7,344 research outputs found
Evidence for Two Distinct Morphological Classes of Gamma-Ray Bursts from their Short Timescale Variability
We have analyzed the 241 bursts for which peak counts \C exist in the
publicly available Burst and Transient Source Experiment (BATSE) catalog.
Introducing peak counts in 1024 ms as a measure of burst brightness \B and
the ratio of peak counts in 64 and 1024 ms as a measure of short timescale
variability \V, we find a statistically significant correlation between the
brightness and the short timescale variability of \g-ray bursts. The bursts
which are smoother on short timescales are both faint and bright, while the
bursts which are variable on short timescales are faint only, suggesting the
existence of two distinct morphological classes of bursts.Comment: 9 pages + 2 Postscript figures available upon request; LATEX v. 2.0
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
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
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
Evidence Against an Association Between Gamma-Ray Bursts and Type I Supernovae
We present a rigorous method, based on Bayesian inference, for calculating
the odds favoring the hypothesis that any particular class of astronomical
transients produce gamma-ray bursts over the hypothesis that they do not. We
then apply this method to a sample of 83 Type Ia supernovae and a sample of 20
Type Ib-Ic supernovae. We find overwhelming odds against the hypothesis that
all Type Ia supernovae produce gamma-ray bursts, whether at low redshift
() or high-redshift (), and very large odds ()
against the hypothesis that all Type Ib, Ib/c, and Ic supernovae produce
observable gamma-ray bursts. We find large odds () against the hypothesis
that a fraction of Type Ia supernovae produce observable gamma-ray bursts, and
moderate odds () against the hypothesis that a fraction of Type Ib-Ic
supernovae produce observable bursts. We have also re-analyzed both a corrected
version of the Wang & Wheeler sample of Type Ib-Ic SNe and our larger sample of
20 Type Ib-Ic SNe, using a generalization of their frequentist method. We find
no significant evidence in either case of a correlation between Type Ib-Ic SNe
and GRBs, consistent with the very strong evidence against such a correlation
that we find from our Bayesian analysis.Comment: 45 pages, 2 PostScript figures. Uses AASTEX macros. Submitted to The
Astrophysical Journa
JT90 thermal barrier coated vanes
The technology of plasma sprayed thermal barrier coatings applied to turbine vane platforms in modern high temperature commercial engines was advanced to the point of demonstrated feasibility for application to commercial aircraft engines. The three thermal barrier coatings refined under this program are zirconia stabilized with twenty-one percent magnesia (21% MSZ), six percent yttria (6% YSZ), and twenty percent yttria (20% YSZ). Improvement in thermal cyclic endurance by a factor of 40 times was demonstrated in rig tests. A cooling system evolved during the program which featured air impingement cooling for the vane platforms rather than film cooling. The impingement cooling system, in combination with the thermal barrier coatings, reduced platform cooling air requirements by 44% relative to the current film cooling system. Improved durability and reduced cooling air requirements were demonstrated in rig and engine endurance tests. Two engine tests were conducted, one of 1000 cycles and the other of 1500 cycles. All three coatings applied to vanes fabricated with the final cooling system configuration completed the final 1500 cycle engine endurance test. Results of this test clearly demonstrated the durability of the 6% YSZ coating which was in very good condition after the test. The 21% MSZ and 20% YSZ coatings had numerous occurrences of significant spalling in the test
Galaxy formation with radiative and chemical feedback
Here we introduce GAMESH, a novel pipeline which implements self-consistent
radiative and chemical feedback in a computational model of galaxy formation.
By combining the cosmological chemical-evolution model GAMETE with the
radiative transfer code CRASH, GAMESH can post process realistic outputs of a
N-body simulation describing the redshift evolution of the forming galaxy.
After introducing the GAMESH implementation and its features, we apply the code
to a low-resolution N-body simulation of the Milky Way formation and we
investigate the combined effects of self-consistent radiative and chemical
feedback. Many physical properties, which can be directly compared with
observations in the Galaxy and its surrounding satellites, are predicted by the
code along the merger-tree assembly. The resulting redshift evolution of the
Local Group star formation rates, reionisation and metal enrichment along with
the predicted Metallicity Distribution Function of halo stars are critically
compared with observations. We discuss the merits and limitations of the first
release of GAMESH, also opening new directions to a full implementation of
feedback processes in galaxy formation models by combining semi-analytic and
numerical methods.Comment: This version has coloured figures not present in the printed version.
Submitted to MNRAS, minor revision
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