3,261 research outputs found
Testing the Invariance of Cooling Rate in Gamma-Ray Burst Pulses
Recent studies have found that the spectral evolution of pulses within
gamma-ray bursts (GRBs) is consistent with simple radiative cooling. Perhaps
more interesting was a report that some bursts may have a single cooling rate
for the multiple pulses that occur within it. We determine the probability that
the observed "cooling rate invariance" is purely coincidental by sampling
values from the observed distribution of cooling rates. We find a 0.1-26%
probability that we would randomly observe a similar degree of invariance based
on a variety of pulse selection methods and pulse comparison statistics. This
probability is sufficiently high to warrant skepticism of any intrinsic
invariance in the cooling rate.Comment: 4 pages, 1 figure, to appear in Proceedings of the Fourth Huntsville
Symposium on Gamma-Ray Burst
Confronting Synchrotron Shock and Inverse Comptonization Models with GRB Spectral Evolution
The time-resolved spectra of gamma-ray bursts (GRBs) remain in conflict with
many proposed models for these events. After proving that most of the bursts in
our sample show evidence for spectral "shape-shifting", we discuss what
restrictions that BATSE time-resolved burst spectra place on current models. We
find that the synchrotron shock model does not allow for the steep low-energy
spectral slope observed in many bursts, including GRB 970111. We also determine
that saturated Comptonization with only Thomson thinning fails to explain the
observed rise and fall of the low-energy spectral slope seen in GRB 970111 and
other bursts. This implies that saturated Comptonization models must include
some mechanism which can cause the Thomson depth to increase intially in
pulses.Comment: (5 pages, 3 figures, to appear in Proceedings of the Fourth
Huntsville Symposium on Gamma-Ray Bursts
Recommended from our members
Multiscale structuring of materials - a hybrid additive, subtractive and directed assembly approach
Extremely hard GRB spectra prune down the forest of emission models
We consider the evidence for very hard low energy spectra during the prompt
phase of Gamma-Ray Bursts (GRB). In particular we examine the spectral
evolution of GRB 980306 together with the detailed analysis of some other
bursts already presented in the literature (GRB 911118, GRB 910807, GRB 910927
and GRB 970111), and check for the significance of their hardness (i.e.
extremely steep spectral slopes below the peak) by applying different
tests. These bursts, detected by the Burst And Transient Source Experiment
(BATSE) in the keV -- 2 MeV energy range, are sufficiently bright to
allow time resolved spectral studies on time intervals of the order of tenths
of a second. We discuss the hard spectra of these bursts and their evolution in
the context of several non--thermal emission models, which all appear
inadequate to account for these cases. The extremely hard spectra, which are
detected in the early part of the BATSE light curve, are also compared with a
black body spectral model: the resulting fits are remarkably good, except for
an excess at high energies (in several cases) which could be simply accounted
for by the presence of a supra--thermal component. The findings on the possible
thermal character of the evolving spectrum and the implications on the GRB
physical scenario are considered in the frameworks of photospheric models for a
fireball which is becoming optically thin, and of Compton drag models, in which
the fireball boosts "ambient" seed photons by its own bulk motion. Both models,
according to simple estimates, appear to be qualitatively and quantitatively
consistent with the found spectral characteristics, although their possible
caveats are discussed.Comment: 14 pages, 10 figures, 2 tables - Accepted for publication in
Astronomy & Astrophysic
Recommended from our members
Breast cancer tumour detection using microwave radar techniques
A breast cancer detection technique using multi-static radar is proposed herein. Images of a breast tumour are produced using this technique, with backscatter data. A wideband antenna design suitable for a breast cancer detection system is also described. Practical measurements are performed using a network analyser and a pair of antennas that are used to simulate an array. These initial images demonstrate the successful detection of a tumour phantom immersed in a liquid phantom with similar dielectric properties as the breast tissues
Recommended from our members
Breast tumour detection using a flat 16 element array
A new experimental prototype of a breast cancer detection technique using real aperture multi-static radar is presented. The system comprises a fully-populated 16 element flat array and an associated system to switch between different transmit and receive elements. 3D images are produced using backscatter signals from a synthetic breast phantom. After suppression of skin reflections, initial images demonstrate the successful detection of 4-mmdiameter tumours
Recommended from our members
Wideband microstrip patch antenna design for breast cancer tumour detection
A patch antenna is presented which has been designed to radiate into human breast tissue. The antenna is shown by means of simulation and practical measurement to possess a wide input bandwidth, stable radiation patterns and a good front-to-back ratio. Consideration is also given to its ability to radiate a pulse, and in this respect it is also found to be suitable for the proposed application
Mathematics in Literature
A lightly edited version of a Public Talk intended for a general audience, this essay examines significant appearances of mathematics, mathematical education, and attitudes to mathematics in, particularly, English and French literature. Both serious and light-hearted sources are considered. No attempt is made to be comprehensive
Recommended from our members
Microwave detection of breast tumours
Copyright @ 2003 European Bioelectromagnetics Association
- …