123 research outputs found
A Model for Short Gamma-Ray Bursts: Heated Neutron Stars in Close Binary Systems
In this paper we present a model for the short (< second) population of
gamma-ray bursts (GRBs). In this model heated neutron stars in a close binary
system near their last stable orbit emit neutrinos at large luminosities (~
10^53 ergs/sec). A fraction of these neutrinos will annihilate to form an
electron-positron pair plasma wind which will, in turn, expand and recombine to
photons which make the gamma-ray burst. We study neutrino annihilation and show
that a substantial fraction (~ 50%) of energy deposited comes from inter-star
neutrinos, where each member of the neutrino pair originates from each neutron
star. Thus, in addition to the annihilation of neutrinos blowing off of a
single star, we have a new source of baryon free energy that is deposited
between the stars. To model the pair plasma wind between stars, we do
three-dimensional relativistic numerical hydrodynamic calculations.
Preliminary results are also presented of new, fully general relativistic
calculations of gravitationally attracting stars falling from infinity with no
angular momentum. These simulations exhibit a compression effect.Comment: 3 pages, 3 postscript figs (2 color), to appear in "Gamma-Ray Burst
and Afterglow Astronomy 2001", Woods Hole; 5-9 Nov, 200
Heterogeneity in Short Gamma-ray Bursts
We analyze the Swift/BAT sample of short gamma-ray bursts, using an objective
Bayesian Block procedure to extract temporal descriptors of the bursts' initial
pulse complexes (IPCs). The sample comprises 12 and 41 bursts with and without
extended emission (EE) components, respectively. IPCs of non-EE bursts are
dominated by single pulse structures, while EE bursts tend to have two or more
pulse structures. The medians of characteristic timescales - durations, pulse
structure widths, and peak intervals - for EE bursts are factors of ~ 2-3
longer than for non-EE bursts. A trend previously reported by Hakkila and
colleagues unifying long and short bursts - the anti-correlation of pulse
intensity and width - continues in the two short burst groups, with non-EE
bursts extending to more intense, narrower pulses. In addition we find that
preceding and succeeding pulse intensities are anti-correlated with pulse
interval. We also examine the short burst X-ray afterglows as observed by the
Swift/XRT. The median flux of the initial XRT detections for EE bursts (~ 6 x
10^-10 erg cm^-2 s^-1) is ~> 20 x brighter than for non-EE bursts, and the
median X-ray afterglow duration for EE bursts (~ 60,000 s) is ~ 30 x longer
than for non-EE bursts.
The tendency for EE bursts toward longer prompt-emission timescales and
higher initial X-ray afterglow fluxes implies larger energy injections powering
the afterglows. The longer-lasting X-ray afterglows of EE bursts may suggest
that a significant fraction explode into more dense environments than non-EE
bursts, or that the sometimes-dominant EE component efficiently powers the
afterglow. Combined, these results favor different progenitors for EE and
non-EE short bursts.Comment: 30 pages, 11 figures, 3 tables; accepted to The Astrophysical Journa
Gamma-Ray Bursts in the Swift Era
GRB research has undergone a revolution in the last two years. The launch of Swift, with its rapid slewing capability, has greatly increased the number and quality of GRB localizations and X-ray and optical afterglow lightcurves. Over 160 GRBs have been detected, and nearly all that have been followed up with the on-board narrow field telescopes. Advances in our understanding of short GRBs have been spectacular. The detection of X-ray afterglows has led to accurate localizations from ground based observatories, which have given host identifications and redshifts. Theoretical models for short GRB progenitors have, for the first time, been placed on a sound foundation. The hosts for the short GRBs differ in a fundamental way from the long GRB hosts: short GRBs tend to occur in non-star forming galaxies or regions, whereas long GRBs are strongly concentrated within star forming regions. Observations are consistent with a binary neutron star merger model, but other models involving old stellar populations are also viable. Swift has greatly increased the redshift range of GRB detection. The highest redshift GRBs, at zeta approx. 5-6, are approaching the era of reionization. Ground-based deep optical spectroscopy of high redshift bursts is giving metallicity measurements and other information on the source environment to much greater distance than other techniques. The localization of GRB 060218 to a nearby galaxy, and association with SN 2006aj, added a valuable member to the class of GRBs with detected supernova. The prospects for future progress are excellent given the >10 year orbital lifetime of the Swift satellite
GRB Spikes Could Resolve Stars
GRBs vary more rapidly than any other known cosmological phenomena. The lower
limits of this variability have not yet been explored. Improvements in
detectors would reveal or limit the actual rate of short GRBs. Were microsecond
"spike" GRBs to exist and be detectable, they would time-resolve stellar mass
objects throughout the universe by their gravitational microlensing effect.
Analyzing the time structure of sufficient numbers of GRB spikes would reveal
or limit , , and/or .Comment: 18 pages, 2 figures, in press: ApJ (Letters
Studies in Astronomical Time Series Analysis. VI. Bayesian Block Representations
This paper addresses the problem of detecting and characterizing local
variability in time series and other forms of sequential data. The goal is to
identify and characterize statistically significant variations, at the same
time suppressing the inevitable corrupting observational errors. We present a
simple nonparametric modeling technique and an algorithm implementing it - an
improved and generalized version of Bayesian Blocks (Scargle 1998) - that finds
the optimal segmentation of the data in the observation interval. The structure
of the algorithm allows it to be used in either a real-time trigger mode, or a
retrospective mode. Maximum likelihood or marginal posterior functions to
measure model fitness are presented for events, binned counts, and measurements
at arbitrary times with known error distributions. Problems addressed include
those connected with data gaps, variable exposure, extension to piecewise
linear and piecewise exponential representations, multi-variate time series
data, analysis of variance, data on the circle, other data modes, and dispersed
data. Simulations provide evidence that the detection efficiency for weak
signals is close to a theoretical asymptotic limit derived by (Arias-Castro,
Donoho and Huo 2003). In the spirit of Reproducible Research (Donoho et al.
2008) all of the code and data necessary to reproduce all of the figures in
this paper are included as auxiliary material.Comment: Added some missing script files and updated other ancillary data
(code and data files). To be submitted to the Astophysical Journa
Correlations Between Lag, Luminosity, and Duration in Gamma-ray Burst Pulses
We derive a new peak lag vs. peak luminosity relation in gamma-ray burst
(GRB) pulses. We demonstrate conclusively that GRB spectral lags are pulse
rather than burst properties and show how the lag vs. luminosity relation
determined from CCF measurements of burst properties is essentially just a
rough measure of this newly derived relation for individual pulses. We further
show that most GRB pulses have correlated properties: short-lag pulses have
shorter durations, are more luminous, and are harder within a burst than
long-lag pulses. We also uncover a new pulse duration vs. pulse peak luminosity
relation, and indicate that long-lag pulses often precede short-lag pulses.
Although most pulse behaviors are supportive of internal shocks (including
long-lag pulses), we identify some pulse shapes that could result from external
shocks.Comment: 14 pages, 4 figures, 1 table; accepted for publication in
Astrophysical Journal Letter
The Correlation of Spectral Lag Evolution with Prompt Optical Emission in GRB 080319B
We report on observations of correlated behavior between the prompt gamma-ray
and optical emission from GRB 080319B, which confirm that (i) they occurred
within the same astrophysical source region and (ii) their respective radiation
mechanisms were dynamically coupled. Our results, based upon a new CCF
methodology for determining the time-resolved spectral lag, are summarized as
follows. First, the evolution in the arrival offset of prompt gamma-ray photon
counts between Swift-BAT 15-25 keV and 50-100 keV energy bands (intrinsic
gamma-ray spectral lag) appears to be anti-correlated with the arrival offset
between prompt 15-350 keV gamma-rays and the optical emission observed by
TORTORA (extrinsic optical/gamma-ray spectral lag), thus effectively
partitioning the burst into two main episodes at ~T+28+/-2 sec. Second, the
rise and decline of prompt optical emission at ~T+10+/-1 sec and ~T+50+/-1 sec,
respectively, both coincide with discontinuities in the hard to soft evolution
of the photon index for a power law fit to 15-150 keV Swift-BAT data at
~T+8+/-2 sec and ~T+48+/-1 sec. These spectral energy changes also coincide
with intervals whose time-resolved spectral lag values are consistent with
zero, at ~T+12+/-2 sec and ~T+50+/-2 sec. These results, which are robust
across heuristic permutations of Swift-BAT energy channels and varying temporal
bin resolution, have also been corroborated via independent analysis of
Konus-Wind data. This potential discovery may provide the first observational
evidence for an implicit connection between spectral lags and GRB emission
mechanisms in the context of canonical fireball phenomenology. Future work
includes exploring a subset of bursts with prompt optical emission to probe the
unique or ubiquitous nature of this result.Comment: 6 pages, 3 figures. Contributed to the Proceedings of the Sixth
Huntsville GRB Symposium. Edited by C.A. Meegan, N. Gehrels, and C.
Kouvelioto
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