3,449 research outputs found
Constraining the unexplored period between reionization and the dark ages with observations of the global 21 cm signal
Observations of the frequency dependence of the global brightness temperature
of the redshifted 21 cm line of neutral hydrogen may be possible with single
dipole experiments. In this paper, we develop a Fisher matrix formalism for
calculating the sensitivity of such instruments to the 21 cm signal from
reionization and the dark ages. We show that rapid reionization histories with
duration delta z< 2 can be constrained, provided that local foregrounds can be
well modelled by low order polynomials. It is then shown that observations in
the range nu = 50 - 100 MHz can feasibly constrain the Lyman alpha and X-ray
emissivity of the first stars forming at z = 15 - 25, provided that systematic
temperature residuals can be controlled to less than 1 mK. Finally, we
demonstrate the difficulty of detecting the 21 cm signal from the dark ages
before star formation.Comment: 11 pages, 14 figures, submitted to PR
Constraints on Off-Axis X-Ray Emission from Beamed GRBs
We calculate the prompt x-ray emission as a function of viewing angle for
beamed Gamma-Ray Burst (GRB) sources. Prompt x-rays are inevitable due to the
less highly blueshifted photons emitted at angles greater than 1/gamma relative
to the beam symmetry axis, where gamma is the expansion Lorentz factor. The
observed flux depends on the combinations (gamma Delta theta) and (gamma
theta_v), where (Delta theta) is the beaming angle and theta_v is the viewing
angle. We use the observed source counts of gamma-ray-selected GRBs to predict
the minimum detection rate of prompt x-ray bursts as a function of limiting
sensitivity. We compare our predictions with the results from the Ariel V
catalog of fast x-ray transients, and find that Ariel's sensitivity is not
great enough to place significant constraints on gamma and (Delta theta). We
estimate that a detector with fluence limit ~10^{-7} erg/cm^2 in the 2-10 keV
channel will be necessary to distinguish between geometries. Because the x-ray
emission is simultaneous with the GRB emission, our predicted constraints do
not involve any model assumptions about the emission physics but simply follow
from special-relativistic considerations.Comment: Submitted to Ap
Global 21cm signal experiments: a designer's guide
[Abridged] The spatially averaged global spectrum of the redshifted 21cm line
has generated much experimental interest, for it is potentially a direct probe
of the Epoch of Reionization and the Dark Ages. Since the cosmological signal
here has a purely spectral signature, most proposed experiments have little
angular sensitivity. This is worrisome because with only spectra, the global
21cm signal can be difficult to distinguish from foregrounds such as Galactic
synchrotron radiation, as both are spectrally smooth and the latter is orders
of magnitude brighter. We establish a mathematical framework for global signal
data analysis in a way that removes foregrounds optimally, complementing
spectra with angular information. We explore various experimental design
trade-offs, and find that 1) with spectral-only methods, it is impossible to
mitigate errors that arise from uncertainties in foreground modeling; 2)
foreground contamination can be significantly reduced for experiments with fine
angular resolution; 3) most of the statistical significance in a positive
detection during the Dark Ages comes from a characteristic high-redshift trough
in the 21cm brightness temperature; and 4) Measurement errors decrease more
rapidly with integration time for instruments with fine angular resolution. We
show that if observations and algorithms are optimized based on these findings,
an instrument with a 5 degree beam can achieve highly significant detections
(greater than 5-sigma) of even extended (high Delta-z) reionization scenarios
after integrating for 500 hrs. This is in contrast to instruments without
angular resolution, which cannot detect gradual reionization. Abrupt ionization
histories can be detected at the level of 10-100's of sigma. The expected
errors are also low during the Dark Ages, with a 25-sigma detection of the
expected cosmological signal after only 100 hrs of integration.Comment: 34 pages, 30 figures. Replaced (v2) to match accepted PRD version
(minor pedagogical additions to text; methods, results, and conclusions
unchanged). Fixed two typos (v3); text, results, conclusions etc. completely
unchange
Gravitational Lensing of the SDSS High-Redshift Quasars
We predict the effects of gravitational lensing on the color-selected
flux-limited samples of z~4.3 and z>5.8 quasars, recently published by the
Sloan Digital Sky Survey (SDSS). Our main findings are: (i) The lensing
probability should be 1-2 orders of magnitude higher than for conventional
surveys. The expected fraction of multiply-imaged quasars is highly sensitive
to redshift and the uncertain slope of the bright end of the luminosity
function, beta_h. For beta_h=2.58 (3.43) we find that at z~4.3 and i*<20.0 the
fraction is ~4% (13%) while at z~6 and z*<20.2 the fraction is ~7% (30%). (ii)
The distribution of magnifications is heavily skewed; sources having the
redshift and luminosity of the SDSS z>5.8 quasars acquire median magnifications
of med(mu_obs)~1.1-1.3 and mean magnifications of ~5-50. Estimates of
the quasar luminosity density at high redshift must therefore filter out
gravitationally-lensed sources. (iii) The flux in the Gunn-Peterson trough of
the highest redshift (z=6.28) quasar is known to be f_lambda<3 10^-19
erg/sec/cm^2/Angstrom. Should this quasar be multiply imaged, we estimate a 40%
chance that light from the lens galaxy would have contaminated the same part of
the quasar spectrum with a higher flux. Hence, spectroscopic studies of the
epoch of reionization need to account for the possibility that a lens galaxy,
which boosts the quasar flux, also contaminates the Gunn-Peterson trough. (iv)
Microlensing by stars should result in ~1/3 of multiply imaged quasars in the
z>5.8 catalog varying by more than 0.5 magnitudes over the next decade. The
median equivalent width would be lowered by ~20% with respect to the intrinsic
value due to differential magnification of the continuum and emission-line
regions.Comment: 27 pages, 10 figures. Expansion on the discussion in
astro-ph/0203116. Replaced with version accepted for publication in Ap
Prospects for Redshifted 21-cm observations of quasar HII regions
The introduction of low-frequency radio arrays over the coming decade is
expected to revolutionize the study of the reionization epoch. Observation of
the contrast in redshifted 21cm emission between a large HII region and the
surrounding neutral IGM will be the simplest and most easily interpreted
signature. We find that an instrument like the planned Mileura Widefield Array
Low-Frequency Demonstrator (LFD) will be able to obtain good signal to noise on
HII regions around the most luminous quasars, and determine some gross
geometric properties, e.g. whether the HII region is spherical or conical. A
hypothetical follow-up instrument with 10 times the collecting area of the LFD
(MWA-5000) will be capable of mapping the detailed geometry of HII regions,
while SKA will be capable of detecting very narrow spectral features as well as
the sharpness of the HII region boundary. The MWA-5000 will discover
serendipitous HII regions in widefield observations. We estimate the number of
HII regions which are expected to be generated by quasars. Assuming a late
reionization at z~6 we find that there should be several tens of quasar HII
regions larger than 4Mpc at z~6-8 per field of view. Identification of HII
regions in forthcoming 21cm surveys can guide a search for bright galaxies in
the middle of these regions. Most of the discovered galaxies would be the
massive hosts of dormant quasars that left behind fossil HII cavities that
persisted long after the quasar emission ended, owing to the long recombination
time of intergalactic hydrogen. A snap-shot survey of candidate HII regions
selected in redshifted 21cm image cubes may prove to be the most efficient
method for finding very high redshift quasars and galaxies.Comment: 14 pages, 8 figures. Submitted to Ap
Observing Lense-Thirring Precession in Tidal Disruption Flares
When a star is tidally disrupted by a supermassive black hole (SMBH), the
streams of liberated gas form an accretion disk after their return to
pericenter. We demonstrate that Lense-Thirring precession in the spacetime
around a rotating SMBH can produce significant time evolution of the disk
angular momentum vector, due to both the periodic precession of the disk and
the nonperiodic, differential precession of the bound debris streams. Jet
precession and periodic modulation of disk luminosity are possible
consequences. The persistence of the jetted X-ray emission in the Swift
J164449.3+573451 flare suggests that the jet axis was aligned with the spin
axis of the SMBH during this event.Comment: 4 pages, 4 figures. Accepted for publication in Physical Review
Letters. Minor changes made to match proof
Probing the Mass Fraction of MACHOs in Extragalactic Halos
Current microlensing searches calibrate the mass fraction of the Milky Way
halo which is in the form of Massive Compact Halo Objects (MACHOs). We show
that surveys like the Sloan Digital Sky Survey (SDSS) can probe the same
quantity in halos of distant galaxies. Microlensing of background quasars by
MACHOs in intervening galaxies would distort the equivalent width distribution
of the quasar emission lines by an amplitude that depends on the projected
quasar-galaxy separation. For a statistical sample of detectable at the >2sigma
level out to a quasar-galaxy impact parameter of several tens of kpc, as long
as extragalactic halos are made of MACHOs. Detection of this signal would test
whether the MACHO fraction inferred for the Milky-Way halo is typical of other
galaxies.Comment: 12 pages, 2 figures, submitted to ApJ Letter
Emission Spectra from Internal Shocks in Gamma-Ray-Burst Sources
Unsteady activity of gamma-ray burst sources leads to internal shocks in
their emergent relativistic wind. We study the emission spectra from such
shocks, assuming that they produce a power-law distribution of relativistic
electrons and posses strong magnetic fields. The synchrotron radiation emitted
by the accelerated electrons is Compton up-scattered multiple times by the same
electrons. A substantial component of the scattered photons acquires high
energies and produces e+e- pairs. The pairs transfer back their kinetic energy
to the radiation through Compton scattering. The generic spectral signature
from pair creation and multiple Compton scattering is highly sensitive to the
radius at which the shock dissipation takes place and to the Lorentz factor of
the wind. The entire emission spectrum extends over a wide range of photon
energies, from the optical regime up to TeV energies. For reasonable values of
the wind parameters, the calculated spectrum is found to be in good agreement
with the burst spectra observed by BATSE.Comment: 12 pages, latex, 2 figures, submitted to ApJ
Evolution of the 21 cm signal throughout cosmic history
The potential use of the redshifted 21 cm line from neutral hydrogen for
probing the epoch of reionization is motivating the construction of several
low-frequency interferometers. There is also much interest in the possibility
of constraining the initial conditions from inflation and the nature of the
dark matter and dark energy by probing the power-spectrum of density
perturbations in three dimensions and on smaller scales than probed by the
microwave background anisotropies. Theoretical understanding of the 21 cm
signal has been fragmented into different regimes of physical interest. In this
paper, we make the first attempt to describe the full redshift evolution of the
21 cm signal between 0<z<300.
We include contributions to the 21 cm signal from fluctuations in the gas
density, temperature and neutral fraction, as well as the Lyman alpha flux, and
allow for a post-reionization signal from damped Ly alpha systems. Our
comprehensive analysis provides a useful foundation for optimizing the design
of future arrays whose goal is to separate the particle physics from the
astrophysics, either by probing the peculiar velocity distortion of the 21 cm
power spectrum, or by extending the 21 cm horizon to z > 25 before the first
galaxies had formed, or to z < 6 when the residual pockets of hydrogen trace
large scale structure.Comment: 15 pages, 11 figures, submitted to PR
The Expected Rate of Gamma-Ray Burst Afterglows In Supernova Searches
We predict the rate at which Gamma-Ray Burst (GRB) afterglows should be
detected in supernova searches as a function of limiting flux. Although GRB
afterglows are rarer than supernovae, they are detectable at greater distances
because of their higher intrinsic luminosity. Assuming that GRBs trace the
cosmic star formation history and that every GRB gives rise to a bright
afterglow, we find that the average detection rate of supernovae and afterglows
should be comparable at limiting magnitudes brighter than K=18. The actual rate
of afterglows is expected to be somewhat lower since only a fraction of all
gamma-ray selected GRBs were observed to have associated afterglows. However,
the rate could also be higher if the initial gamma-ray emission from GRB
sources is more beamed than their late afterglow emission. Hence, current and
future supernova searches can place strong constraints on the afterglow
appearance fraction and the initial beaming angle of GRB sources.Comment: 13 pages, submitted to ApJ
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