622 research outputs found
Neutrino oscillations and gamma-ray bursts
If the ordinary neutrinos oscillate into a sterile flavor in a manner
consistent with the Super-Kamiokande data on the zenith-angle dependence of
atmospheric mu-neutrino flux, an energy sufficient to power a typical cosmic
gamma-ray burst (GRB) (about 10^{52} erg) can be carried by sterile neutrinos
away from the source and deposited in a region relatively free of baryons.
Hence, ultra-relativistic bulk motion (required by the theory of and
observations of GRBs and their afterglows) can easily be achieved in the
vicinity of plausible sources of GRBs. Oscillations between sterile and
ordinary neutrinos would thus provide a solution to the ``baryon-loading
problem'' in the theory of GRBs
Euclidean vs. non-Euclidean Gamma-Ray Bursts
We classify gamma-ray bursts (GRBs) according to their observed durations and
physical properties of their spectra. We find that long/hard bursts (of
duration T_90 > 2.5 s, and typical photon energy E_p > 0.8 MeV corresponding to
BATSE's energy fluence hardness H^e_{32} > 3) show the strongest deviation from
the three-dimensional Euclidean brightness distribution. The majority of GRBs,
i.e., short bursts (T_90 2.5 s, and
H^e_{32} < 3) show little, if any, deviations from the Euclidean distribution.
These results contradict the prediction of simple extragalactic GRB models that
the most distant bursts should be the most affected by cosmological energy
redshift and time-dilation (long/soft GRBs). The strongly non-Euclidean GRB
subclass has very hard spectra of typical photon energy above 1 MeV, i.e.,
outside the ideal energy range for optimal detection by BATSE. We discuss
possible explanations of this puzzling feature of GRBs.Comment: 15 pages, LATEX text plus two postscript figures included. Submitted
to ApJ Letters on November 24, 1997. Accepted on February 13, 199
Constraints on the Redshift and Luminosity Distributions of Gamma Ray Bursts in an Einstein-de Sitter Universe
Two models of the gamma ray burst population, one with a standard candle
luminosity and one with a power law luminosity distribution, are chi^2-fitted
to the union of two data sets: the differential number versus peak flux
distribution of BATSE's long duration bursts, and the time dilation and energy
shifting versus peak flux information of pulse duration time dilation factors,
interpulse duration time dilation factors, and peak energy shifting factors.
The differential peak flux distribution is corrected for threshold effects at
low peak fluxes and at short burst durations, and the pulse duration time
dilation factors are also corrected for energy stretching and similar effects.
Within an Einstein-de Sitter cosmology, we place strong bounds on the evolution
of the bursts, and these bounds are incompatible with a homogeneous population,
assuming a power law spectrum and no luminosity evolution. Additionally, under
the implied conditions of moderate evolution, the 90% width of the observed
luminosity distribution is shown to be < 10^2, which is less constrained than
others have demonstrated it to be assuming no evolution. Finally, redshift
considerations indicate that if the redshifts of BATSE's faintest bursts are to
be compatible with that which is currently known for galaxies, a standard
candle luminosity is unacceptable, and in the case of the power law luminosity
distribution, a mean luminosity < 10^57 ph s^-1 is favored.Comment: Accepted to the Astrophysical Journal, 18 pages, LaTe
Carbon-poor stellar cores as supernova progenitors
Exploring stellar models which ignite carbon off-center (in the mass range of
about 1.05 - 1.25 Msun, depending on the carbon mass fraction) we find that
they may present an interesting SN I progenitor scenario, since whereas in the
standard scenario runaway always takes place at the same density of about 2 X
10^9 gr/cm^3, in our case, due to the small amount of carbon ignited, we get a
whole range of densities from 1 X 10^9 up to 6 X 10^9 gr/cm^3. These results
could contribute in resolving the emerging recognition that at least some
diversity among SNe I exists, since runaway at various central densities is
expected to yield various outcomes in terms of the velocities and composition
of the ejecta, which should be modeled and compared to observations.Comment: 49 pages, 20 figure
Calorimetry of gamma-ray bursts: echos in gravitational waves
Black holes surrounded by a disk or torus may drive the enigmatic
cosmological gamma-ray bursts (GRBs). Equivalence in poloidal topology to
pulsar magnetospheres shows a high incidence of the black hole-luminosity
into the surrounding magnetized matter. We argue that this emission is
re-radiated into gravitational waves at in frequencies of
order 1kHz, winds and, potentially, MeV neutrinos. The total energy budget and
input to the GRB from baryon poor jets are expected to be standard in this
scenario, consistent with recent analysis of afterglow data. Collimation of
these outflows by baryon rich disk or torus winds may account for the observed
spread in opening angles up to about . This model may be tested by future
LIGO/VIRGO observations.Comment: To appear in ApJ
Nucleation of quark matter in neutron stars cores
We consider the general conditions of quark droplets formation in high
density neutron matter. The growth of the quark bubble (assumed to contain a
sufficiently large number of particles) can be described by means of a
Fokker-Planck equation. The dynamics of the nucleation essentially depends on
the physical properties of the medium it takes place. The conditions for quark
bubble formation are analyzed within the frameworks of both dissipative and
non-dissipative (with zero bulk and shear viscosity coefficients) approaches.
The conversion time of the neutron star to a quark star is obtained as a
function of the equation of state of the neutron matter and of the microscopic
parameters of the quark nuclei. As an application of the obtained formalism we
analyze the first order phase transition from neutron matter to quark matter in
rapidly rotating neutron stars cores, triggered by the gravitational energy
released during the spinning down of the neutron star. The endothermic
conversion process, via gravitational energy absorption, could take place, in a
very short time interval, of the order of few tens seconds, in a class of dense
compact objects, with very high magnetic fields, called magnetars.Comment: 31 pages, 2 figures, to appear in Ap
A gamma ray burst with small contamination
We present a scenario (SupraNova) for the formation of GRBs occurring when a
supramassive neutron star (SMNS) loses so much angular momentum that
centrifugal support against self--gravity becomes impossible, and the star
implodes to a black hole. This may be the baryon--cleanest environment proposed
so far, because the SN explosion in which the SMNS formed swept the medium
surrounding the remnant, and the quickly spinning remnant loses energy through
magnetic dipole radiation at a rate exceeding its Eddington luminosity by some
four orders of magnitude. The implosion is adiabatic because neutrinos have
short mean free paths, and silent, given the prompt collapse of the polar caps.
However, a mass ~ 0.1 M_solar in the equatorial belt can easily reach
centrifugal equilibrium. The mechanism of energy extraction is via the
conversion of the Poynting flux (due to the large--scale magnetic field locked
into the minitorus) into a magnetized relativistic wind. Occasionally this
model will produce quickly decaying, or non--detectable afterglows.Comment: To appear in The Astrophysical Journal Letters. AASTeX LateX, no
figure
Radiation Pressure Supported Starburst Disks and AGN Fueling
We consider the structure of marginally Toomre-stable starburst disks under
the assumption that radiation pressure on dust grains provides the dominant
vertical support against gravity. This is particularly appropriate when the
disk is optically thick to its own IR radiation, as in the central regions of
ULIRGs. Because the disk radiates at its Eddington limit, the Schmidt-law for
star formation changes in the optically-thick limit, with the star formation
rate per unit area scaling as Sigma_g/kappa, where Sigma_g is the gas surface
density and kappa is the mean opacity. We show that optically thick starburst
disks have a characteristic flux and dust effective temperature of F ~ 10^{13}
L_sun/kpc^2 and T_eff ~ 90K, respectively. We compare our predictions with
observations and find good agreement. We extend our model from many-hundred
parsec scales to sub-parsec scales and address the problem of fueling AGN. We
assume that angular momentum transport proceeds via global torques rather than
a local viscosity. We account for the radial depletion of gas due to star
formation and find a strong bifurcation between two classes of disk models: (1)
solutions with a starburst on large scales that consumes all of the gas with
little fueling of a central AGN and (2) models with an outer large-scale
starburst accompanied by a more compact starburst on 1-10 pc scales and a
bright central AGN. The luminosity of the latter models is in many cases
dominated by the AGN. We show that the vertical thickness of the starburst disk
on pc scales can approach h ~ r, perhaps accounting for the nuclear obscuration
in some Type 2 AGN. We also argue that the disk of young stars in the Galactic
Center may be the remnant of such a compact nuclear starburst.Comment: 26 pages, 9 figures, emulateapj, accepted to ApJ, minor changes,
discussion tightened, references adde
Observational Prospects for Afterglows of Short Duration Gamma-ray Bursts
If the efficiency for producing -rays is the same in short duration
(\siml 2 s) Gamma-Ray Bursts (GRBs) as in long duration GRBs, then the
average kinetic energy of short GRBs must be times less than that of
long GRBs. Assuming further that the relativistic shocks in short and long
duration GRBs have similar parameters, we show that the afterglows of short
GRBs will be on average 10--40 times dimmer than those of long GRBs. We find
that the afterglow of a typical short GRB will be below the detection limit
(\siml 10 \microJy) of searches at radio frequencies. The afterglow would be
difficult to observe also in the optical, where we predict R \simg 23 a few
hours after the burst. The radio and optical afterglow would be even fainter if
short GRBs occur in a low-density medium, as expected in NS-NS and NS-BH merger
models. The best prospects for detecting short-GRB afterglows are with early
(\siml 1 day) observations in X-rays.Comment: 5 pages, 2 figures, submitted to ApJ lette
Phase-Dependent Properties of Extrasolar Planet Atmospheres
Recently the Spitzer Space Telescope observed the transiting extrasolar
planets, TrES-1 and HD209458b. These observations have provided the first
estimates of the day side thermal flux from two extrasolar planets orbiting
Sun-like stars. In this paper, synthetic spectra from atmospheric models are
compared to these observations. The day-night temperature difference is
explored and phase-dependent flux densities are predicted for both planets. For
HD209458b and TrES-1, models with significant day-to-night energy
redistribution are required to reproduce the observations. However, the
observational error bars are large and a range of models remains viable.Comment: 8 pages, 7 figures, accepted for publication in the Astrophysical
Journa
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