1,798 research outputs found
Particle decay in the early universe: predictions for 21 cm
The influence of ultra-high energy cosmic rays (UHECRs) and decaying dark
matter particles on the emission and absorption characteristics of neutral
hydrogen in 21 cm at redshifts is considered. In presence of UHECRs
21 cm can be seen in absorption with the brightness temperature
mK in the range . Decayng particles can stimulate a 21 cm signal in
emission with mK at , and mK at . Characteristics of the fluctuations of the brightness temperature, in
particular, its power spectrum are also calculated. The maps of the power
spectrum of the brightness temperature on the plane {\it wavenumber-redshift}
are shown to be sensitive to the parameters of UHECRs and decaying dark matter.
Observational possibilities to detect manifestations of UHECRs and/or decaying
particles in 21 cm with the future radio telescopes (LOFAR, 21CMA and SKA), and
to distinguish contributions from them are briefly discussed.Comment: 10 pages, 9 figures, accepted in MNRA
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 () \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 Imprint of Lithium Recombination on the Microwave Background Anisotropies
Following Loeb (2001), we explore the imprint of the resonant 6708A line
opacity of neutral lithium on the temperature and polarization anisotropies of
the cosmic microwave background (CMB) at observed wavelengths of 250-350 micron
(0.9-1.2 THz). We show that if lithium recombines in the redshift range of
z=400-500 as expected, then the standard CMB anisotropies would be
significantly modified in this wavelength band. The modified polarization
signal could be comparable to the expected polarization anisotropies of the
far-infrared background on sub-degree angular scales (l>100). Detection of the
predicted signal can be used to infer the primordial abundance of lithium, and
to probe structure in the Universe at z~500.Comment: 17 pages, 8 figures, submitted to ApJ (5/18/01
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
Large scale distribution of total mass versus luminous matter from Baryon Acoustic Oscillations: First search in the SDSS-III BOSS Data Release 10
Baryon Acoustic Oscillations (BAOs) in the early Universe are predicted to
leave an as yet undetected signature on the relative clustering of total mass
versus luminous matter. A detection of this effect would provide an important
confirmation of the standard cosmological paradigm and constrain alternatives
to dark matter as well as non-standard fluctuations such as Compensated
Isocurvature Perturbations (CIPs). We conduct the first observational search
for this effect, by comparing the number-weighted and luminosity-weighted
correlation functions, using the SDSS-III BOSS Data Release 10 CMASS sample.
When including CIPs in our model, we formally obtain evidence at of
the relative clustering signature and a limit that matches the existing upper
limits on the amplitude of CIPs. However, various tests suggest that these
results are not yet robust, perhaps due to systematic biases in the data. The
method developed in this Letter, used with more accurate future data such as
that from DESI, is likely to confirm or disprove our preliminary evidence.Comment: 6 pages, 2 figures, accepted for publication in PR
Synthesis of a new polypyridinic highly conjugated ligand with electron-acceptor properties
A new acceptor polypyridinic ligand functionalized with a quinone fragment is reported. The ligand, dipyrido[3,2-a:2′,3′-c]-benzo[3,4]-phenazine-11,16-quinone, Nqphen, was synthesized by condensation of 1,10-phenanthroline-5,6-dione and 2,3-diamino-1,4-naphthoquinone. The syntheses of two rhenium complexes with this ligand are also reported
Redshift determination in the X-ray band of gamma-ray bursts
If gamma-ray bursts originate in dense stellar forming regions, the
interstellar material can imprint detectable absorption features on the
observed X-ray spectrum. Such features can be detected by existing and planned
X-ray satellites, as long as the X-ray afterglow is observed after a few
minutes from the burst. If the column density of the interstellar material
exceeds ~10^{23} cm^{-2} there exists the possibility to detect the K_alpha
fluorescent iron line, which should be visible for more than one year, long
after the X-ray afterglow continuum has faded away. Detection of these X-ray
features will make possible the determination of the redshift of gamma-ray
bursts even when their optical afterglow is severely dimmed by extinction.Comment: 15 pages with 5 figures. Submitted to Ap
To rescue a star
Massless neutrinos are exchanged in a neutron star, leading to long range
interactions. Many body forces of this type follow and we resum them. Their net
contribution to the total energy is negligible as compared to the star mass.
The stability of the star is not in danger, contrary to recent assertions.Comment: 10 pages, Latex2e, two figure
Decompositions of Bn and \u3a0n using symmetric chains
We review the Green/Kleitman/Leeb interpretation of de Bruijn\u2019s symmetric chain decomposition of Bn, and explain how it can be used to find a maximal collection of disjoint symmetric chains in the nonsymmetric lattice of partitions of a set
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