707 research outputs found
Possible Magnetic Activity in the Low Mass X-ray Binary EXO 0748-676
We report evidence of magnetic activity associated with the secondary star in
the EXO 0748-676 low mass X-ray binary system. An analysis of a sequence of
five consecutive X-ray eclipses observed during December 2003 with the RXTE
satellite brings out a feature occurring during ingress we interpret as the
X-ray photoelectric absorption shadow, as seen by an observer at Earth, of a
plasma structure suspended above the surface of the secondary star. The light
curve feature consists of an initial drop in count rate to near zero (the
absorption shadow) with a very short rebound to a significant fraction of the
pre-ingress count rate and then a final plunge to totality over a total time
scale of ~25 s. The ingress feature persists for at least 5 consecutive orbital
periods (a total of ~19 hr), and possibly up to 5 days in our data. Our data
also show significant post-egress dipping during this eclipse sequence, unusual
for this source, indicating possible secondary star mass ejection during this
episode.Comment: Accepted by Astrophysical Journal Letters; 11 Pages including 3
figures and 1 tabl
Scaling and Asymptotic Scaling in the SU(2) Gauge Theory
We determine the critical couplings for the deconfinement phase transition in
gauge theory on lattices with
and 16 and varying between 16 and 48. A comparison with string
tension data shows scaling of the ratio in the entire
coupling regime , while the individual quantities still
exhibit large scaling violations. We find . We
also discuss in detail the extrapolation of and to the continuum
limit. Our result, which is consistent with the above ratio, is and . We also comment upon corresponding
results for gauge theory and four flavour QCD.Comment: 27 pages with 9 postscript figures included. Plain TeX file (needed
macros are included). BI-TP 92-26, FSU-SCRI-92-103, HLRZ-92-39 (Quote of
UKQCD string tension, and accordingly Figs. 5 and 7a, plus a few typo's
corrected.
Dust aerosol, clouds, and the atmospheric optical depth record over 5 Mars years of the Mars Exploration Rover mission
Dust aerosol plays a fundamental role in the behavior and evolution of the
Martian atmosphere. The first five Mars years of Mars Exploration Rover data
provide an unprecedented record of the dust load at two sites. This record is
useful for characterization of the atmosphere at the sites and as ground truth
for orbital observations. Atmospheric extinction optical depths have been
derived from solar images after calibration and correction for time-varying
dust that has accumulated on the camera windows. The record includes local,
regional, and globally extensive dust storms. Comparison with contemporaneous
thermal infrared data suggests significant variation in the size of the dust
aerosols, with a 1 {\mu}m effective radius during northern summer and a 2
{\mu}m effective radius at the onset of a dust lifting event. The solar
longitude (LS) 20-136{\deg} period is also characterized by the presence of
cirriform clouds at the Opportunity site, especially near LS=50 and 115{\deg}.
In addition to water ice clouds, a water ice haze may also be present, and
carbon dioxide clouds may be present early in the season. Variations in dust
opacity are important to the energy balance of each site, and work with
seasonal variations in insolation to control dust devil frequency at the Spirit
site.Comment: 60 pages, 12 figures, to be published in Icaru
The Ever Changing Circumstellar Nebula Around UW Centauri
We present new images of the reflection nebula surrounding the R Coronae
Borealis Star, UW Cen. This nebula, first detected in 1990, has changed its
appearance significantly. At the estimated distance of UW Cen, this nebula is
approximately 0.6 ly in radius so the nebula cannot have physically altered in
only 8 years. Instead, the morphology of the nebula appears to change as
different parts are illuminated by light from the central star modulated by
shifting thick dust clouds near its surface. These dust clouds form and
dissipate at irregular intervals causing the well-known declines in the R
Coronae Borealis (RCB) stars. In this way, the central star acts like a
lighthouse shining through holes in the dust clouds and lighting up different
portions of the nebula. The existence of this nebula provides clues to the
evolutionary history of RCB stars possibly linking them to the Planetary
Nebulae and the final helium shell flash stars.Comment: To be published in ApJ Letters. 5 pages, 3 figures (2 in color
Optical/IR from ground
Optical/infrared (O/IR) astronomy in the 1990's is reviewed. The following subject areas are included: research environment; science opportunities; technical development of the 1980's and opportunities for the 1990's; and ground-based O/IR astronomy outside the U.S. Recommendations are presented for: (1) large scale programs (Priority 1: a coordinated program for large O/IR telescopes); (2) medium scale programs (Priority 1: a coordinated program for high angular resolution; Priority 2: a new generation of 4-m class telescopes); (3) small scale programs (Priority 1: near-IR and optical all-sky surveys; Priority 2: a National Astrometric Facility); and (4) infrastructure issues (develop, purchase, and distribute optical CCDs and infrared arrays; a program to support large optics technology; a new generation of large filled aperture telescopes; a program to archive and disseminate astronomical databases; and a program for training new instrumentalists
No First-Order Phase Transition in the Gross-Neveu Model?
Within a variational calculation we investigate the role of baryons for the
structure of dense matter in the Gross-Neveu model. We construct a trial ground
state at finite baryon density which breaks translational invariance. Its
scalar potential interpolates between widely spaced kinks and antikinks at low
density and the value zero at infinite density. Its energy is lower than the
one of the standard Fermi gas at all densities considered. This suggests that
the discrete gamma_5 symmetry of the Gross-Neveu model does not get restored in
a first order phase transition at finite density, at variance with common
wisdom.Comment: 16 pages, 7 figures, LaTe
AXTAR: Mission Design Concept
The Advanced X-ray Timing Array (AXTAR) is a mission concept for X-ray timing
of compact objects that combines very large collecting area, broadband spectral
coverage, high time resolution, highly flexible scheduling, and an ability to
respond promptly to time-critical targets of opportunity. It is optimized for
submillisecond timing of bright Galactic X-ray sources in order to study
phenomena at the natural time scales of neutron star surfaces and black hole
event horizons, thus probing the physics of ultradense matter, strongly curved
spacetimes, and intense magnetic fields. AXTAR's main instrument, the Large
Area Timing Array (LATA) is a collimated instrument with 2-50 keV coverage and
over 3 square meters effective area. The LATA is made up of an array of
supermodules that house 2-mm thick silicon pixel detectors. AXTAR will provide
a significant improvement in effective area (a factor of 7 at 4 keV and a
factor of 36 at 30 keV) over the RXTE PCA. AXTAR will also carry a sensitive
Sky Monitor (SM) that acts as a trigger for pointed observations of X-ray
transients in addition to providing high duty cycle monitoring of the X-ray
sky. We review the science goals and technical concept for AXTAR and present
results from a preliminary mission design study.Comment: 19 pages, 10 figures, to be published in Space Telescopes and
Instrumentation 2010: Ultraviolet to Gamma Ray, Proceedings of SPIE Volume
773
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