4,097 research outputs found
Evidence for Circumburst Extinction of Gamma-Ray Bursts with Dark Optical Afterglows and Evidence for a Molecular Cloud Origin of Gamma-Ray Bursts
First, we show that the gamma-ray bursts with dark optical afterglows (DOAs)
cannot be explained by a failure to image deeply enough quickly enough, and
argue that circumburst extinction is the most likely solution. If so, many DOAs
will be ``revived'' with rapid follow up and NIR searches in the HETE-2 and
Swift eras. Next, we consider the effects of dust sublimation and
fragmentation, and show that DOAs occur in clouds of size R > 10L_{49}^{1/2} pc
and mass M > 3x10^5L_{49} M_{sun}, where L is the luminosity of the optical
flash. Stability considerations show that such clouds cannot be diffuse, but
must be molecular. Consequently, we compute the expected column density
distribution of bursts that occur in Galactic-like molecular clouds, and show
that the column density measurements from X-ray spectra of afterglows, DOAs and
otherwise, satisfy this expectation in the source frame.Comment: Invited Review. To appear in Procs. of Gamma-Ray Burst and Afterglow
Astronomy 2001: A Workshop Celebrating the First Year of the HETE Mission, 8
pages, 8 figures, LaTe
The Detectability of Gamma-Ray Bursts and Their Afterglows at Very High Redshifts
There is increasingly strong evidence that gamma-ray bursts (GRBs) are
associated with star-forming galaxies, and occur near or in the star-forming
regions of these galaxies. These associations provide indirect evidence that at
least the long GRBs detected by BeppoSAX are a result of the collapse of
massive stars. The recent evidence that the light curves and the spectra of the
afterglows of GRB 970228 and GRB 980326 appear to contain a supernova
component, in addition to a relativistic shock wave component, provide more
direct clues that this is the case. Here we establish that GRBs and their
afterglows are both detectable out to very high redshifts (z > 5).Comment: To appear in Proc. of the 10th Annual October Astrophysics Conference
in Maryland: Cosmic Explosions, 4 pages, LaTe
Gamma-Ray Burst Dust Echoes Revisited: Expectations at Early Times
Gamma-ray burst (GRB) dust echoes were first proposed as an alternative
explanation for the supernova-like (SN-like) components to the afterglows of
GRB 980326 and GRB 970228. However, the spectroscopic identification of Type Ic
SN 2003dh associated with GRB 030329, as well as the identification of SN-like
components to the afterglows of other GRBs, appears to have confirmed the
GRB/SN paradigm. However, the likely progenitors of Type Ic SNe are Wolf-Rayet
WC stars, and late-type WC stars have been observed to be surrounded by dust,
at a distance of 10^14 -- 10^15 cm from the star. Consequently, we revisit the
possibility of GRB dust echoes, not on a timescale of weeks after the burst but
on a timescale of minutes to hours. We find that if the optical flash is
sufficiently bright and the jet sufficiently wide, GRB afterglows may be
accompanied by chromatic variations on this timescale. From these signatures,
such model parameters as the inner radius of the dust distribution, the initial
opening angle of the jet, etc., may be deduced. With rapid and regular
localizations of GRBs by HETE-2, Integral, and now Swift, and new and improved
robotic telescope systems, these early-time GRB dust echoes may soon be
detected. We describe one such robotic telescope system, called PROMPT, that
the University of North Carolina at Chapel Hill is building at the Cerro Tololo
Inter-American Observatory in greater detail.Comment: Accepted to The Astrophysical Journal, 15 pages, 5 figures, LaTe
SimLex-999: Evaluating Semantic Models with (Genuine) Similarity Estimation
We present SimLex-999, a gold standard resource for evaluating distributional
semantic models that improves on existing resources in several important ways.
First, in contrast to gold standards such as WordSim-353 and MEN, it explicitly
quantifies similarity rather than association or relatedness, so that pairs of
entities that are associated but not actually similar [Freud, psychology] have
a low rating. We show that, via this focus on similarity, SimLex-999
incentivizes the development of models with a different, and arguably wider
range of applications than those which reflect conceptual association. Second,
SimLex-999 contains a range of concrete and abstract adjective, noun and verb
pairs, together with an independent rating of concreteness and (free)
association strength for each pair. This diversity enables fine-grained
analyses of the performance of models on concepts of different types, and
consequently greater insight into how architectures can be improved. Further,
unlike existing gold standard evaluations, for which automatic approaches have
reached or surpassed the inter-annotator agreement ceiling, state-of-the-art
models perform well below this ceiling on SimLex-999. There is therefore plenty
of scope for SimLex-999 to quantify future improvements to distributional
semantic models, guiding the development of the next generation of
representation-learning architectures
Construction of the Variability -> Luminosity Estimator
We present a possible Cepheid-like luminosity estimator for the long-duration
gamma-ray bursts based on the variability of their light curves.Comment: To appear in Procs. of Gamma-Ray Bursts in the Afterglow Era: 2nd
Workshop, 3 pages, 2 figures, LaTe
Gamma-Ray Bursts as a Probe of Cosmology
We show that, if the long GRBs are produced by the collapse of massive stars,
GRBs and their afterglows may provide a powerful probe of cosmology and the
early universe.Comment: 6 pages, 5 PostScript figures. To appear in the proceedings of the
October 2000 Rome Workshop on Gamma-Ray Bursts in the Afterglow Er
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