138 research outputs found

### Polarization of the \lya Halos Around Sources Before Cosmological Reionization

In Loeb & Rybicki (1999; paper I) it was shown that before reionization, the
scattering of \lya photons from a cosmological source might lead to a fairly
compact ($\sim 15''$) \lya halo around the source. Observations of such halos
could constrain the properties of the neutral intergalactic medium (IGM), and
in particular yield the cosmological density parameters of baryons and matter
on scales where the Hubble flow is unperturbed. Paper I did not treat the
polarization of this scattered radiation, but did suggest that the degree of
such polarization might be large. In this Letter we report on improved
calculations for these \lya halos, now accounting for the polarization of the
radiation field. The polarization is linear and is oriented tangentially to the
projected displacement from the center of the source. The degree of
polarization is found to be 14% at the core radius, where the intensity has
fallen to half of the central value. It rises to 32% and 45% at the radii where
the intensity has fallen to one-tenth and one-hundreth of the central
intensity, respectively. At larger radii the degree of polarization rises
further, asymptotically to 60%. Such high values of polarization should be
easily observable and provide a clear signature of the phenomenon of \lya halos
surrounding sources prior to reionization.Comment: 8 pages, 2 Postscript figures, accepted by Astrophysical Journal
Letters; some typos corrected; added two paragraphs at the end of section 3
concerning detectability of Lyman alpha halo

### The Time Development of a Resonance Line in the Expanding Universe

The time-dependent spectral profile of a resonance line in a homogeneous
expanding medium is studied by numerically solving an improved Fokker-Planck
diffusion equation. The solutions are used to determine the time required to
reach a quasi-static solution near the line center. A simple scaling law for
this relaxation time is derived and is fitted to the numerical results. The
results are applied to the case of Lyman alpha scattering during primordial
recombination of hydrogen. For a wide range of cosmological models it is found
that the relaxation times are smaller than the recombination timescale,
although not by a very large factor. Thus the standard assumption of a
quasi-static solution in cosmological recombination calculations is reasonably
valid, and should not cause substantial errors in the solutions.Comment: 20 pages text and 10 figures, in 30 pages of uuencoded, compressed
postscript. CFA preprint no. 375

### Magnification Ratio of the Fluctuating Light in Gravitational Lens 0957+561

Radio observations establish the B/A magnification ratio of gravitational
lens 0957+561 at about 0.75. Yet, for more than 15 years, the optical
magnfication ratio has been between 0.9 and 1.12. The accepted explanation is
microlensing of the optical source. However, this explanation is mildly
discordant with (i) the relative constancy of the optical ratio, and (ii)
recent data indicating possible non-achromaticity in the ratio. To study these
issues, we develop a statistical formalism for separately measuring, in a
unified manner, the magnification ratio of the fluctuating and constant parts
of the light curve. Applying the formalism to the published data of Kundi\'c et
al. (1997), we find that the magnification ratios of fluctuating parts in both
the g and r colors agrees with the magnification ratio of the constant part in
g-band, and tends to disagree with the r-band value. One explanation could be
about 0.1 mag of consistently unsubtracted r light from the lensing galaxy G1,
which seems unlikely. Another could be that 0957+561 is approaching a caustic
in the microlensing pattern.Comment: 12 pages including 1 PostScript figur

### Scattered Lyman-alpha Radiation Around Sources Before Cosmological Reionization

The spectra of the first galaxies and quasars in the Universe should be
strongly absorbed shortward of their rest-frame Lyman-alpha wavelength by
neutral hydrogen (HI) in the intervening intergalactic medium. However, the
Lyman-alpha line photons emitted by these sources are not eliminated but rather
scatter until they redshift out of resonance and escape due to the Hubble
expansion of the surrounding intergalactic HI. We calculate the resulting
brightness distribution and the spectral shape of the diffuse Lyman-alpha line
emission around high redshift sources, before the intergalactic medium was
reionized. Typically, the Lyman-alpha photons emitted by a source at z=10
scatter over a characteristic angular radius of order 15 arcseconds around the
source and compose a line which is broadened and redshifted by about a thousand
km/s relative to the source. The scattered photons are highly polarized.
Detection of the diffuse Lyman-alpha halos around high redshift sources would
provide a unique tool for probing the neutral intergalactic medium before the
epoch of reionization. On sufficiently large scales where the Hubble flow is
smooth and the gas is neutral, the Lyman-alpha brightness distribution can be
used to determine the cosmological mass densities of baryons and matter.Comment: 21 pages, 5 Postscript figures, accepted by ApJ; figures 1--3
corrected; new section added on the detectability of Lyman alpha halos;
conclusions update

### A New Kinetic Equation for Compton Scattering

A kinetic equation for Compton scattering is given that differs from the
Kompaneets equation in several significant ways. By using an inverse
differential operator this equation allows treatment of problems for which the
radiation field varies rapidly on the scale of the width of the Compton kernel.
This inverse operator method describes, among other effects, the thermal
Doppler broadening of spectral lines and continuum edges, and automatically
incorporates the process of Compton heating/cooling. It is well adapted for
inclusion into a numerical iterative solution of radiative transfer problems.
The equivalent kernel of the new method is shown to be a positive function and
with reasonable accuracy near the intitial frequency, unlike the Kompaneets
kernel, which is singular and not wholly positive. It is shown that iterates of
the inverse operator kernel can be easily calculated numerically, and a simple
summation formula over these iterates is derived that can be efficiently used
to compute Comptonized spectra. It is shown that the new method can be used for
initial value and other problems with no more numerical effort than the
Kompaneets equation, and that it more correctly describes the solution over
times comparable to the mean scattering time.Comment: 27 pages, 5 figures, to be published in ApJ. Minor changes, including
one reference correcte

### Properties of High-Redshift Lyman Alpha Clouds I. Statistical Analysis of the SSG Quasars

Techniques for the statistical analysis of the \Lya\ forest in high redshift
quasars are developed, and applied to the low resolution (25 \AA) spectra of 29
of the 33 quasars in the Schneider-Schmidt-Gunn (SSG) sample.We find that the
mean absorption increases with $z$ approximately as a power law
$(1+z)^{\gamma+1}$ with $\gamma = 2.46\pm 0.37$. The mean ratio of \Lya\ to
Lyman $\beta$ absorption in the clouds is $0.476\pm 0.054$. We also detect, and
obtain ratios, for Lyman $\beta$, $\gamma$, and possibly $\epsilon$. We are
also able to quantify the fluctuations of the absorption around its mean, and
find that these are comparable to, or perhaps slightly larger than, that
expected from an uncorrelated distribution of clouds. The techniques in this
paper, which include the use of bootstrap resampling of the quasar sample to
obtain estimated errors and error covariances, and a mathematical treatment of
absorption from a (possibly non-uniform) stochastic distribution of lines,
should be applicable to future, more extensive, data sets.Comment: 29 pages, LaTeX using aastex30 macros, forthcoming as CfA preprin

### Thermal X-rays from Millisecond Pulsars: Constraining the Fundamental Properties of Neutron Stars

Abridged) We model the X-ray properties of millisecond pulsars (MSPs) by
considering hot spot emission from a weakly magnetized rotating neutron star
(NS) covered by an optically-thick hydrogen atmosphere. We investigate the
limitations of using the thermal X-ray pulse profiles of MSPs to constrain the
mass-to-radius ($M/R$) ratio of the underlying NS. The accuracy is strongly
dependent on the viewing angle and magnetic inclination. For certain systems,
the accuracy is ultimately limited only by photon statistics implying that
future X-ray observatories could, in principle, achieve constraints on $M/R$
and hence the NS equation of state to better than $\sim$5%. We demonstrate that
valuable information regarding the basic properties of the NS can be extracted
even from X-ray data of fairly limited photon statistics through modeling of
archival spectroscopic and timing observations of the nearby isolated PSRs
J0030+0451 and J2124--3358. The X-ray emission from these pulsars is consistent
with the presence of a hydrogen atmosphere and a dipolar magnetic field
configuration, in agreement with previous findings for PSR J0437--4715. For
both MSPs, the favorable geometry allows us to place interesting limits on the
allowed $M/R$ of NSs. Assuming 1.4 M$_{\odot}$, the stellar radius is
constrained to be $R > 9.4$ km and $R > 7.8$ km (68% confidence) for PSRs
J0030+0451 and J2124--3358, respectively. We explore the prospects of using
future observatories such as \textit{Constellation-X} and \textit{XEUS} to
conduct blind X-ray timing searches for MSPs not detectable at radio
wavelengths due to unfavorable viewing geometry. Using the observational
constraints on the pulsar obliquities we are also able to place strong
constraints on the magnetic field evolution model proposed by Ruderman.Comment: 9 pages, 7 figures, published in the Astrophysical Journal (Volume
689, Issue 1, pp. 407-415

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