2,747 research outputs found
The Burst and Transient Source Experiment Earth Occultation Technique
An Earth orbiting detector sensitive to gamma ray photons will see step-like
occultation features in its counting rate when a gamma ray point source crosses
the Earth's limb. This is due to the change in atmospheric attenuation of the
gamma rays along the line of sight. In an uncollimated detector, these
occultation features can be used to locate and monitor astrophysical sources
provided their signals can be individually separated from the detector
background. We show that the Earth occultation technique applied to the Burst
and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory
(CGRO) is a viable and flexible all-sky monitor in the low energy gamma ray and
hard X-ray energy range (20 keV - 1 MeV). The method is an alternative to more
sophisticated photon imaging devices for astronomy, and can serve well as a
cost-effective science capability for monitoring the high energy sky.
Here we describe the Earth occultation technique for locating new sources and
for measuring source intensity and spectra without the use of complex
background models. Examples of transform imaging, step searches, spectra, and
light curves are presented. Systematic uncertainties due to source confusion,
detector response, and contamination from rapid background fluctuations are
discussed and analyzed for their effect on intensity measurements. A sky
location-dependent average systematic error is derived as a function of
galactic coordinates. The sensitivity of the technique is derived as a function
of incident photon energy and also as a function of angle between the source
and the normal to the detector entrance window. Occultations of the Crab Nebula
by the Moon are used to calibrate Earth occultation flux measurements
independent of possible atmospheric scattering effects.Comment: 39 pages, 24 figures. Accepted for publication in the Astrophysical
Journal Supplement
Monte-Carlo simulations of thermal/nonthermal radiation from a neutron-star magnetospheric accretion shell
We discuss the space-and-time-dependent Monte Carlo code we have developed to
simulate the relativistic radiation output from compact astrophysical objects,
coupled to a Fokker-Planck code to determine the self-consistent lepton
populations. We have applied this code to model the emission from a magnetized
neutron star accretion shell near the Alfven radius, reprocessing the radiation
from the neutron sar surface. We explore the parameter space defined by the
accretion rate, stellar surface field and the level of wave turbulence in the
shell. Our results are relevant to the emission from atoll sources, soft-X-ray
transient X-ray binaries containing weakly magnetized neutron stars, and to
recently suggested models of accretion-powered emission from anomalous X-ray
pulsars.Comment: 24 pages, including 7 figures; uses epsf.sty. final version, accepted
for publication in ApJ. Extended introduction and discussio
The unusual volatile composition of the Halley-type comet 8P/Tuttle: Addressing the existence of an Inner Oort Cloud
We measured organic volatiles (CH4, CH3OH, C2H6, H2CO), CO, and water in
comet 8P/Tuttle, a comet from the Oort cloud reservoir now in a short-period
Halley-type orbit. We compare its composition with two other comets in
Halley-type orbits, and with comets of the "organics-normal" and
"organics-depleted" classes. Chemical gradients are expected in the
comet-forming region of the proto-planetary disk, and an individual comet
should reflect its specific heritage. If Halley-type comets came from the inner
Oort cloud as proposed, we see no common characteristics that could distinguish
such comets from those that were stored in the outer Oort cloud.Comment: 14 pages, including 1 figure and 2 Table
Neel state of antiferromagnet as a result of a local measurement in the distributed quantum system
Single-site measurement in a distributed macroscopic antiferromagnet is
considered; we show that it can create antiferromagnetic sublattices at
macroscopic scale. We demonstrate that the result of measurement depends on the
symmetry of the ground state: for the easy-axis case the Neel state is formed,
while for the easy-plane case unusual ``fan'' sublattices appear with unbroken
rotational symmetry, and a decoherence wave is generated. For the latter case,
a macroscopically large number of measurements is needed to pin down the
orientation of the sublattices, in spite of the high degeneracy of the ground
state. We note that the type of the final state and the appearance of the
decoherence wave are governed by the degree of entanglement of spins in the
system.Comment: 4 REVTeX pages, 1 figure in PostScrip
Correlation Between BATSE Hard X-ray Spectral and Timing Properties of Cygnus X-1
We have analyzed approximately 1100 days of Cygnus X-1 hard X-ray data
obtained with BATSE to study its rapid variability. We find for the first time
correlations between the slope of the spectrum and the hard X-ray intensity,
and between the spectral slope and the amplitude of the rapid variations of the
hard X-ray flux. We compare our results with expectations from current theories
of accretion onto black holes.Comment: 17 pages, 3 Postscript figures, uses aasms4.sty. Accepted for
publication in Astrophysical Journal Letter
Integrating ecology into macroevolutionary research
On 9 March, over 150 biologists gathered in London for the Centre for Ecology and Evolution spring symposium, âIntegrating Ecology into Macroevolutionary Researchâ. The event brought together researchers from London-based institutions alongside others from across the UK, Europe and North America for a day of talks. The meeting highlighted methodological advances and recent analyses of exemplar datasets focusing on the exploration of the role of ecological processes in shaping macroevolutionary patterns
The Burst and Transient Source Experiment (BATSE) Earth Occultation Catalog of Low-Energy Gamma-Ray Sources
The Burst and Transient Source Experiment (BATSE), aboard the Compton Gamma
Ray Observatory (CGRO), provided a record of the low-energy gamma-ray sky
(20-1000 keV) between 1991 April and 2000 May (9.1y). Using the Earth
Occultation Technique to extract flux information, a catalog of sources using
data from the BATSE large area detectors has been prepared. The first part of
the catalog consists of results from the monitoring of 58 sources, mostly
Galactic. For these sources, we have included tables of flux and spectral data,
and outburst times for transients. Light curves (or flux histories) have been
placed on the world wide web. We then performed a deep-sampling of 179 objects
(including the aforementioned 58 objects) combining data from the entire 9.1y
BATSE dataset. Source types considered were primarily accreting binaries, but a
small number of representative active galaxies, X-ray-emitting stars, and
supernova remnants were also included. The deep sample results include definite
detections of 83 objects and possible detections of 36 additional objects. The
definite detections spanned three classes of sources: accreting black hole and
neutron star binaries, active galaxies and supernova remnants. Flux data for
the deep sample are presented in four energy bands: 20-40, 40-70, 70-160, and
160-430 keV. The limiting average flux level (9.1 y) for the sample varies from
3.5 to 20 mCrab (5 sigma) between 20 and 430 keV, depending on systematic
error, which in turn is primarily dependent on the sky location. To strengthen
the credibility of detection of weaker sources (5-25 mCrab), we generated Earth
occultation images, searched for periodic behavior using FFT and epoch folding
methods, and critically evaluated the energy-dependent emission in the four
flux bands.Comment: 64 pages, 17 figures, abstract abridged, Accepted by ApJ
The X-ray Cluster Dipole
We estimate the dipole of the whole sky X-ray flux-limited sample of
Abell/ACO clusters (XBACs) and compare it to the optical Abell/ACO cluster
dipole. The X-ray cluster dipole is well aligned () with the
CMB dipole, while it follows closely the radial profile of its optical cluster
counterpart although its amplitude is per cent lower. In view of
the fact that the the XBACs sample is not affected by the volume incompleteness
and the projection effects that are known to exist at some level in the optical
parent Abell/ACO cluster catalogue, our present results confirm the previous
optical cluster dipole analysis that there are significant contributions to the
Local Group motion from large distances (Mpc). In order to
assess the expected contribution to the X-ray cluster dipole from a purely
X-ray selected sample we compare the dipoles of the XBACs and the Brightest
Cluster Sample (Ebeling et al. 1997a) in their overlap region. The resulting
dipoles are in mutual good aggreement with an indication that the XBACs sample
slightly underestimates the full X-ray dipole (by per cent) while the
Virgo cluster contributes about 10 - 15 per cent to the overall X-ray cluster
dipole. Using linear perturbation theory to relate the X-ray cluster dipole to
the Local group peculiar velocity we estimate the density parameter to be
.Comment: 16 pages, latex, + 4 ps figures, submitted to Ap
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