508 research outputs found
Swift and Fermi observations of X-ray flares: the case of Late Internal Shock
Simultaneous Swift and Fermi observations of gamma-ray bursts (GRBs) offer a
unique broadband view of their afterglow emission, spanning more than ten
decades in energy. We present the sample of X-ray flares observed by both Swift
and Fermi during the first three years of Fermi operations. While bright in the
X-ray band, X-ray flares are often undetected at lower (optical), and higher
(MeV to GeV) energies. We show that this disfavors synchrotron self-Compton
processes as origin of the observed X-ray emission. We compare the broadband
properties of X-ray flares with the standard late internal shock model, and
find that, in this scenario, X-ray flares can be produced by a late-time
relativistic (Gamma>50) outflow at radii R~10^13-10^14 cm. This conclusion
holds only if the variability timescale is significantly shorter than the
observed flare duration, and implies that X-ray flares can directly probe the
activity of the GRB central engine.Comment: 13 pages, 4 figures, accepted for publication in Ap
BATSE observations of BL Lac Objects
The Burst and Transient Source Experiment (BATSE) on the Compton Gamma-Ray Observatory has been shown to be sensitive to non-transient hard X-ray sources in our galaxy, down to flux levels of 100 mCrab for daily measurements, 3 mCrab for integrations over several years. We use the continuous BATSE database and the Earth Occultation technique to extract average flux values between 20 and 200 keV from complete radio- and X-ray- selected BL Lac samples over a 2 year period
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
Acoustic characteristics of synthesized signals of Chinese bahaba (Bahaba taipingensis)
As a member of Sciaenidae, Chinese bahaba (Bahaba taipingensis) generate sounds using sonic muscles to drive the swim bladder. In this study, the drumming sounds of Chinese bahaba in two groups differing in body size were recorded in an indoor aquarium and an outdoor pond. A piecewise exponential oscillation function was developed to synthesize the signals with a good agreement. Statistical comparisons found that the oscillation frequency and damping coefficient (part 1) of synthesized signals from larger-sized fish were lower. The results suggest that the acoustic characteristics of Chinese bahaba signals are related to fish morphology as the physiological age alters
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&
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