1,261 research outputs found
Yet Another Model of Gamma-Ray Bursts
Sari and Piran have demonstrated that the time structure of gamma-ray bursts
must reflect the time structure of their energy release. A model which
satisfies this condition uses the electrodynamic emission of energy by the
magnetized rotating ring of dense matter left by neutron star coalescence; GRB
are essentially fast, high field, differentially rotating pulsars. The energy
densities are large enough that the power appears as an outflowing equilibrium
pair plasma, which produces the burst by baryon entrainment and subsequent
internal shocks. I estimate the magnetic field and characteristic time scale
for its rearrangement, which determines the observed time structure of the
burst. There may be quasi-periodic oscillations at the rotational frequencies,
which are predicted to range up to 5770 Hz (in a local frame). This model is
one of a general class of electrodynamic accretion models which includes the
Blandford and Lovelace model of AGN, and which can also be applied to black
hole X-ray sources of stellar mass. The apparent efficiency of nonthermal
particle acceleration is predicted to be 10--50%, but higher values are
possible if the underlying accretion flow is super-Eddington. Applications to
high energy gamma-ray observations of AGN are briefly discussed.Comment: 21pp, latex, uses aaspp4.st
Charged Scalar-Tensor Boson Stars: Equilibrium, Stability and Evolution
We study charged boson stars in scalar-tensor (ST) gravitational theories. We
analyse the weak field limit of the solutions and analytically show that there
is a maximum charge to mass ratio for the bosons above which the weak field
solutions are not stable. This charge limit can be greater than the GR limit
for a wide class of ST theories. We numerically investigate strong field
solutions in both the Brans Dicke and power law ST theories. We find that the
charge limit decreases with increasing central boson density. We discuss the
gravitational evolution of charged and uncharged boson stars in a cosmological
setting and show how, at any point in its evolution, the physical properties of
the star may be calculated by a rescaling of a solution whose asymptotic value
of the scalar field is equal to its initial asymptotic value. We focus on
evolution in which the particle number of the star is conserved and we find
that the energy and central density of the star decreases as the cosmological
time increases. We also analyse the appearance of the scalarization phenomenon
recently discovered for neutron stars configurations and, finally, we give a
short discussion on how making the correct choice of mass influences the
argument over which conformal frame, the Einstein frame or the Jordan frame, is
physical.Comment: RevTeX, 27 pages, 9 postscript figures. Minor revisions and updated
references. Accepted for publication in Phys. Rev.
New Test of Supernova Electron Neutrino Emission using Sudbury Neutrino Observatory Sensitivity to the Diffuse Supernova Neutrino Background
Supernovae are rare nearby, but they are not rare in the Universe, and all
past core-collapse supernovae contributed to the Diffuse Supernova Neutrino
Background (DSNB), for which the near-term detection prospects are very good.
The Super-Kamiokande limit on the DSNB electron {\it antineutrino} flux,
cm s, is just above the
range of recent theoretical predictions based on the measured star formation
rate history. We show that the Sudbury Neutrino Observatory should be able to
test the corresponding DSNB electron {\it neutrino} flux with a sensitivity as
low as cm s,
improving the existing Mont Blanc limit by about three orders of magnitude.
While conventional supernova models predict comparable electron neutrino and
antineutrino fluxes, it is often considered that the first (and
forward-directed) SN 1987A event in the Kamiokande-II detector should be
attributed to electron-neutrino scattering with an electron, which would
require a substantially enhanced electron neutrino flux. We show that with the
required enhancements in either the burst or thermal phase fluxes, the
DSNB electron neutrino flux would generally be detectable in the Sudbury
Neutrino Observatory. A direct experimental test could then resolve one of the
enduring mysteries of SN 1987A: whether the first Kamiokande-II event reveals a
serious misunderstanding of supernova physics, or was simply an unlikely
statistical fluctuation. Thus the electron neutrino sensitivity of the Sudbury
Neutrino Observatory is an important complement to the electron antineutrino
sensitivity of Super-Kamiokande in the quest to understand the DSNB.Comment: 10 pages, 3 figure
Ozone detection by differential absorption spectroscopy at ambient pressure with a 9.6ÎŒm pulsed quantum-cascade laser
We report direct absorption spectroscopic detection of ozone at ambient pressure with a pulsed, DFB quantum-cascade laser (QCL) tuned within 1044-1050cm-1 by temperature scanning. Wavelength calibration curves were derived from FTIR and CO2 spectra and interpreted with respect to the heat transfer from the heterostructure to the sink. The laser linewidth (âŒ0.13cm-1 FWHM) was found to decrease with temperature, probably as a result of operation at constant current. Spurious spectral features due to baseline inaccuracies were successfully filtered out from the QCL O3 spectra using differential absorption. Reference O3 concentrations were obtained by applying the same method to UV spectra, simultaneously measured with a differential optical absorption spectrometer (DOAS). Column densities retrieved from QCL spectra are in fairly good agreement (±20%) with the DOAS values above 28ppmâm. The estimated QCL lowest detectable, absolute and differential absorptions, (7Ă10-3 and 2Ă10-3, respectively), entail effective detection limits of 14 and 25ppmâm, respectively. Ongoing improvements in the acquisition system should allow the achievement of detection limits at the level of commercial open-path DOAS systems (âŒ2ppmâm) in the near future. Our results demonstrate the applicability of the differential absorption method to QCL spectroscopy at ambient pressure, and encourage its use for open path detectio
The Environments of Supernovae in Post-Refurbishment Hubble Space Telescope Images
The locations of supernovae in the local stellar and gaseous environment in
galaxies contain important clues to their progenitor stars. Access to this
information, however, has been hampered by the limited resolution achieved by
ground-based observations. High spatial resolution Hubble Space Telescope (HST)
images of galaxy fields in which supernovae had been observed can improve the
situation considerably. We have examined the immediate environments of a few
dozen supernovae using archival post-refurbishment HST images. Although our
analysis is limited due to signal-to-noise ratio and filter bandpass
considerations, the images allow us for the first time to resolve individual
stars in, and to derive detailed color-magnitude diagrams for, several
environments. We are able to place more rigorous constraints on the masses of
these supernovae. A search was made for late-time emission from supernovae in
the archival images, and for the progenitor stars in presupernova images of the
host galaxies. We have detected SN 1986J in NGC 891 and, possibly, SN 1981K in
NGC 4258. We have also identified the progenitor of the Type IIn SN 1997bs in
NGC 3627. By removing younger resolved stars in the environments of SNe Ia, we
can measure the colors of the unresolved stellar background and attribute these
colors generally to an older, redder population. HST images ``accidentally''
caught the Type Ia SN 1994D in NGC 4526 shortly after its outburst; we measure
its brightness. Finally, we add to the statistical inferences that can be made
from studying the association of SNe with recent star-forming regions.Comment: 20 pages, 29 figures, to appear in A
Stars of extragalactic origin in the solar neighborhood
We computed the spatial velocities and the galactic orbital elements using
Hipparcos data for 77 nearest main-sequence F-G-stars with published the iron,
magnesium, and europium abundances determined from high dispersion spectra and
with the ages estimated from theoretical isochrones. A comparison with the
orbital elements of the globular clusters that are known was accreted by our
Galaxy in the past reveals stars of extragalactic origin. We show that the
relative elemental abundance ratios of r- and \alpha- elements in all the
accreted stars differ sharply from those in the stars that are genetically
associated with the Galaxy. According to current theoretical models, europium
is produced mainly in low mass Type II supernovae (SNe II), while magnesium is
synthesized in larger amounts in high mass SN II progenitors. Since all the old
accreted stars of our sample exhibit a significant Eu overabundance relative to
Mg, we conclude that the maximum masses of the SNII progenitors outside the
Galaxy were much lower than those inside it are. On the other hand, only a
small number of young accreted stars exhibit low negative ratios .
The delay of primordial star formation burst and the explosions of high mass
SNe II in a relatively small part of extragalactic space can explain this
situation. We provide evidence that the interstellar medium was weakly mixed at
the early evolutionary stages of the Galaxy formed from a single proto-galactic
cloud and that the maximum mass of the SN II progenitors increased in it with
time simultaneously with the increase in mean metallicity.Comment: Accepted for 2004, Astronomy Letters, Vol. 30, No. 3, P.148-158 15
pages, 3 figure
Specific features of the luminescence and conductivity of zinc selenide on exposure to X-ray and optical excitation
The set of experimental data on the X-ray-excited luminescence and X-ray
induced conductivity of ZnSe are compared to the data on the photoluminescence
and photoconductivity. It is experimentally established that the
current-voltage characteristics and the kinetics of phosphorescence and current
relaxation depend on the type of excitation. It is found that the external
electric field influences the intensity and shape of bands in the luminescence
spectra. It is shown that the character of excitation defines the kinetics of
recombination, charge carrier trapping, and conductivity in wide-gap
semiconductors.Comment: 7 pages, 7 figures, published in Fizika i Tekhnika Poluprovodnikov,
2010, Vol. 44, No. 5, pp. 594-59
Helium Emission in the Type Ic SN 1999cq
We present the first unambiguous detection of helium emission lines in
spectra of Type Ic supernovae (SNe Ic). The presence of He I lines, with full
width at half maximum ~ 2000 km/s, and the distinct absence of any other
intermediate-width emission (e.g., Halpha), implies that the ejecta of SN Ic
1999cq are interacting with dense circumstellar material composed of almost
pure helium. This strengthens the argument that the progenitors of SNe Ic are
core-collapse events in stars that have lost both their hydrogen and helium
envelopes, either through a dense wind or mass-transfer to a companion. In this
way, SN 1999cq is similar to supernovae such as SN 1987K and SN 1993J that
helped firmly establish a physical connection between Type Ib and Type II
supernovae. The light curve of SN 1999cq is very fast, with an extremely rapid
rise followed by a quick decline. SN 1999cq is also found to exhibit a high
level of emission at blue wavelengths (< 5500 A), likely resulting from either
an unusually large amount of iron and iron-group element emission or
uncharacteristically low reddening compared with other SNe Ic.Comment: 17 pages (AASTeX V5.0), 4 figures, accepted for publication in the
Astronomical Journa
Generation of single-mode SU(1,1) intelligent states and an analytic approach to their quantum statistical properties
We discuss a scheme for generation of single-mode photon states associated
with the two-photon realization of the SU(1,1) algebra. This scheme is based on
the process of non-degenerate down-conversion with the signal prepared
initially in the squeezed vacuum state and with a measurement of the photon
number in one of the output modes. We focus on the generation and properties of
single-mode SU(1,1) intelligent states which minimize the uncertainty relations
for Hermitian generators of the group. Properties of the intelligent states are
studied by using a ``weak'' extension of the analytic representation in the
unit disk. Then we are able to obtain exact analytical expressions for
expectation values describing quantum statistical properties of the SU(1,1)
intelligent states. Attention is mainly devoted to the study of photon
statistics and linear and quadratic squeezing.Comment: to appear in Quantum Semiclass. Opt., LaTeX, epsf style, 21 pages
including 5 Postscript figures. More information on
http://www.technion.ac.il/~brif/science.htm
Ellipsoidal configurations in the de Sitter spacetime
The cosmological constant modifies certain properties of large
astrophysical rotating configurations with ellipsoidal geometries, provided the
objects are not too compact. Assuming an equilibrium configuration and so using
the tensor virial equation with we explore several equilibrium
properties of homogeneous rotating ellipsoids. One shows that the bifurcation
point, which in the oblate case distinguishes the Maclaurin ellipsoid from the
Jacobi ellipsoid, is sensitive to the cosmological constant. Adding to that,
the cosmological constant allows triaxial configurations of equilibrium
rotating the minor axis as solutions of the virial equations. The significance
of the result lies in the fact that minor axis rotation is indeed found in
nature. Being impossible for the oblate case, it is permissible for prolate
geometries, with zero and positive. For the triaxial case, however,
an equilibrium solution is found only for non-zero positive . Finally,
we solve the tensor virial equation for the angular velocity and display
special effects of the cosmological constant there.Comment: 15 pages, 11 figures, published in Class. Quant. Grav. References
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