7,588 research outputs found
Mu and Tau Neutrino Thermalization and Production in Supernovae: Processes and Timescales
We investigate the rates of production and thermalization of and
neutrinos at temperatures and densities relevant to core-collapse
supernovae and protoneutron stars. Included are contributions from electron
scattering, electron-positron annihilation, nucleon-nucleon bremsstrahlung, and
nucleon scattering. For the scattering processes, in order to incorporate the
full scattering kinematics at arbitrary degeneracy, the structure function
formalism developed by Reddy et al. (1998) and Burrows and Sawyer (1998) is
employed. Furthermore, we derive formulae for the total and differential rates
of nucleon-nucleon bremsstrahlung for arbitrary nucleon degeneracy in
asymmetric matter. We find that electron scattering dominates nucleon
scattering as a thermalization process at low neutrino energies
( MeV), but that nucleon scattering is always faster
than or comparable to electron scattering above MeV. In
addition, for g cm, MeV, and
neutrino energies MeV, nucleon-nucleon bremsstrahlung always
dominates electron-positron annihilation as a production mechanism for
and neutrinos.Comment: 29 pages, LaTeX (RevTeX), 13 figures, submitted to Phys. Rev. C. Also
to be found at anonymous ftp site http://www.astrophysics.arizona.edu; cd to
pub/thompso
Strong Water Absorption in the Dayside Emission Spectrum of the Planet HD 189733b
Recent observations of the extrasolar planet HD 189733b did not reveal the
presence of water in the emission spectrum of the planet. Yet models of such
'Hot Jupiter' planets predict an abundance of atmospheric water vapour.
Validating and constraining these models is crucial for understanding the
physics and chemistry of planetary atmospheres in extreme environments.
Indications of the presence of water in the atmosphere of HD 189733b have
recently been found in transmission spectra, where the planet's atmosphere
selectively absorbs the light of the parent star, and in broadband photometry.
Here we report on the detection of strong water absorption in a high
signal-to-noise, mid-infrared emission spectrum of the planet itself. We find
both a strong downturn in the flux ratio below 10 microns and discrete spectral
features that are characteristic of strong absorption by water vapour. The
differences between these and previous observations are significant and admit
the possibility that predicted planetary-scale dynamical weather structures
might alter the emission spectrum over time. Models that match the observed
spectrum and the broadband photometry suggest that heat distribution from the
dayside to the night side is weak. Reconciling this with the high night side
temperature will require a better understanding of atmospheric circulation or
possible additional energy sources.Comment: 11 pages, 1 figure, published in Natur
Theoretical Spectral Models of the Planet HD 209458b with a Thermal Inversion and Water Emission Bands
We find that a theoretical fit to all the HD 209458b data at secondary
eclipse requires that the dayside atmosphere of HD 209458b have a thermal
inversion and a stratosphere. This inversion is caused by the capture of
optical stellar flux by an absorber of uncertain origin that resides at
altitude. One consequence of stratospheric heating and temperature inversion is
the flipping of water absorption features into emission features from the near-
to the mid-infrared and we see evidence of such a water emission feature in the
recent HD 209458b IRAC data of Knutson et al. In addition, an upper-atmosphere
optical absorber may help explain both the weaker-than-expected Na D feature
seen in transit and the fact that the transit radius at 24 m is smaller
than the corresponding radius in the optical. Moreover, it may be a factor in
why HD 209458b's optical transit radius is as large as it is. We speculate on
the nature of this absorber and the planets whose atmospheres may, or may not,
be affected by its presence.Comment: Accepted to the Astrophysical Journal Letters on August 28, 2007, six
pages in emulateapj forma
Theoretical Interpretation of the Measurements of the Secondary Eclipses of TrES-1 and HD209458b
We calculate the planet-star flux-density ratios as a function of wavelength
from 0.5 microns to 25 microns for the transiting extrasolar giant planets
TrES-1 and HD209458b and compare them with the recent Spitzer/IRAC-MIPS
secondary eclipse data in the 4.5, 8.0, and 24 micron bands. With only three
data points and generic calibration issues, detailed conclusions are difficult,
but inferences regarding atmospheric composition, temperature, and global
circulation can be made. Our models reproduce the observations reasonably well,
but not perfectly, and we speculate on the theoretical consequences of
variations around our baseline models. One preliminary conclusion is that we
may be seeing in the data indications that the day side of a close-in
extrasolar giant planet is brighter in the mid-infrared than its night side,
unlike Jupiter and Saturn. This correspondence will be further tested when the
data anticipated in other Spitzer bands are acquired, and we make predictions
for what those data may show.Comment: 15 pages, including 3 color figures, submitted to the Astrophysical
Journa
GRB Energetics in the Swift Era
We examine the rest frame energetics of 76 gamma-ray bursts (GRBs) with known
redshift that were detected by the Swift spacecraft and monitored by the
satellite's X-ray Telescope (XRT). Using the bolometric fluence values
estimated in Butler et al. 2007b and the last XRT observation for each event,
we set a lower limit the their collimation corrected energy Eg and find that a
68% of our sample are at high enough redshift and/or low enough fluence to
accommodate a jet break occurring beyond the last XRT observation and still be
consistent with the pre-Swift Eg distribution for long GRBs. We find that
relatively few of the X-ray light curves for the remaining events show evidence
for late-time decay slopes that are consistent with that expected from post jet
break emission. The breaks in the X-ray light curves that do exist tend to be
shallower and occur earlier than the breaks previously observed in optical
light curves, yielding a Eg distribution that is far lower than the pre-Swift
distribution. If these early X-ray breaks are not due to jet effects, then a
small but significant fraction of our sample have lower limits to their
collimation corrected energy that place them well above the pre-Swift Eg
distribution. Either scenario would necessitate a much wider post-Swift Eg
distribution for long cosmological GRBs compared to the narrow standard energy
deduced from pre-Swift observations. We note that almost all of the pre-Swift
Eg estimates come from jet breaks detected in the optical whereas our sample is
limited entirely to X-ray wavelengths, furthering the suggestion that the
assumed achromaticity of jet breaks may not extend to high energies.Comment: 30 pages, 10 figures, Accepted to Ap
Theory for the Secondary Eclipse Fluxes, Spectra, Atmospheres, and Light Curves of Transiting Extrasolar Giant Planets
We have created a general methodology for calculating the
wavelength-dependent light curves of close-in extrasolar giant planets (EGPs)
as they traverse their orbits. Focussing on the transiting EGPs HD189733b,
TrES-1, and HD209458b, we calculate planet/star flux ratios during secondary
eclipse and compare them with the Spitzer data points obtained so far in the
mid-infrared. We introduce a simple parametrization for the redistribution of
heat to the planet's nightside, derive constraints on this parameter (P_n), and
provide a general set of predictions for planet/star contrast ratios as a
function of wavelength, model, and phase. Moreover, we calculate average
dayside and nightside atmospheric temperature/pressure profiles for each
transiting planet/P_n pair with which existing and anticipated Spitzer data can
be used to probe the atmospheric thermal structure of severely irradiated EGPs.
We find that the baseline models do a good job of fitting the current secondary
eclipse dataset, but that the Spitzer error bars are not yet small enough to
discriminate cleanly between all the various possibilities.Comment: 14 figures, 7 text pages (in two-column emulateapj format); Accepted
to the Ap.J. June 26, 2006; one cosmetic change made to astro-ph version
Analysis of the X-ray Emission of Nine Swift Afterglows
The X-ray light-curves of 9 Swift XRT afterglows (050126, 050128, 050219A,
050315, 050318, 050319, 050401, 050408, 050505) display a complex behaviour: a
steep t^{-3.0 \pm 0.3} decay until ~400 s, followed by a significantly slower
t^{-0.65+/-0.20} fall-off, which at 0.2--2 d after the burst evolves into a
t^{-1.7+/-0.5} decay. We consider three possible models for the geometry of
relativistic blast-waves (spherical outflows, non-spreading jets, and spreading
jets), two possible dynamical regimes for the forward shock (adiabatic and
fully radiative), and we take into account a possible angular structure of the
outflow and delayed energy injection in the blast-wave, to identify the models
which reconcile the X-ray light-curve decay with the slope of the X-ray
continuum for each of the above three afterglow phases. By piecing together the
various models for each phase in a way that makes physical sense, we identify
possible models for the entire X-ray afterglow. The major conclusion of this
work is that a long-lived episode of energy injection in the blast-wave, during
which the shock energy increases at t^{1.0+/-0.5}, is required for five
afterglows and could be at work in the other four as well. Optical observations
in conjunction with the X-ray can distinguish among these various models. Our
simple tests allow the determination of the location of the cooling frequency
relative to the X-ray domain and, thus, of the index of the electron power-law
distribution with energy in the blast-wave. The resulting indices are clearly
inconsistent with an universal value.Comment: 10 pages, minor changes, to be published in the MNRA
Experimental study of acoustic displays of flight parameters in a simulated aerospace vehicle
Evaluating acoustic displays of target location in target detection and of flight parameters in simulated aerospace vehicle
The rapid decline of the prompt emission in Gamma-Ray Bursts
Many gamma ray bursts (GRBs) have been observed with the Burst-Alert and
X-Ray telescopes of the Swift satellite. The successive `pulses' of these GRBs
end with a fast decline and a fast spectral softening, until they are overtaken
by another pulse, or the last pulse's decline is overtaken by a less
rapidly-varying `afterglow'. The fast decline-phase has been attributed, in the
currently-explored standard fireball model of GRBs, to `high-latitude'
synchrotron emission from a collision of two conical shells. This high latitude
emission does not explain the observed spectral softening. In contrast, the
temporal behaviour and the spectral evolution during the fast-decline phase
agree with the predictions of the cannonball model of GRBs.Comment: Four added figures comparing the evolution of the inferred effective
photon spectral index during the fast decline phase of the prompt emission in
14 selected Swift GRBS and the cannonball (CB) model predictio
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