58,860 research outputs found
Reionization by Hard Photons: I. X-rays from the First Star Clusters
Observations of the Ly-alpha forest at z~3 reveal an average metallicity
Z~0.01 Z_solar. The high-redshift supernovae that polluted the IGM also
accelerated relativistic electrons. Since the energy density of the CMB scales
as (1+z)^4, at high redshift these electrons cool via inverse Compton
scattering. Thus, the first star clusters emit X-rays. Unlike stellar UV
ionizing photons, these X-rays can escape easily from their host galaxies. This
has a number of important physical consequences: (i) Due to their large mean
free path, these X-rays can quickly establish a universal ionizing background
and partially reionize the universe in a gradual, homogeneous fashion. If
X-rays formed the dominant ionizing background, the universe would have more
closely resembled a single-phase medium, rather than a two-phase medium. (ii)
X-rays can reheat the universe to higher temperatures than possible with UV
radiation. (iii) X-rays counter the tendency of UV radiation to
photo-dissociate H2, an important coolant in the early universe, by promoting
gas phase H2 formation. The X-ray production efficiency is calibrated to local
observations of starburst galaxies, which imply that ~10% of the supernova
energy is converted to X-rays. While direct detection of sources in X-ray
emission is difficult, the presence of relativistic electrons at high redshift
and thus a minimal level of X-ray emission may be inferred by synchrotron
emission observations with the Square Kilometer Array. These sources may
constitute a significant fraction of the unresolved hard X-ray background, and
can account for both the shape and amplitude of the gamma-ray background. This
paper discusses the existence and observability of high-redshift X-ray sources,
while a companion paper models the detailed reionization physics and chemistry.Comment: Final version accepted by ApJ. 32 pages, 3 figure
Polarization enhancement in ,He)n reaction: Nuclear teleportation
I show that an experimental technique used in nuclear physics may be
successfully applied to quantum teleportation (QT) of spin states of massive
matter. A new non-local physical effect the `quantum-teleportation-effect' is
discovered for the nuclear polarization measurement. Enhancement of the neutron
polarization is expected in the proposed experiment for QT that discriminates
{\it only} one of the Bell states.Comment: 3 pages, accepted for publication in J. Phys.
Maneuvering strategies using CMGs
This paper considers control strategies for maneuvering spacecraft using Single-Gimbal Control Momentum Gyros (CMGs). A pyramid configuration using four gyros is utilized. Preferred initial gimbal angles for maximum utilization of CMG momentum are obtained for some known torque commands. Feedback control laws are derived from the stability point of view by using the Liapunov's Second Theorem. The gyro rates are obtained by the pseudo-inverse technique. The effect of gimbal rate bounds on controllability are studied for an example maneuver. Singularity avoidance is based on limiting the gyro rates depending on a singularity index
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