147 research outputs found
Search for Non-Triggered Gamma Ray Bursts in the BATSE Continuous Records: Preliminary Results
We present preliminary results of an off-line search for non-triggered
gamma-ray bursts (GRBs) in the BATSE daily records for about 5.7 years of
observations. We found more GRB-like events than the yield of the similar
search of Kommers et al. (1998) and extended the Log N - log P distribution
down to 0.1 ph cm s. The indication of a turnover of the
log N - log P at a small P is not confirmed: the distribution is straight at
1.5 decades with the power law index -.6 and cannot be fitted with a standard
candle cosmological model.Comment: 4 pages, LaTeX, to appear in Proceedings "Gamma Ray Bursts in the
Afterglow Era", Rome, November 1998, A&AS, 199
A New Window on Primordial non-Gaussianity
We know very little about primordial curvature perturbations on scales
smaller than about a Mpc. Measurements of the mu-type distortion of the CMB
spectrum provide the unique opportunity to probe these scales over the
unexplored range from 50 to 10^4 Mpc^-1. This is a very clean probe, in that it
relies only on well-understood linear evolution. We point out that correlations
between mu-distortion and temperature anisotropies can be used to test
Gaussianity at these very small scales. In particular the mu-T cross
correlation is proportional to the very squeezed limit of the primordial
bispectrum and hence measures fNL^loc, while mu-mu is proportional to the
primordial trispectrum and measures tauNL. We present a Fisher matrix forecast
of the observational constraints.Comment: 5 pages, one figure. v2: added clarifying comments and references,
fixed typo
Modifications to the Cosmic 21-cm Background Frequency Spectrum by Scattering via electrons in Galaxy Clusters
The cosmic 21-cm background frequency spectrum related to the spin-flip
transition of neutral Hydrogen present during and before the era of
reionization is rich in features associated with physical processes that govern
transitions between the two spin states. The intervening electrons in
foreground galaxy clusters inversely Compton scatter the 21-cm background
spectrum and modify it just as the cosmic microwave background (CMB) spectrum
is modified by inverse-Compton scattering. Towards typical galaxy clusters at
low redshifts, the resulting modification is a few tenths milli-Kelvin
correction to the few tens milli-Kelvin temperature of 21-cm signal relative to
that of the cosmic microwave background black body spectrum. The modifications
are mostly associated with sharp changes in the cosmic 21-cm background
spectrum such as due to the onset of a Lyman- radiation field or
heating of neutral gas. Though low frequency radio interferometers that are now
planned for 21-cm anisotropy measurements are insensitive to the mean 21-cm
spectrum, differential observations of galaxy clusters with these
interferometers can be utilized to indirectly establish global features in the
21-cm frequency spectrum. We discuss the feasibility to detect the spectrum
modified by clusters and find that for upcoming interferometers, while a
detection towards an individual cluster is challenging, one can average signals
over a number of clusters, selected based on the strength of the
Sunyave-Zel'dovich effect at high radio frequencies involving CMB scattering
alone, to establish the mean 21-cm spectrum.Comment: 6 pages, 3 figures, PRD in press; expanded and title changed from v1.
Final version in pres
Analytical Study on the Sunyaev-Zeldovich Effect for Clusters of Galaxies. II. comparison of covariant formalisms
We study a covariant formalism for the Sunyaev-Zeldovich effects developed in
the previous papers by the present authors, and derive analytic expressions for
the redistribution functions in the Thomson approximation. We also explore
another covariant formalism recently developed by Poutanen and Vurm. We show
that the two formalisms are mathematically equivalent in the Thomson
approximation which is fully valid for the cosmic microwave background photon
energies. The present finding will establish a theoretical foundation for the
analysis of the Sunyaev-Zeldovich effects for the clusters of galaxies.Comment: Accepted version, 7 pages, 1 figure, accepted by Physical Review D
for publicatio
Supernova Remnant in a Stratified Medium: Explicit, Analytical Approximations for Adiabatic Expansion and Radiative Cooling
We propose simple, explicit, analytical approximations for the kinematics of
an adiabatic blast wave propagating in an exponentially stratified ambient
medium, and for the onset of radiative cooling, which ends the adiabatic era.
Our method, based on the Kompaneets implicit solution and the Kahn
approximation for the radiative cooling coefficient, gives straightforward
estimates for the size, expansion velocity, and progression of cooling times
over the surface, when applied to supernova remnants (SNRs). The remnant shape
is remarkably close to spherical for moderate density gradients, but even a
small gradient in ambient density causes the cooling time to vary substantially
over the remnant's surface, so that for a considerable period there will be a
cold dense expanding shell covering only a part of the remnant. Our
approximation provides an effective tool for identifying the approximate
parameters when planning 2-dimensional numerical models of SNRs, the example of
W44 being given in a subsequent paper.Comment: ApJ accepted, 11 pages, 2 figures embedded, aas style with
ecmatex.sty and lscape.sty package
Are HI Supershells the Remnants of Gamma-Ray Bursts?
Gamma-Ray Bursts (GRBs) are thought to originate at cosmological distances
from the most powerful explosions in the Universe. If GRBs are not beamed then
the distribution of their number as a function of Gamma-ray flux implies that
they occur once per (0.3-40) million years per bright galaxy and that they
deposit >10^{53} ergs into their surrounding interstellar medium. The blast
wave generated by a GRB explosion would be washed out by interstellar
turbulence only after tens of millions of years when it finally slows down to a
velocity of 10 km/s. This rather long lifetime implies that there could be up
to several tens of active GRB remnants in each galaxy at any given time. For
many years, radio observations have revealed the enigmatic presence of
expanding neutral-hydrogen (HI) supershells of kpc radius in the Milky Way and
in other nearby galaxies. The properties of some supershells cannot be easily
explained in terms of conventional sources such as stellar winds or supernova
explosions. However, the inferred energy and frequency of the explosions
required to produce most of the observed supershells agree with the above GRB
parameters. More careful observations and analysis might reveal which fraction
of these supershells are GRB remnants. We show that if this link is
established, the data on HI supershells can be used to constrain the energy
output, the rate per galaxy, the beaming factor, and the environment of GRB
sources in the Universe.Comment: 8 pages, final version, ApJ Letters, in pres
Dispersion and damping of potential surface waves in a degenerate plasma
Potential (electrostatic) surface waves in plasma half-space with degenerate
electrons are studied using the quasi-classical mean-field kinetic model. The
wave spectrum and the collisionless damping rate are obtained numerically for a
wide range of wavelengths. In the limit of long wavelengths, the wave frequency
approaches the cold-plasma limit with
being the plasma frequency, while at short wavelengths, the wave
spectrum asymptotically approaches the spectrum of zero-sound mode propagating
along the boundary. It is shown that the surface waves in this system remain
weakly damped at all wavelengths (in contrast to strongly damped surface waves
in Maxwellian electron plasmas), and the damping rate nonmonotonically depends
on the wavelength, with the maximum (yet small) damping occuring for surface
waves with wavelength of , where is the
Thomas-Fermi length.Comment: 22 pages, 6 figure
Shielding of a moving test charge in a quantum plasma
The linearized potential of a moving test charge in a one-component fully
degenerate fermion plasma is studied using the Lindhard dielectric function.
The motion is found to greatly enhance the Friedel oscillations behind the
charge, especially for velocities larger than a half of the Fermi velocity, in
which case the asymptotic behavior of their amplitude changes from 1/r^3 to
1/r^2.5. In the absence of the quantum recoil (tunneling) the potential reduces
to a form similar to that in a classical Maxwellian plasma, with a difference
being that the plasma oscillations behind the charge at velocities larger than
the Fermi velocity are not Landau-damped.Comment: 9 pages, 11 figures. v3: Fixed typo, updated abstrac
Bound states near a moving charge in a quantum plasma
It is investigated how the shielding of a moving point charge in a
one-component fully degenerate fermion plasma affects the bound states near the
charge at velocities smaller than the Fermi one. The shielding is accounted for
by using the Lindhard dielectric function, and the resulting potential is
substituted into the Schr\"odinger equation in order to obtain the energy
levels. Their number and values are shown to be primarily determined by the
value of the charge and the quantum plasma coupling parameter, while the main
effect of the motion is to split certain energy levels. This provides a link
between quantum plasma theory and possible measurements of spectra of ions
passing through solids.Comment: Published in EPL, see
http://epljournal.edpsciences.org/articles/epl/abs/2011/09/epl13478/epl13478.htm
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