163 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
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
Light Curves and Spectra of Dust Echoes From Gamma-Ray Bursts and their Afterglows: Continued Evidence that GRB 970228 is Associated with a Supernova
The relative brightening and reddening of the optical afterglows of GRB
970228 and GRB 980326 around 20 - 30 days after these bursts have been
attributed to supernovae, which are red (blueward of their spectral peak), and
which peak in time after about 20(1+z) days. However, this direct evidence for
a GRB/SN connection has recently been challenged. It has been suggested that
the late afterglows of these bursts can be explained by dust echoes, of which
we consider two cases: (1) the scattering of light from the afterglow (the
forward shock), the optical flash (the reverse shock), and/or optical light
from the burst itself by a shell of dust at a radius R from the progenitor, and
(2) the absorption and thermal re-emission of this light by this shell of dust.
In this paper, we model and compute dust echo light curves and spectra for both
cases. Although the late-time afterglow of GRB 980326 was not sufficiently well
sampled to rule out a dust echo description, we find that the late-time
afterglow of GRB 970228 cannot be explained by a dust echo.Comment: Accepted to The Astrophysical Journal, 32 pages, 14 figures, LaTe
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