594 research outputs found
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
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
A multi-data source surveillance system to detect a bioterrorism attack during the G8 summit in Scotland
In 18 weeks, Health Protection Scotland (HPS) deployed a syndromic surveillance system to early-detect natural or intentional disease outbreaks during the G8 Summit 2005 at Gleneagles, Scotland. The system integrated clinical and non-clinical datasets. Clinical datasets included Accident and Emergency (A and E) syndromes, and General Practice (GPs) codes grouped into syndromes. Non-clinical data included telephone calls to a nurse helpline, laboratory test orders, and hotel staff absenteeism. A cumulative sum-based detection algorithm and a log-linear regression model identified signals in the data. The system had a fax-based track for real-time identification of unusual presentations. Ninety-five signals were triggered by the detection algorithms and four forms were faxed to HPS. Thirteen signals were investigated. The system successfully complemented a traditional surveillance system in identifying a small cluster of gastroenteritis among the police force and triggered interventions to prevent further cases
GRB Redshift Evolution Within the Unified Jet Model
HETE-2 has provided new evidence that gamma-ray bursts may evolve with
redshift. We investigate the consequences of this possibility for the unified
jet model of XRFs and GRBs. We find that burst evolution with redshift can be
naturally explained within the unified jet model, and the resulting model
provides excellent agreement with existing HETE-2 and BeppoSAX data sets. In
addition, this evolution model produces reasonable fits to the BATSE peak
photon number flux distribution -- something that cannot be easily done without
redshift evolution.Comment: 5 pages, 4 figures, to appear in proc. 2003 GRB Conference, Santa Fe,
N
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
Unifying XRFs and GRBs with a Fisher-Shaped Universal Jet Model
We show analytically that GRB jets with an emissivity profile given by the
Fisher distribution, epsilon(theta) = A * e^(B cos(theta)), have the unique
property of producing equal numbers of bursts per logarithmic interval in Eiso,
and therefore in most burst properties. Since this broad distribution of burst
properties is a key feature found by HETE-2, a Fisher-shaped universal jet
model can explain many of the observed properties of XRFs, X-ray-rich GRBs, and
GRBs reasonably well, in contrast to a power-law universal model. For small
viewing angles, the Fisher distribution can be approximated by a Gaussian,
whose properties have been explored by Zhang et al. (2004). We also show that
the Fisher universal jet model produces a broad distribution in the inferred
radiated energy Egamma_inf, in contrast to the narrow distribution predicted by
the uniform variable opening-angle jet model Lamb, Donaghy & Graziani (2005).
Here we present Monte Carlo simulations of both a Fisher-shaped universal jet
model and a Fisher-shaped variable opening-angle jet model.Comment: 4 pages, 2 figures. 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
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