8,149 research outputs found
Gamma-Ray Burst Jet Profiles And Their Signatures
HETE-II and BeppoSAX have produced a sample of GRBs and XRFs with known
redshifts and . This sample provides four important empirical
constraints on the nature of the source jets: Log is approximately
uniformly distributed over several orders of magnitude; the inferred prompt
energy Log is narrowly distributed; the Amati relation holds
between and ; and the Ghirlanda relation holds between
and .
We explore the implications of these constraints for GRB jet structure during
the prompt emission phase. We infer the underlying angular profiles from the
first two of the above constraints assuming all jets have the same profile and
total energy, and show that such ``universal jet'' models cannot satisfy both
constraints.
We introduce a general and efficient method for calculating relativistic
emission distributions and distributions from jets with arbitrary
(smooth) angular jet profiles. We also exhibit explicit analytical formulas for
emission from top-hat jets (which are not smooth). We use these methods to
exhibit and as a function of viewing angle, for several
interesting families of GRB jet profiles. We use the same methods to calculate
expected frequency distributions of and for the same
families of models.
We then proceed to explore the behavior of universal jet models under a range
of profile shapes and parameters, to map the extent to which these models can
conform to the above four empirical constraints.Comment: 71 page, 33 figures. Submitted to Ap
Jet Models of X-Ray Flashes
One third of all HETE-2--localized bursts are X-Ray Flashes (XRFs), a class
of events first identified by Heise in which the fluence in the 2-30 keV energy
band exceeds that in the 30-400 keV energy band. We summarize recent HETE-2 and
other results on the properties of XRFs. These results show that the properties
of XRFs, X-ray-rich gamma-ray bursts (GRBs), and GRBs form a continuum, and
thus provide evidence that all three kinds of bursts are closely related
phenomena. As the most extreme burst population, XRFs provide severe
constraints on burst models and unique insights into the structure of GRB jets,
the GRB rate, and the nature of Type Ib/Ic supernovae. We briefly mention a
number of the physical models that have been proposed to explain XRFs. We then
consider two fundamentally different classes of phenomenological jet models:
universal jet models, in which it is posited that all GRBs jets are identical
and that differences in the observed properties of the bursts are due entirely
to differences in the viewing angle; and variable-opening angle jet models, in
which it is posited that GRB jets have a distribution of jet opening angles and
that differences in the observed properties of the bursts are due to
differences in the emissivity and spectra of jets having different opening
angles. We consider three shapes for the emissivity as a function of the
viewing angle theta_v from the axis of the jet: power-law, top hat (or
uniform), and Gaussian (or Fisher). We then discuss the effect of relativistic
beaming on each of these models. We show that observations can distinguish
between these various models.Comment: 8 pages, 7 figures, 1 table. Invited review talk at the 4th Workshop
Gamma-Ray Bursts in the Afterglow Era, Rome,18-22 October 2004. Editors: L.
Piro, L. Amati, S. Covino, and B. Gendre. Il Nuovo Cimento, in pres
Likelihood Analysis of GRB Evolution with Redshift
We present a likelihood approach to modeling multi-dimensional GRB
Epeak--fluence--redshift data that naturally incorporates instrument detection
thresholds. The treatment of instrument thresholds is essential for analyzing
evidence for GRB evolution. The method described here compares the data to a
uniform jet model, in which the jet parameters are allowed to vary with
redshift. Data from different experiments may be modeled jointly. In addition,
BATSE data (for which no redshift information is available) may be incorporated
by ascribing to each event a likelihood derived from the full model by
integrating the probability density over the unknown redshift. The loss of
redshift information is mitigated by the large number of available bursts. We
discuss the implementation of the method, and validation of it using simulated
data.Comment: 4 pages, 1 figure. Poster presented at the 4th Workshop Gamma-Ray
Bursts in the Afterglow Era, Rome,18-22 October 2004. Editors: L. Piro, L.
Amati, S. Covino, and B. Gendre. Il Nuovo Cimento, in pres
Three New Long Period X-ray Pulsars Discovered in the Small Magellanic Cloud
The Small Magellanic Cloud is increasingly an invaluable laboratory for
studying accreting and isolated X-ray pulsars. We add to the class of compact
SMC objects by reporting the discovery of three new long period X-ray pulsars
detected with the {\it Chandra X-ray Observatory}. The pulsars, with periods of
152, 304 and 565 seconds, all show hard X-ray spectra over the range from 0.6 -
7.5 keV. The source positions of the three pulsars are consistent with known
H-alpha emission sources, indicating they are likely to be Be type X-ray binary
star systems.Comment: Accepted for publication in the Astrophysical Journa
Gamma-Ray Bursts are Produced Predominately in the Early Universe
It is known that some observed gamma-ray bursts (GRBs) are produced at
cosmological distances and that the GRB production rate may follow the star
formation rate. We model the BATSE-detected intensity distribution of long GRBs
in order to determine their space density distribution and opening angle
distribution. Our main results are: the lower and upper distance limits to the
GRB production are z 0.24 and >10, respectively; the GRB opening angle follows
an exponential distribution and the mean opening angle is about 0.03 radians;
and the peak luminosity appears to be a better standard candle than the total
energy of a GRB.Comment: 12 pages, 2 figur
Gamma-Ray Bursts as a Probe of the Very High Redshift Universe
We show that, if many GRBs are indeed produced by the collapse of massive
stars, GRBs and their afterglows provide a powerful probe of the very high
redshift (z > 5) universe.Comment: To appear in Proc. of the 5th Huntsville Gamma-Ray Burst Symposium, 5
pages, LaTe
Evidence From HETE-2 For GRB Evolution With Redshift
After taking into account threshold effects, we find that the
isotropic-equivalent energies E_iso and luminosities L_iso of gamma-ray bursts
(GRBs) are correlated with redshift at the 5% and 0.9% signficance levels,
respectively. Our results are based on 10 BeppoSAX GRBs and 11 HETE-2 GRBs with
known redshifts. Our results suggest that the isotropic-equivalent energies and
luminosities of GRBs increase with redshift. They strengthen earlier clues to
this effect from analyses of the BATSE catalog of GRBs, using the variability
of burst time histories as an estimator of burst luminosities (and therefore
redshifts), and from an analysis of BeppoSAX bursts only. If the
isotropic-equivalent energies and luminosities of GRBs really do increase with
redshift, it suggests that GRB jets at high redshifts may be narrower and thus
the cores of GRB progenitor stars at high redshifts may be rotating more
rapidly. It also suggests that GRBs at very high redshifts may be more luminous
-- and therefore easier to detect -- than has been thought, which would make
GRBs a more powerful probe of cosmology and the early universe than has been
thought.Comment: 5 pages, 3 figures, to appear in proc. 2003 GRB Conference, Santa Fe,
N
Discovery of Pulsed X-ray Emission from the SMC Transient RX J0117.6-7330
We report on the detection of pulsed, broad-band, X-ray emission from the
transient source RX J0117.6-7330. The pulse period of 22 seconds is detected by
the ROSAT/PSPC instrument in a 1992 Sep 30 - Oct 2 observation and by the
CGRO/BATSE instrument during the same epoch. Hard X-ray pulsations are
detectable by BATSE for approximately 100 days surrounding the ROSAT
observation (1992 Aug 28 - Dec 8). The total directly measured X-ray luminosity
during the ROSAT observation is 1.0E38 (d/60 kpc)^2 ergs s-1. The pulse
frequency increases rapidly during the outburst, with a peak spin-up rate of
1.2E-10 Hz s-1 and a total frequency change 1.8%. The pulsed percentage is
11.3% from 0.1-2.5 keV, increasing to at least 78% in the 20-70 keV band. These
results establish RX J0117.6-7330 as a transient Be binary system.Comment: 17 pages, Latex, aasms, accepted for publication in ApJ Letter
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