49 research outputs found
Non-equilibrium effects in steady relativistic winds
We consider an ultra-relativistic wind consisting of electron-positron pairs
and photons with the principal goal of finding the asymptotic Lorentz factor
for zero baryon number. The wind is assumed to originate at
radius where it has a Lorentz factor and a temperature
sufficiently high to maintain pair equilibrium. As increases, decreases
and becomes less than the temperature corresponding to the electron mass ,
after which non-equilibrium effects become important. Further out in the flow
the optical depth drops below one, but the pairs may still be
accelerated by the photons until falls below . Radiative transfer calculations show that only at this point
do the radiation flux and pressure start to deviate significantly from their
blackbody values. The acceleration of the pairs increases by a factor
as compared to its value at the photosphere; it is shown to approach
\gamma_{\infty} \sim 1.4\times 10^3 (r_i/10^6\mbox{cm})^{1/4} \gamma_{i}^{3/4}
T_i/m_e.Comment: 41 pages, 9 figures. Submitted to MNRA
Inferring the Spatial and Energy Distribution of Gamma Ray Burst Sources. II. Isotropic Models
We use Bayesian methods to analyze the distribution of gamma ray burst
intensities reported in the Third BATSE Catalog (3B catalog) of gamma ray
bursts, presuming the distribution of burst sources (``bursters'') is
isotropic. We study both phenomenological and cosmological source distribution
models, using Bayes's theorem both to infer unknown parameters in the models,
and to compare rival models. We analyze the distribution of the time-averaged
peak photon number flux, F, measured on both 64 ms and 1024 ms time scales,
performing the analysis of data based on each time scale independently. Several
of our findings differ from those of previous analyses that modeled burst
detection less completely. In particular, we find that the width of the
intrinsic luminosity function for bursters is unconstrained, and the luminosity
function of the actually observed bursts can be extremely broad, in contrast to
the findings of all previous studies. Useful constraints probably require
observation of bursts significantly fainter than those visible to BATSE. We
also find that the 3B peak flux data do not usefully constrain the redshifts of
burst sources; useful constraints require the analysis of data beyond that in
the 3B catalog (such as burst time histories), or data from brighter bursts
than have been seen by BATSE (such as those observed by the Pioneer Venus
Orbiter). In addition, we find that an accurate understanding of the peak flux
distributions reported in the 3B almost certainly requires consideration of
data on the temporal and spectral properties of bursts beyond that reported in
the 3B catalog, and more sophisticated modeling than has so far been attempted.Comment: 66 pages, 33 figures; submitted to The Astrophysical Journal, 12/9
Luminosity distance in Swiss cheese cosmology with randomized voids. II. Magnification probability distributions
We study the fluctuations in luminosity distances due to gravitational
lensing by large scale (> 35 Mpc) structures, specifically voids and sheets. We
use a simplified "Swiss cheese" model consisting of a \Lambda -CDM
Friedman-Robertson-Walker background in which a number of randomly distributed
non-overlapping spherical regions are replaced by mass compensating comoving
voids, each with a uniform density interior and a thin shell of matter on the
surface. We compute the distribution of magnitude shifts using a variant of the
method of Holz & Wald (1998), which includes the effect of lensing shear. The
standard deviation of this distribution is ~ 0.027 magnitudes and the mean is ~
0.003 magnitudes for voids of radius 35 Mpc, sources at redshift z_s=1.0, with
the voids chosen so that 90% of the mass is on the shell today. The standard
deviation varies from 0.005 to 0.06 magnitudes as we vary the void size, source
redshift, and fraction of mass on the shells today. If the shell walls are
given a finite thickness of ~ 1 Mpc, the standard deviation is reduced to ~
0.013 magnitudes. This standard deviation due to voids is a factor ~ 3 smaller
than that due to galaxy scale structures. We summarize our results in terms of
a fitting formula that is accurate to ~ 20%, and also build a simplified
analytic model that reproduces our results to within ~ 30%. Our model also
allows us to explore the domain of validity of weak lensing theory for voids.
We find that for 35 Mpc voids, corrections to the dispersion due to lens-lens
coupling are of order ~ 4%, and corrections to due shear are ~ 3%. Finally, we
estimate the bias due to source-lens clustering in our model to be negligible
Modification to the Luminosity Distance Redshift Relation in Modified Gravity Theories
We derive an expression for the luminosity distance as a function of redshift
for a flat Robertson-Walker spacetime perturbed by arbitrary scalar
perturbations possibly produced by a modified gravity theory with two different
scalar perturbation potentials. Measurements of the luminosity distance as
function of redshift provide a constraint on a combination of the scalar
potentials and so they can complement weak lensing and other measurements in
trying to distinguish among the various alternative theories of gravity.Comment: 15 pages, we discuss in more detail how the luminosity distance
expression can be used to differentiate among various theories of gravit
Bounds on Cosmic Strings from WMAP and SDSS
We find the constraints from WMAP and SDSS data on the fraction of
cosmological fluctuations sourced by local cosmic strings using a Markov Chain
Monte Carlo (MCMC) analysis. In addition to varying the usual 6 cosmological
parameters and the string tension (), we also varied the amount of
small-scale structure on the strings. Our results indicate that cosmic strings
can account for up to 7 (14)% of the total power of the microwave anisotropy at
68 (95)% confidence level. The corresponding bound on the string mass per unit
length, within our string model, is at 68
(95)% c.l., where this constraint has been altered from what appears below
following the correction of errors in our cosmic string code outlined in a
recent erratum, astro-ph/0604141. We also calculate the B-type polarization
spectra sourced by cosmic strings and discuss the prospects of their detection.Comment: 11 pages, 7 figures. A few errors in the computer code used to
calculated CMB anistotropy from strings are fixed, resulting in a somewhat
tighter bound on G\mu and an enhanced B-mode polarization. Details of the
corrected errors and their implications can be found in astro-ph/0604141. Go
to http://physics.syr.edu/~lepogosi/cmbact.html for our now-corrected cosmic
string CMB and LSS cod
The Evolution Of LMC X-4 Flares: Evidence For Super-Eddington Radiation Oozing Through Inhomogeneous Polar Cap Accretion Flows ?
We present the results of two extensive Rossi X-ray Timing Explorer
observations of large X-ray flaring episodes from the high-mass X-ray binary
pulsar LMC X-4. Light curves during the flaring episodes comprise bright peaks
embedded in relatively fainter regions, with complex patterns of recurrence and
clustering of flares. We identify precursors preceding the flaring activity.
Pulse profiles during the flares appear to be simple sinusoids, and pulsed
fractions are proportional to the flare intensities. We fit Gaussian functions
to flare peaks to estimate the mean full-width-half-maximum to be 68 s.
Significant rapid aperiodic variability exists up to a few hertz during the
flares, which is related to the appearance of narrow, spiky peaks in the light
curves. While spectral fits and softness ratios show overall spectral softening
as the flare intensity increases, the narrow, spiky peaks do not follow this
trend. The mean fluence of the flare peaks is (3.1 2.9)
10 ergs in the 2.5--25 keV energy range, with its maximum at 1.9
10 ergs. The flare peak luminosity reaches up to (2.1
0.2) 10 ergs s, far above the Eddington luminosity of a
neutron star. We discuss possible origins of the flares, and we also propose
that inhomogeneous accretion columns onto the neutron star polar caps are
responsible for the observed properties.Comment: 39 pages (including figures and tables), accepted for publication in
Ap
Bayesian Analysis of the Polarization of Distant Radio Sources: Limits on Cosmological Birefringence
A recent study of the rotation of the plane of polarization of light from 160
cosmological sources claims to find significant evidence for cosmological
anisotropy. We point out methodological weaknesses of that study, and reanalyze
the same data using Bayesian methods that overcome these problems. We find that
the data always favor isotropic models for the distribution of observed
polarizations over counterparts that have a cosmological anisotropy of the type
advocated in the earlier study. Although anisotropic models are not completely
ruled out, the data put strong lower limits on the length scale (in
units of the Hubble length) associated with the anisotropy; the lower limits of
95% credible regions for lie between 0.43 and 0.62 in all anisotropic
models we studied, values several times larger than the best-fit value of
found in the earlier study. The length scale is not
constrained from above. The vast majority of sources in the data are at
distances closer than 0.4 Hubble lengths (corresponding to a redshift of
0.8); the results are thus consistent with there being no significant
anisotropy on the length scale probed by these data.Comment: 8 pages, 3 figures; submitted to Phys. Rev.
Systematic corrections to the measured cosmological constant as a result of local inhomogeneity
We calculate the systematic inhomogeneity-induced correction to the
cosmological constant that one would infer from an analysis of the luminosities
and redshifts of Type Ia supernovae, assuming a homogeneous universe. The
calculation entails a post-Newtonian expansion within the framework of second
order perturbation theory, wherein we consider the effects of subhorizon
density perturbations in a flat, dust dominated universe. Within this
formalism, we calculate luminosity distances and redshifts along the past light
cone of an observer. The resulting luminosity distance-redshift relation is fit
to that of a homogeneous model in order to deduce the best-fit cosmological
constant density Omega_Lambda. We find that the luminosity distance-redshift
relation is indeed modified, by a small fraction of order 10^{-5}. When fitting
this perturbed relation to that of a homogeneous universe, we find that the
inferred cosmological constant can be surprisingly large, depending on the
range of redshifts sampled. For a sample of supernovae extending from z=0.02
out to z=0.15, we find that Omega_Lambda=0.004. The value of Omega_Lambda has a
large variance, and its magnitude tends to get larger for smaller redshifts,
implying that precision measurements from nearby supernova data will require
taking this effect into account. However, we find that this effect is likely
too small to explain the observed value of Omega_Lambda=0.7. There have been
previous claims of much larger backreaction effects. By contrast to those
calculations, our work is directly related to how observers deduce cosmological
parameters from astronomical data.Comment: 28 pages, 3 figures, revtex4; v2: corrected comments and the section
on previous work; v3: clarified wording. References adde