32 research outputs found
The Unique Signature of Shell Curvature in Gamma-Ray Bursts
As a result of spherical kinematics, temporal evolution of received gamma-ray
emission should demonstrate signatures of curvature from the emitting shell.
Specifically, the shape of the pulse decay must bear a strict dependence on the
degree of curvature of the gamma-ray emitting surface. We compare the spectral
evolution of the decay of individual GRB pulses to the evolution as expected
from curvature. In particular, we examine the relationship between photon flux
intensity (I) and the peak of the \nu F\nu distribution (E_{peak}) as predicted
by colliding shells. Kinematics necessitate that E_{peak} demonstrate a
power-law relationship with I described roughly as: I=E_{peak}^{(1-\zeta)}
where \zeta represents a weighted average of the low and high energy spectral
indices. Data analyses of 24 BATSE gamma-ray burst pulses provide evidence that
there exists a robust relationship between E_{peak} and I in the decay phase.
Simulation results, however, show that a sizable fraction of observed pulses
evolve faster than kinematics allow. Regardless of kinematic parameters, we
found that the existence of curvature demands that the I - E_{peak} function
decay be defined by \sim (1-\zeta). Efforts were employed to break this
curvature dependency within simulations through a number of scenarios such as
anisotropic emission (jets) with angular dependencies, thickness values for the
colliding shells, and various cooling mechanisms. Of these, the only method
successful in dominating curvature effects was a slow cooling model. As a
result, GRB models must confront the fact that observed pulses do not evolve in
the manner which curvature demands.Comment: 3 pages, To appear in Proc. from the 2nd Workshop on Gamma-Ray Bursts
in the Afterglow Er
Constraints on the Gamma-ray Burst Luminosity Function from PVO and BATSE
We examine the width of the gamma-ray burst luminosity function through the
distribution of GRB peak fluxes as detected by the Pioneer Venus Orbiter (PVO)
and the Burst and Transient Source Experiment (BATSE). The strength of the
analysis is greatly enhanced by using a merged catalog of peak fluxes from both
instruments with good cross-calibration of their sensitivities. The range of
peak fluxes is increased by approximately a factor of 20 relative to the BATSE
catalog. Thus, more sensitive investigations of the
distribution are possible. We place constraints on the width of the luminosity
function of gamma-ray bursts brighter than the BATSE completeness limit by
comparing the intensity distribution in the merged catalog with those produced
by a variety of spatial density and luminosity functions. For the models
examined, of the {\em detectable\/} bursts have peak luminosities within
a range of 10, indicating that the peak luminosities of gamma-ray bursts span a
markedly less wide range of values than many other of their measurable
properties. We also discuss for which slopes of a power-law luminosity function
the observed width is at the upper end of the constrained range. This is
important in determining the power-law slopes for which luminosity-duration
correlations could be important.Comment: 10 pages latex + 2 uuencoded figures; APJL accepte
The Corrected Log N-Log Fluence Distribution of Cosmological Gamma-Ray Bursts
Recent analysis of relativistically expanding shells of cosmological
gamma-ray bursts has shown that if the bursts are cosmological, then most
likely total energy (E_0) is standard and not peak luminosity (L_0). Assuming a
flat Friedmann cosmology (q_o = 1/2, Lambda = 0) and constant rate density
(rho_0) of bursting sources, we fit a standard candle energy to a uniformly
selected log N-log S in the BATSE 3B catalog correcting for fluence efficiency
and averaging over 48 observed spectral shapes. We find the data consistent
with E_0 = 7.3^{+0.7}_{-1.0} X 10^{51} ergs and discuss implications of this
energy for cosmological models of gamma-ray bursts.Comment: A five page LateX file that uses the Revtex conference proceedings
macro aipbook.sty, and includes three postscript figures using psfig. To Be
published in the Proceedings of the Third Hunstville Symposium on Gamma-Ray
Bursts, eds. C. Kouveliotou, M.S. Briggs and G.J. Fishman (New York:AIP).
Postscript version availible at http://nis-www.lanl.gov/~jsbloom/LOG_S.p
Cosmic Gamma-Ray Bursts as a Probe of Star Formation History
The cosmic gamma-ray burst (GRB) formation rate, as derived from the
variability-luminosity relation for long-duration GRBs, is compared with the
cosmic star formation rate. If GRBs are related to the collapse of massive
stars, one expects the GRB rate to be approximately proportional to the star
formation rate. We found that these two rates have similar slopes at low
redshift. This suggests that GRBs do indeed track the star formation rate of
the Universe, which in turn implies that the formation rate of massive stars
that produce GRBs is proportional to the total star formation rate. It also
implies that we might be able to use GRBs as a probe of the cosmic star
formation rate at high redshift. We find that the cosmic star formation rate
inferred from the variability-luminosity relation increases steeply with
redshift at z > 3. This is in apparent contrast to what is derived from
measurements of the cosmic star formation rate at high redshift from optical
observations of field galaxies, suggesting that much high-z star formation is
being missed in the optical surveys, even after corrections for dust extinction
have been made.Comment: 4 pages, 1 figure, talk given at the CAPP2000 Conference on Cosmology
and Particle Physics, Verbier, Switzerland, eds. J. Garcia-Bellido, R. Durrer
and M. Shaposhnikov, (AIP,2001
A Possible Cepheid-Like Luminosity Estimator for the Long Gamma-Ray Bursts
We present a possible Cepheid-like luminosity estimator for the long
gamma-ray bursts based on the variability of their light curves. To construct
the luminosity estimator, we use CGRO/BATSE data for 13 bursts, Wind/KONUS data
for 5 bursts, Ulysses/GRB data for 1 burst, and NEAR/XGRS data for 1 burst.
Spectroscopic redshifts, peak fluxes, and high resolution light curves are
available for 11 of these bursts; partial information is available for the
remaining 9 bursts. We find that the isotropic-equivalent luminosities L of
these bursts positively correlate with a rigorously-constructed measure V of
the variability of their light curves. We fit a model to these data that
accommodates both intrinsic scatter (statistical variance) and extrinsic
scatter (sample variance). If one excludes GRB 980425 from the fit on the
grounds that its association with SN 1998bw at a redshift of z = 0.0085 is not
secure, the luminosity estimator spans approx. 2.5 orders of magnitude in L,
and the slope of the correlation between L and V is positive with a probability
of 1 - 1.4 x 10^-4 (3.8 sigma). Although GRB 980425 is excluded from this fit,
its L and V values are consistent with the fitted model, which suggests that
GRB 980425 may well be associated with SN 1998bw, and that GRB 980425 and the
cosmological bursts may share a common physical origin. If one includes GRB
980425 in the fit, the luminosity estimator spans approx. 6.3 orders of
magnitude in L, and the slope of the correlation is positive with a probability
of 1 - 9.3 x 10^-7 (4.9 sigma). Independently of whether or not GRB 980425
should be included in the fit, its light curve is unique in that it is much
less variable than the other approx. 17 light curves in our sample for which
the signal-to-noise is reasonably good.Comment: Accepted to The Astrophysical Journal, 31 pages, 13 figures, LaTe
Expanding Relativistic Shells and Gamma-Ray Burst Temporal Structure
Many models of cosmological gamma-ray bursts involve the sudden release of
erg which produce shells which expand at relativistic speeds
(Lorentz factors of ). We investigate the kinematic limits
on the source size due to the observed time structure in three types of bursts:
short spikes, FREDs (Fast Rise, Exponentail decay), and long complex bursts.
The emitting shell keeps up with the photons it produces reducing apparent
durations by so that source sizes can be very large (c\Delta T\Delta T\Gamma^{-1}$, we show that the
curvature of the shell within that angle creates delays comparable to those
associated with the duration of the event. As a result, most bursts should be
like FREDs with sharp rises related to how long the shell emits and power law
decays related to how long the shell expanded before becoming gamma-ray active.
Few bursts have the long decay phases required for large shells resulting in
unacceptable high densities for ISM objects to cause the observed subpeaks. To
be consistent with the observations, perhaps very thick shells (which act as
parallel slabs) are required to avoid the effects of the curvature, or the
duration is dictated by a central engine.Comment: Tex file, 30 pages, 7 Postscript figures, in press ApJ, Vol 47
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Neutron sensors for locating sites of planetary water deposits
This is the final report of a six-month, Laboratory Directed Research and Development (LDRD) project at the Los Alamos National Laboratory (LANL). This project helped in exploration of the value and feasibility of use of collimated neutron detection methods for improving the sensitivity of neutron spectrometers specifically designed for deep-space missions to detect and identify both present-day deposits of near-surface water ice. The authors believed that this result helped enable a decision to include a Los Alamos-designed neutron sensor as a component of the NASA Mars Global Surveyor-01 Gamma-Ray/Neutron Spectrometer
An Optically Dark GRB Observed by HETE-2: GRB 051022
GRB 051022 was detected at 13:07:58 on 22 October 2005 by HETE-2. The
location of GRB 051022 was determined immediately by the flight localization
system. This burst contains multiple pulses and has a rather long duration of
about 190 seconds. The detections of candidate X-ray and radio afterglows were
reported, whereas no optical afterglow was found. The optical spectroscopic
observations of the host galaxy revealed the redshift z = 0.8. Using the data
derived by HETE-2 observation of the prompt emission, we found the absorption
N_H = 8.8 -2.9/+3.1 x 10^22 cm^-2 and the visual extinction A_V = 49 -16/+17
mag in the host galaxy. If this is the case, no detection of any optical
transient would be quite reasonable. The absorption derived by the Swift XRT
observations of the afterglow is fully consistent with those obtained from the
early HETE-2 observation of the prompt emission. Our analysis implies an
interpretation that the absorbing medium could be outside external shock at R ~
10^16 cm, which may be a dusty molecular cloud.Comment: 6 pages, 2 figures, accepted for publication in PASJ lette
The Lantern Vol. 5, No. 1, December 1936
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