496 research outputs found
High column densities and low extinctions of gamma-ray bursts: Evidence for hypernovae and dust destruction
We analyze a complete sample of gamma-ray burst afterglows, and find X-ray evidence for high column densities of gas around them. The column densities are in the range 1e+22-1e+23/cm2, which is right around the average column density of Galactic giant molecular clouds. We also estimate the cloud sizes to be 10-30 pc, implying masses greater than about 1e+5 solar masses. This strongly suggests that gamma-ray bursts lie within star forming regions, and therefore argues against neutron star mergers and for collapses of massive stars as their sources. The optical extinctions, however, are 10-100 times smaller than expected from the high column densities. This confirms theoretical findings that the early hard radiation from gamma-ray bursts and their afterglows can destroy the dust in their environment, thus carving a path for the afterglow light out of the molecular cloud. Because of the self-created low extinction and location in star-forming regions, we expect gamma-ray bursts to provide a relatively unbiased sample of high-redshift star formation. Thus they may help resolve what is the typical environment of high-redshift star formation
Continued Radio Monitoring of the Gamma Ray Burst 991208
We present radio observations of the afterglow of the bright gamma-ray burst
GRB 991208 at frequencies of 1.4, 4.9 and 8.5 GHz, taken between two weeks and
300 days after the burst. The well-sampled radio light curve at 8.5 GHz shows
that the peak flux density peaked about 10 days after the burst and decayed
thereafter as a power-law t^-1.07. This decay rate is more shallow than the
optical afterglow with t^-2.2, which was measured during the first week. These
late-time data are combined with extensive optical, millimeter and centimeter
measurements and fitted to the standard relativistic blast wave model. In
agreement with previous findings, we find that an isotropic explosion in a
constant density or wind-blown medium cannot explain these broadband data
without modifying the assumption of a single power-law slope for the electron
energy distribution. A jet-like expansion provides a reasonable fit to the
data. In this case, the flatter radio light curve compared to the optical may
be due to emission from an underlying host galaxy, or due to the blastwave
making a transition to non-relativistic expansion. The model that best
represents the data is a free-form model in which it is assumed that the
broadband emission originates from a synchrotron spectrum, while the
time-evolution of the break frequencies and peak flux density are solved for
explicitly. Although the decay indices for most of the synchrotron parameters
are similar to the jet model, the evolution of the cooling break is unusually
rapid, and therefore requires some non-standard evolution in the shock.
(abridged)Comment: ApJ, in pres
Physical parameters of GRB 970508 and GRB 971214 from their afterglow synchrotron emission
We have calculated synchrotron spectra of relativistic blast waves, and find
predicted characteristic frequencies that are more than an order of magnitude
different from previous calculations. For the case of an adiabatically
expanding blast wave, which is applicable to observed gamma-ray burst (GRB)
afterglows at late times, we give expressions to infer the physical properties
of the afterglow from the measured spectral features.
We show that enough data exist for GRB970508 to compute unambiguously the
ambient density, n=0.03/cm**3, and the blast wave energy per unit solid angle,
E=3E52 erg/4pi sr. We also compute the energy density in electrons and magnetic
field. We find that they are 12% and 9%, respectively, of the nucleon energy
density and thus confirm for the first time that both are close to but below
equipartition.
For GRB971214, we discuss the break found in its spectrum by Ramaprakash et
al. (1998). It can be interpreted either as the peak frequency or as the
cooling frequency; both interpretations have some problems, but on balance the
break is more likely to be the cooling frequency. Even when we assume this, our
ignorance of the self-absorption frequency and presence or absence of beaming
make it impossible to constrain the physical parameters of GRB971214 very well.Comment: very strongly revised analysis of GRB971214 and discussion, submitted
to ApJ, 11 pages LaTeX, 4 figures, uses emulateapj.sty (included
The 1.4 GHz light curve of GRB 970508
We report on Westerbork 1.4 GHz radio observations of the radio counterpart
to -ray burst GRB~970508, between 0.80 and 138 days after this event.
The 1.4 GHz light curve shows a transition from optically thick to thin
emission between 39 and 54 days after the event. We derive the slope of the
spectrum of injected electrons () in two
independent ways which yield values very close to . This is in agreement
with a relativistic dynamically near-adiabatic blast wave model whose emission
is dominated by synchrotron radiation and in which a significant fraction of
the electrons cool fast.Comment: Paper I. Accepted for publication in the Astrophysical Journal
Letter
Relational Prior Knowledge Graphs for Detection and Instance Segmentation
Humans have a remarkable ability to perceive and reason about the world around them by understanding the relationships between objects. In this paper, we investigate the effectiveness of using such relationships for object detection and instance segmentation. To this end, we propose a Relational Prior-based Feature Enhancement Model (RP-FEM), a graph transformer that enhances object proposal features using relational priors. The proposed architecture operates on top of scene graphs obtained from initial proposals and aims to concurrently learn relational context modeling for object detection and instance segmentation.Experimental evaluations on COCO show that the utilization of scene graphs, augmented with relational priors, offer benefits for object detection and instance segmentation. RP-FEM demonstrates its capacity to suppress improbable class predictions within the image while also preventing the model from generating duplicate predictions, leading to improvements over the baseline model on which it is built.</p
Characterization of seedling and adult-plant resistance to stem rust race Ug99 in Iranian bread wheat landraces
The full-length infectious cDNA clone was constructed and sequenced from the strain DM of echovirus 9, which was recently isolated from a 6-week-old child at the clinical onset of type 1 diabetes. Parallel with the isolate DM, the full-length infectious cDNA clone of the prototype strain echovirus 9 Barty (Barty-INF), was constructed and sequenced. Genetic relationships of the sequenced echo 9 viruses to the other members of the human enterovirus type B species were studied by phylogenetic analyses. Comparison of capsid protein sequences showed that the isolate DM was closely related to both prototype strains: Hill and Barty-INF. The only exception was the inner capsid protein VP4 where serotype specificity was not evident and the isolate DM clustered with the strain Hill and the strain Barty-INF with echovirus 30 Bastianni. Likewise, the nonstructural protein coding region, P2P3, of isolate DM was more similar to strain Hill than to strain Barty-INF. However, like echovirus 9 Barty, the isolate DM contained the RGD-motif in the carboxy terminus of capsid protein VP1. By blocking experiments using an RGD-containing peptide and a polyclonal rabbit antiserum to the alpha(v)beta(3)-integrin, it was shown that this molecule works as a cellular receptor for isolate DM. By using primary human islets, it was shown that the isolate DM is capable of infecting insulin-producing beta-cells like the corresponding prototype strains did. However, only isolate DM was clearly cytolytic for beta-cells. The infectious clones that were made allow further investigations of the molecular features responsible for the diabetogenicity of the isolate DM
The Enigmatic Radio Afterglow of GRB 991216
We present wide-band radio observations spanning from 1.4 GHz to 350 GHz of
the afterglow of GRB 991216, taken from 1 to 80 days after the burst. The
optical and X-ray afterglow of this burst were fairly typical and are explained
by a jet fireball. In contrast, the radio light curve is unusual in two
respects: (a) the radio light curve does not show the usual rise to maximum
flux on timescales of weeks and instead appears to be declining already on day
1 and (b) the power law indices show significant steepening from the radio
through the X-ray bands. We show that the standard fireball model, in which the
afterglow is from a forward shock, is unable to account for (b) and we conclude
that the bulk of the radio emission must arise from a different source. We
consider two models, neither of which can be ruled out with the existing data.
In the first (conventional) model, the early radio emission is attributed to
emission from the reverse shock as in the case of GRB 990123. We predict that
the prompt optical emission would have been as bright (or brighter) than 8th
magnitude. In the second (exotic) model, the radio emission originates from the
forward shock of an isotropically energetic fireball (10^54 erg) expanding into
a tenuous medium (10^-4 cm^-3). The resulting fireball would remain
relativistic for months and is potentially resolvable with VLBI techniques.
Finally, we note that the near-IR bump of the afterglow is similar to that seen
in GRB 971214 and no fireball model can explain this bump.Comment: ApJ, submitte
The complete catalogue of gamma-ray bursts observed by the Wide Field Cameras on board BeppoSAX
We present the complete on-line catalogue of gamma-ray bursts observed by the
two Wide Field Cameras on board \sax in the period 1996-2002. Our aim is to
provide the community with the largest published data set of GRB's prompt
emission X-ray light curves and other useful data. This catalogue
(BS-GRBWFCcat) contains data on 77 bursts and a collection of the X-ray light
curves of 56 GRB discovered or noticed shortly after the event and of other
additional bursts detected in subsequent searches. Light curves are given in
the three X-ray energy bands (2-5, 5-10, 10-26 keV). The catalogue can be
accessed from the home web page of the ASI Science Data Center-ASDC
(http://www.asdc.asi.it)Comment: 4 pages, 3 figure
SCUBA sub-millimeter observations of gamma-ray bursters: II. GRB 991208, 991216, 000301C, 000630, 000911, 000926
We discuss our ongoing program of Target of Opportunity sub-millimeter
observations of gamma-ray bursts (GRBs) using the Sub-millimetre Common-User
Bolometer Array (SCUBA) on the James Clerk Maxwell Telescope (JCMT).
Sub-millimeter observations of the early afterglows are of interest because
this is where the emission peaks in some bursts in the days to weeks following
the burst. Of increasing interest is to look for underlying quiescent
sub-millimeter sources that may be dusty star-forming host galaxies. In this
paper, we present observations of GRB 991208, 991216, 000301C, 000630, 000911,
and 000926. For all these bursts, any sub-millimeter emission is consistent
with coming from the afterglow. This means that we did not conclusively detect
quiescent sub-millimeter counterparts to any of the bursts that were studied
from 1997 through 2000. The inferred star formation rates (M > 5 Msun) are
typically < 300 Msun/yr. If GRBs are due to the explosions of high-mass stars,
this may indicate that the relatively small population of extremely luminous
dusty galaxies does not dominate the total star formation in the universe at
early epochs. Instead, the GRBs may be predominantly tracing slightly lower
luminosity galaxies. The optical faintness of some host galaxies is unlikely to
be explained as due to dust absorption in the host.Comment: 9 pages. 0 figures. Astronomy and Astrophysics, in pres
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