1,507 research outputs found
A Beaming-Independent Estimate of the Energy Distribution of Long Gamma-Ray Bursts: Initial Results and Future Prospects
We present single-epoch radio afterglow observations of 24 long-duration
gamma-ray burst (GRB) on a timescale of >100 d after the burst. These
observations trace the afterglow evolution when the blastwave has decelerated
to mildly- or non-relativistic velocities and has roughly isotropized. We infer
beaming-independent kinetic energies using the Sedov-Taylor self-similar
solution, and find a median value for the sample of detected bursts of about
7x10^51 erg, with a 90% confidence range of 1.1x10^50-3.3x10^53 erg. Both the
median and 90% confidence range are somewhat larger than the results of
multi-wavelength, multi-epoch afterglow modeling (including large beaming
corrections), and the distribution of beaming-corrected gamma-ray energies.
This is due to bursts in our sample with only a single-frequency observation
for which we can only determine an upper bound on the peak of the synchrotron
spectrum. This limitation leads to a wider range of allowed energies than for
bursts with a well-measured spectral peak. Our study indicates that
single-epoch centimeter-band observations covering the spectral peak on a
timescale of ~1 yr can provide a robust estimate of the total kinetic energy
distribution with a small investment of telescope time. The substantial
increase in bandwidth of the EVLA (up to 8 GHz simultaneously with full
coverage at 1-40 GHz) will provide the opportunity to estimate the kinetic
energy distribution of GRBs with only a few hours of data per burst.Comment: Submitted to ApJ; 11 pages, 5 figures, 2 table
Gamma-Ray Burst Afterglows as Analogues of High Frequency-Peaked BL Lac Objects
The spectral properties from radio to optical bands are compared between the
18 optically bright Gamma-ray burst afterglows and well established
power-spectrum sequence in Blazars. The comparison shows that the afterglows
are well agreement with the well known Blazar sequence (i.e., the - correlation, where
is the broad-band spectral slope from radio to optical
bands). The afterglows are, however, clustered at the low luminosity end of the
sequence, which is typically occupied by high frequency-peaked BL Lac objects.
The correlation suggests that Gamma-ray burst afterglows share the similar
emission process with high frequency-peaked BL Lac objects. We further identify
a deviation at a significance level larger than 2 from the sequence for
three typical optically "dark" bursts. The deviation favors a heavy extinction
in optical bands for the "dark" bursts. The extinction is estimated to be
larger than 0.5-0.6 magnitude from the - sequence.Comment: 13 pages, 2 figures and 2 tables, to be published in ApJ
Curvature Effects in Gamma Ray Burst Colliding Shells
An elementary kinematic model for emission produced by relativistic spherical
colliding shells is studied. The case of a uniform blast-wave shell with jet
opening angle is considered, where is the
Lorentz factor of the emitting shell. The shell, with comoving width , is assumed to be illuminated for a comoving time
and to radiate a broken power-law spectrum peaking at comoving
photon energy \e_{pk,0}^{\prime}. Synthetic GRB pulses are calculated, and
the relation between energy flux and internal comoving energy density is
quantified. Curvature effects dictate that the measured flux at the
measured peak photon energy \e_{pk} is proportional to \e^3_{pk} in the
declining phase of a GRB pulse. Possible reasons for discrepancy with
observations are discussed, including adiabatic and radiative cooling processes
that extend the decay timescale, a nonuniform jet, or the formation of pulses
by external shock processes. A prediction of a correlation between prompt
emission properties and times of the optical afterglow beaming breaks is made
for a cooling model, which can be tested with Swift.Comment: 13 pages, 5 figures, added back-of-envelope estimate of curvature
relation, minor corrections, ApJ, in press, v. 614, 10 Oct 200
Variability of sub-mJy radio sources
We present 1.4 GHz VLA observations of the variability of radio sources in
the Lockman Hole region at the level of > 100 uJy on timescales of 17 months
and 19 days. These data indicate that the areal density of highly variable
sources at this level is < 0.005 arcmin^{-2}. We set an upper limit of 2% to
the fraction of 50 to 100uJy sources that are highly variable (> 50%). These
results imply a lower limit to the beaming angle for GRBs of 1deg, and give a
lower limit of 200 arcmin^2 to the area that can be safely searched for GRB
radio afterglows before confusion might become an issue.Comment: aastex 2 postscript figures. to appear in the Astrophysical Journa
High Resolution Observations of GRB 030329
The nearby (z=0.1685) gamma-ray burst of 29 March 2003 has presented us with a unique opportunity to study an event with unprecedented physical resolution. This burst reached flux density levels at centimeter wavelengths more than 50 times brighter than any previously studied event. Here we present the results of VLBI observations that have resolved the radio afterglow, and constrain its proper motion in the sky to <0.3 mas. The size of the afterglow is measured to be ~0.08 mas 24 days after the burst, consistent with expectations of the standard fireball model. In observations taken 51 days after the burst we detect an additional compact, "jet", component at a distance from the main component of 0.28 ± 0.05 mas. The presence of this jet component is not consistent with the standard model
A Possible Explanation of the Radio Afterglow of GRB980519: The Dense Medium Effect
GRB{980519} is characterized by its rapidly declining optical and X-ray
afterglows. Explanations of this behavior include models invoking a dense
medium environment which makes the shock wave evolve quickly into the
sub-relativistic phase, a jet-like outflow, and a wind-shaped circumburst
medium environment. Recently, Frail {et al}. (1999a) found that the latter two
cases are consistent with the radio afterglow of this burst. Here, by
considering the trans-relativistic shock hydrodynamics, we show that the dense
medium model can also account for the radio light curve quite well. The
potential virtue of the dense medium model for GRB{980519} is that it implies a
smaller angular size of the afterglow, which is essential for interpreting the
strong modulation of the radio light curve. Optical extinction due to the dense
medium is not important if the prompt optical-UV flash accompanying the
-ray emission can destroy dust by sublimation out to an appreciable
distance. Comparisons with some other radio afterglows are also discussed.Comment: 5 pages, 1 figure, a few minor changes made and references up dated,
MNRAS, in pres
SN1998bw: The Case for a Relativistic Shock
SN1998bw shot to fame by claims of association with GRB980425. Independent of
its presumed association with a GRB, this SN is unusual in its radio
properties. A simple interpretation of the unusually bright radio emission
leads us to the conclusion that there are two shocks in this SN: a slow moving
shock containing most of the ejecta and a relativistic shock (Gamma=2) which is
responsible for the radio emission. This is the first evidence for the
existence of relativistic shocks in supernovae. It is quite plausible that this
shock may produce high energy emission (at early times and by inverse Compton
scattering). As with other supernovae, we expect radio emission at much later
times powered primarily by the slow moving ejecta. This expectation has
motivated us to continue monitoring this unusual SN.Comment: A&A (in press), Rome GRB Symposium, Nov. 199
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