620 research outputs found
Spectral Evolution of Two High-Energy Gamma-Ray Bursts
The prompt emission of the gamma-ray bursts is found to be very energetic,
releasing ~10^51 ergs in a flash. However, their emission mechanism remains
unclear and understanding their spectra is a key to determining the emission
mechanism. Many GRB spectra have been analyzed in the sub-MeV energy band, and
are usually well described with a smoothly broken power-law model. We present a
spectral analysis of two bright bursts (GRB910503 and GRB930506), using BATSE
and EGRET spectra that cover more than four decades of energy (30 keV - 200
MeV). Our results show time evolutions of spectral parameters (low-energy &
high-energy photon indices and break energy) that are difficult to reconcile
with a simple shock-acceleration model.Comment: 8 pages, 2 figures, to appear in the proceedings of "Astrophysical
Particle Acceleration in Geospace and Beyond", Chattanooga, 2002, AGU
monograp
Neutrino afterglow from Gamma-Ray Bursts: ~10^{18} eV
We show that a significant fraction of the energy of a gamma-ray burst(GRB)
is probably converted to a burst of 10^{17}-10^{19} eV neutrinos and multiple
GeV gammas that follow the GRB by > 10 s . If, as previously suggested, GRB's
accelerate protons to ~10^{20} eV, then both the neutrinos and the gammas may
be detectable.Comment: Accepted ApJ; added sentence re: sterile neutrinos; related material
at http://www.sns.ias.edu/~jn
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Radiant transmittance of cerium doped quartz from 300 to 1270K
The transmittance of curved slabs of cerium doped quartz is reported as a function of wavelength and temperature. The spectral range of measurement is 0.25 to 0.725 {micro}m and temperature varies from 300K to 1270K. The short wavelength cutoff for transmission shifts to longer wavelengths monotonically with temperature at a rate of {approximately}3nm/l 00K. The tmnstnittance data for wavelengths less than 0.36 {micro}m are fit to a classical pole fit model using 8 modes (Oscillators) and the temperature dependence of the modes is given. For wavelengths beyond 0.36 {micro}m the data are fit to an ``Urbach rule.`` The bandgap parameter in the Urbach rule decreases linearly with temperature to 1270K and varies from 3.394eV at 300K to 3,183 eV at 1270K, while the steepness parameter also decreases approximately linearly from 8.51 eV{sup -1} to 5.80 eV{sup -1}. The fits are used to compute the spectral and temperature dependent absorption coefficient
Limits on Supersymmetric Dark Matter From EGRET Observations of the Galactic Center Region
In most supersymmetic models, neutralino dark matter particles are predicted
to accumulate in the Galactic center and annihilate generating, among other
products, gamma rays. The EGRET experiment has made observations in this
region, and is sensitive to gamma rays from 30 MeV to 30 GeV. We have
used an improved point source analysis including an energy dependent point
spread function and an unbinned maximum likelihood technique, which has allowed
us to significantly lower the limits on gamma ray flux from the Galactic
center. We find that the present EGRET data can limit many supersymmetric
models if the density of the Galactic dark matter halo is cuspy or spiked
toward the Galactic center. We also discuss the ability of GLAST to test these
models.Comment: 4 pages, 3 figure
Observations of GRB 990123 by the Compton Gamma-Ray Observatory
GRB 990123 was the first burst from which simultaneous optical, X-ray and
gamma-ray emission was detected; its afterglow has been followed by an
extensive set of radio, optical and X-ray observations. We have studied the
gamma-ray burst itself as observed by the CGRO detectors. We find that
gamma-ray fluxes are not correlated with the simultaneous optical observations,
and the gamma-ray spectra cannot be extrapolated simply to the optical fluxes.
The burst is well fit by the standard four-parameter GRB function, with the
exception that excess emission compared to this function is observed below ~15
keV during some time intervals. The burst is characterized by the typical
hard-to-soft and hardness-intensity correlation spectral evolution patterns.
The energy of the peak of the nu f_nu spectrum, E_p, reaches an unusually high
value during the first intensity spike, 1470 +/- 110 keV, and then falls to
\~300 keV during the tail of the burst. The high-energy spectrum above ~MeV is
consistent with a power law with a photon index of about -3. By fluence, GRB
990123 is brighter than all but 0.4% of the GRBs observed with BATSE, clearly
placing it on the -3/2 power-law portion of the intensity distribution.
However, the redshift measured for the afterglow is inconsistent with the
Euclidean interpretation of the -3/2 power-law. Using the redshift value of >=
1.61 and assuming isotropic emission, the gamma-ray fluence exceeds 10E54 ergs.Comment: Submitted to The Astrophysical Journal. 16 pages including 4 figure
Gamma-Ray Bursts: Progress, Problems & Prospects
The cosmological gamma-ray burst (GRB) phenomenon is reviewed. The broad
observational facts and empirical phenomenological relations of the GRB prompt
emission and afterglow are outlined. A well-tested, successful fireball shock
model is introduced in a pedagogical manner. Several important uncertainties in
the current understanding of the phenomenon are reviewed, and prospects of how
future experiments and extensive observational and theoretical efforts may
address these problems are discussed.Comment: 86 pages, 17 figures, 566 references, an invited review for
International Journal of Modern Physics A, in pres
Can Fireball or Firecone Models Explain Gamma Ray Bursts?
The observed afterglows of gamma ray bursts, in particular that of GRB 970228
six months later, seem to rule out relativistic fireballs and relativistic
firecones driven by merger or accretion induced collapse of compact stellar
objects in galaxies as the origin of GRBs. GRBs can be produced by superluminal
jets from such events.Comment: A short summary of the main properties of GRBs which are produced by
relativistic jets from merger/AIC of compact stellar objects is included.
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