints of such continued
activity in other GRB, and future observations can decide if this is true of
GRB in general. The observed multi-band spectrum of GRB970228 agrees with the
predictions of relativistic shock theory when the flux is integrated over a
time longer than that required for a radiating electron to lose its energy.Comment: 5 pp., tex, 1 figur
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
Discovery of the peculiar supernova 1998bw in the error box of GRB980425
The discovery of X-ray, optical and radio afterglows of gamma-ray bursts
(GRBs) and the measurements of the distances to some of them have established
that these events come from Gpc distances and are the most powerful photon
emitters known in the Universe, with peak luminosities up to 10^52 erg/s. We
here report the discovery of an optical transient, in the BeppoSAX Wide Field
Camera error box of GRB980425, which occurred within about a day of the
gamma-ray burst. Its optical light curve, spectrum and location in a spiral arm
of the galaxy ESO 184-G82, at a redshift z = 0.0085, show that the transient is
a very luminous type Ic supernova, SN1998bw. The peculiar nature of SN1998bw is
emphasized by its extraordinary radio properties which require that the radio
emitter expand at relativistical speed. Since SN1998bw is very different from
all previously observed afterglows of GRBs, our discovery raises the
possibility that very different mechanisms may give rise to GRBs, which differ
little in their gamma-ray properties.Comment: Under press embargo at Nature (submitted June 10, 1998
Discovery of intense gamma-ray flashes of atmospheric origin
Observations have been made of a new terrestrial phenomenon: brief (approx. millisecond), intense flashes of gamma rays, observed with space-borne detectors. These flashes must originate at altitudes in the atmosphere above at least 30 km in order to be observable by orbiting detectors aboard the Compton Gamma-Ray Observatory (CGRO). At least a dozen events have been detected over the past 2 years. The photon spectra from the events are very hard and are consistent with bremsstrahlung emission from energetic (MeV) electrons. The most likely origin of these high energy electrons, while speculative at this time, is a rare type of high altitude electrical discharge above thunderstorm regions
The Burst and Transient Source Experiment Earth Occultation Technique
An Earth orbiting detector sensitive to gamma ray photons will see step-like
occultation features in its counting rate when a gamma ray point source crosses
the Earth's limb. This is due to the change in atmospheric attenuation of the
gamma rays along the line of sight. In an uncollimated detector, these
occultation features can be used to locate and monitor astrophysical sources
provided their signals can be individually separated from the detector
background. We show that the Earth occultation technique applied to the Burst
and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory
(CGRO) is a viable and flexible all-sky monitor in the low energy gamma ray and
hard X-ray energy range (20 keV - 1 MeV). The method is an alternative to more
sophisticated photon imaging devices for astronomy, and can serve well as a
cost-effective science capability for monitoring the high energy sky.
Here we describe the Earth occultation technique for locating new sources and
for measuring source intensity and spectra without the use of complex
background models. Examples of transform imaging, step searches, spectra, and
light curves are presented. Systematic uncertainties due to source confusion,
detector response, and contamination from rapid background fluctuations are
discussed and analyzed for their effect on intensity measurements. A sky
location-dependent average systematic error is derived as a function of
galactic coordinates. The sensitivity of the technique is derived as a function
of incident photon energy and also as a function of angle between the source
and the normal to the detector entrance window. Occultations of the Crab Nebula
by the Moon are used to calibrate Earth occultation flux measurements
independent of possible atmospheric scattering effects.Comment: 39 pages, 24 figures. Accepted for publication in the Astrophysical
Journal Supplement
Extended Emission from Short Gamma-Ray Bursts Detected with SPI-ACS/INTEGRAL
The short duration (T90 < 2 s) gamma-ray bursts (GRBs) detected in the
SPI-ACS experiment onboard the INTEGRAL observatory are investigated. Averaged
light curves have been constructed for various groups of events, including
short GRBs and unidentified short events. Extended emission has been found in
the averaged light curves of both short GRBs and unidentified short events. It
is shown that the fraction of the short GRBs in the total number of SPI-ACS
GRBs can range from 30 to 45%, which is considerably larger than has been
thought previously.Comment: 27 pages, 10 figure
A Tale of Two Transients: GW 170104 and GRB 170105A
We present multi-wavelength follow-up campaigns by the AstroSat CZTI and GROWTH collaborations in search of an electromagnetic counterpart to the gravitational wave event GW 170104. At the time of the GW 170104 trigger, the AstroSat CZTI field of view covered 50.3% of the sky localization. We do not detect any hard X-ray (>100 keV) signal at this time, and place an upper limit of , for a 1 s timescale. Separately, the ATLAS survey reported a rapidly fading optical source dubbed ATLAS17aeu in the error circle of GW 170104. Our panchromatic investigation of ATLAS17aeu shows that it is the afterglow of an unrelated long, soft GRB 170105A, with only a fortuitous spatial coincidence with GW 170104. We then discuss the properties of this transient in the context of standard long GRB afterglow models