426 research outputs found
Variability in GRBs - A Clue
We show that external shocks cannot produce a variable GRB, unless they are
produced by an extremely narrow jets (angular opening of < ~10^{-4}) or if only
a small fraction of the shell emits the radiation and the process is very
inefficient. Internal shocks can produce the observed complex temporal
structure provided that the source itself is variable. In this case, the
observed temporal structure reflects the activity of the ``inner engine'' that
drives the bursts. This sets direct constraints on it.Comment: 15 page latex file with 5 PS figure. Complete uuencoded compressed PS
file is available at ftp://shemesh.fiz.huji.ac.il or at
http://shemesh.fiz.huji.ac.il/papers/SaP_aclue.u
Statistical Analysis of Spectral Line Candidates in Gamma-Ray Burst GRB870303
The Ginga data for the gamma-ray burst GRB870303 exhibit low-energy dips in
two temporally distinct spectra, denoted S1 and S2. S1, spanning 4 s, exhibits
a single line candidate at ~ 20 keV, while S2, spanning 9 s, exhibits
apparently harmonically spaced line candidates at ~ 20 and 40 keV. We evaluate
the statistical evidence for these lines, using phenomenological continuum and
line models which in their details are independent of the distance scale to
gamma-ray bursts. We employ the methodologies based on both frequentist and
Bayesian statistical inference that we develop in Freeman et al. (1999b). These
methodologies utilize the information present in the data to select the
simplest model that adequately describes the data from among a wide range of
continuum and continuum-plus-line(s) models. This ensures that the chosen model
does not include free parameters that the data deem unnecessary and that would
act to reduce the frequentist significance and Bayesian odds of the
continuum-plus-line(s) model. We calculate the significance of the
continuum-plus-line(s) models using the Chi-Square Maximum Likelihood Ratio
test. We describe a parametrization of the exponentiated Gaussian absorption
line shape that makes the probability surface in parameter space
better-behaved, allowing us to estimate analytically the Bayesian odds. The
significance of the continuum-plus-line models requested by the S1 and S2 data
are 3.6 x 10^-5 and 1.7 x 10^-4 respectively, with the odds favoring them being
114:1 and 7:1. We also apply our methodology to the combined (S1+S2) data. The
significance of the continuum-plus-lines model requested by the combined data
is 4.2 x 10^-8, with the odds favoring it being 40,300:1.Comment: LaTeX2e (aastex.cls included); 41 pages text, 10 figures (on 11
pages); accepted by ApJ (to be published 1 Nov 1999, v. 525
The hidden X-ray breaks in afterglow light curves
Gamma-Ray Burst (GRB) afterglow observations in the Swift era have a
perceived lack of achromatic jet breaks compared to the BeppoSAX, or pre-Swift
era. Specifically, relatively few breaks, consistent with jet breaks, are
observed in the X-ray light curves of these bursts. If these breaks are truly
missing, it has serious consequences for the interpretation of GRB jet
collimation and energy requirements, and the use of GRBs as standard candles.
Here we address the issue of X-ray breaks which are possibly 'hidden' and
hence the light curves are misinterpreted as being single power-laws. We show
how a number of precedents, including GRB 990510 & GRB 060206, exist for such
hidden breaks and how, even with the well sampled light curves of the Swift
era, these breaks may be left misidentified. We do so by synthesising X-ray
light curves and finding general trends via Monte Carlo analysis. Furthermore,
in light of these simulations, we discuss how to best identify achromatic
breaks in afterglow light curves via multi-wavelength analysis.Comment: 4 pages, contributed talk, submitted to the proceedings of Gamma Ray
Bursts 2007, Santa Fe, New Mexico, November 5-9 200
SCUBA Observations of the Host Galaxies of Gamma-Ray Bursts
In recent years, a population of galaxies with huge infrared luminosities and dust masses has been discovered in the submillimetre. Observations suggest that the AGN contribution to the luminosities of these submillimetre-selected galaxies is low; instead their luminosities are thought to be mainly due to strong episodes of star formation following merger events. Our current understanding of GRBs as the endpoints in the life of massive stars suggest that they will be located in such galaxies.We have observed a sample of well-located GRB host galaxies in the submillimetre. Comparing the results with the general submillimetre-selected galaxy population, we find that at low fluxes (S850 †4 mJy), the two agree well. However, there is a lack of bright GRB hosts in the submillimetre. This finding is reinforced when the results of other groups are included. Possible explanations are discussed. These results help us assess the roles of both GRB host galaxies and submillimetre-selected galaxies in the evolution of the Universe
The Halo Beaming Model for Gamma-Ray Bursts
We consider a model for gamma-ray bursts (GRBs) from high-velocity neutron
stars in the galactic halo. In this model, bursters are born in the galactic
disk with large recoil velocities V_r, and GRBs are beamed to within emission
cones of half-angle \phi centered on V_r. We describe scenarios for
magnetically -channeled GRBs that have such beaming characteristics. We then
make detailed comparisons of this halo beaming model (HBM) to BATSE and PVO
data for GRB intensity & angular position distributions. Acceptable fits to
observations of over 1000 bursts are obtained for \phi = 15 - 30 degrees and
for a BATSE sampling depth ~ 180 kpc. Present data favor a truly isotropic
(cosmological) model over the HBM, but not by a statistically compelling
margin. Bursters born in nearby external galaxies, such as M31, are almost
entirely undetectable in the HBM because of misdirected beaming. We analyze
several refinements of the basic HBM: gamma-ray intensities that vary with
angle from the beam axis; non-standard-candle GRB luminosity functions; and
models including a subset of bursters that do not escape from the galaxy. We
also discuss the energy budgets for the bursters, the origins of their recoils,
and the physics of burst beaming and alignment. One possible physical model is
based on the magnetar model of soft gamma repeaters (SGRs). Empirical bounds on
the rate of formation and peculiar velocities of SGRs imply that there exist ~
10^4 to ~ 10^7 aged SGRs in the galactic halo within a distance of 100 kpc. The
HBM gives an acceptable fit to observations only if it satisfies certain
conditions (e.g. \phi ~ 20 deg) which are possible, but for which there exist
no clear & compelling theoretical justifications. The cosmological burster
hypothesis is more generic and thus more attractive in this sense. (Abbreviated
Abstract).Comment: ApJ accepted, 9 figures, AASTE
Early multi-wavelength emission from Gamma-ray Bursts: from Gamma-ray to X-ray
The study of the early high-energy emission from both long and short
Gamma-ray bursts has been revolutionized by the Swift mission. The rapid
response of Swift shows that the non-thermal X-ray emission transitions
smoothly from the prompt phase into a decaying phase whatever the details of
the light curve. The decay is often categorized by a steep-to-shallow
transition suggesting that the prompt emission and the afterglow are two
distinct emission components. In those GRBs with an initially steeply-decaying
X-ray light curve we are probably seeing off-axis emission due to termination
of intense central engine activity. This phase is usually followed, within the
first hour, by a shallow decay, giving the appearance of a late emission hump.
The late emission hump can last for up to a day, and hence, although faint, is
energetically very significant. The energy emitted during the late emission
hump is very likely due to the forward shock being constantly refreshed by
either late central engine activity or less relativistic material emitted
during the prompt phase. In other GRBs the early X-ray emission decays
gradually following the prompt emission with no evidence for early temporal
breaks, and in these bursts the emission may be dominated by classical
afterglow emission from the external shock as the relativistic jet is slowed by
interaction with the surrounding circum-burst medium. At least half of the GRBs
observed by Swift also show erratic X-ray flaring behaviour, usually within the
first few hours. The properties of the X-ray flares suggest that they are due
to central engine activity. Overall, the observed wide variety of early
high-energy phenomena pose a major challenge to GRB models.Comment: Accepted for publication in the New Journal of Physics focus issue on
Gamma Ray Burst
Energy input and response from prompt and early optical afterglow emission in gamma-ray bursts
The taxonomy of optical emission detected during the critical first few
minutes after the onset of a gamma-ray burst (GRB) defines two broad classes:
prompt optical emission correlated with prompt gamma-ray emission, and early
optical afterglow emission uncorrelated with the gamma-ray emission. The
standard theoretical interpretation attributes prompt emission to internal
shocks in the ultra-relativistic outflow generated by the internal engine;
early afterglow emission is attributed to shocks generated by interaction with
the surrounding medium. Here we report on observations of a bright GRB that,
for the first time, clearly show the temporal relationship and relative
strength of the two optical components. The observations indicate that early
afterglow emission can be understood as reverberation of the energy input
measured by prompt emission. Measurements of the early afterglow reverberations
therefore probe the structure of the environment around the burst, whereas the
subsequent response to late-time impulsive energy releases reveals how earlier
flaring episodes have altered the jet and environment parameters. Many GRBs are
generated by the death of massive stars that were born and died before the
Universe was ten per cent of its current age, so GRB afterglow reverberations
provide clues about the environments around some of the first stars.Comment: 13 pages, 4 figures, 1 table. Note: This paper has been accepted for
publication in Nature, but is embargoed for discussion in the popular press
until formal publication in Natur
Conformational Dependence of a Protein Kinase Phosphate Transfer Reaction
Atomic motions and energetics for a phosphate transfer reaction catalyzed by
the cAMP-dependent protein kinase (PKA) are calculated by plane-wave density
functional theory, starting from structures of proteins crystallized in both
the reactant conformation (RC) and the transition-state conformation (TC). In
the TC, we calculate that the reactants and products are nearly isoenergetic
with a 0.2 eV barrier; while phosphate transfer is unfavorable by over 1.2 eV
in the RC, with an even higher barrier. With the protein in the TC, the motions
involved in reaction are small, with only P and the catalytic proton
moving more than 0.5 \AA. Examination of the structures reveals that in the RC
the active site cleft is not completely closed and there is insufficient space
for the phosphorylated serine residue in the product state. Together, these
observations imply that the phosphate transfer reaction occurs rapidly and
reversibly in a particular conformation of the protein, and that the reaction
can be gated by changes of a few tenths of an \AA in the catalytic site.Comment: revtex4, 7 pages, 4 figures, to be submitted to Scienc
BAT X-ray Survey - III: X-ray Spectra and Statistical Properties
In this concluding part of the series of three papers dedicated to the
Swift/BAT hard X-ray survey (BXS), we focus on the X-ray spectral analysis and
statistical properties of the source sample. Using a dedicated method to
extract time-averaged spectra of BAT sources we show that Galactic sources
have, generally, softer spectra than extragalactic objects and that Seyfert 2
galaxies are harder than Seyfert 1s. The averaged spectrum of all Seyfert
galaxies is consistent with a power-law with photon index of 2.00 (+/-0.07).
The cumulative flux-number relation for the extragalactic sources in the 14-170
keV band is best described by a power-law with a slope alpha=1.55 (+/-0.20) and
a normalization of 9.6 AGN deg (or 396(+/-80) AGN
all-sky) above a flux level of 2erg cm s (~0.85
mCrab). The integration of the cumulative flux per unit area indicates that BAT
resolves 1-2% of the X-ray background emission in the 14-170 keV band. A
sub-sample of 24 extragalactic sources above the 4.5 sigma detection limit is
used to study the statistical properties of AGN. This sample comprises local
Seyfert galaxies (z=0.026, median value) and ~10% blazars. We find that 55% of
the Seyfert galaxies are absorbed by column densities of Log(N_H)>22, but that
none is a bona fide Compton-thick. This study shows the capabilities of BAT to
probe the hard X-ray sky to the mCrab level.Comment: Accepted for publication in The Astrophysical Journal; 42 pages, 4
tables, 51 figure
The Gamma Ray Burst section of the White Paper on the Status and Future of Very High Energy Gamma Ray Astronomy: A Brief Preliminary Report
Original paper can be found at: http://proceedings.aip.org/proceedings/ Copyright American Institute of Physics DOI: 10.1063/1.2943545otherPeer reviewe
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