100 research outputs found
Detecting solar axions using Earth's magnetic field
We show that solar axion conversion to photons in the Earth's magnetosphere
can produce an x-ray flux, with average energy \sim 4 keV, which is measurable
on the dark side of the Earth. The smallness of the Earth's magnetic field is
compensated by a large magnetized volume. For axion masses < 10^{-4} eV, a
low-Earth-orbit x-ray detector with an effective area of 10^4 cm^2, pointed at
the solar core, can probe the photon-axion coupling down to 10^{-11} GeV^{-1},
in one year. Thus, the sensitivity of this new approach will be an order of
magnitude beyond current laboratory limits.Comment: 3 pages, 1 figure, typos corrected, references adde
MOXE: An X-ray all-sky monitor for Soviet Spectrum-X-Gamma Mission
A Monitoring Monitoring X-Ray Equipment (MOXE) is being developed for the Soviet Spectrum-X-Gamma Mission. MOXE is an X-ray all-sky monitor based on array of pinhole cameras, to be provided via a collaboration between Goddard Space Flight Center and Los Alamos National Laboratory. The objectives are to alert other observers on Spectrum-X-Gamma and other platforms of interesting transient activity, and to synoptically monitor the X-ray sky and study long-term changes in X-ray binaries. MOXE will be sensitive to sources as faint as 2 milliCrab (5 sigma) in 1 day, and cover the 2 to 20 KeV band
High-resolution Elemental Mapping of the Lunar Surface
New instruments and missions are being proposed to study the lunar surface as a result of the resurgence of interest in returning to the Moon. One instrument recently proposed is similar in concept to the x-ray fluorescence detectors flown on Apollo, but utilizes fluorescence from the L- and M-shells rather than the K-shell. This soft X-Ray Flourescence Imager (XRFI) is discussed
On the Correlated X-ray and Optical Evolution of SS Cygni
We have analyzed the variability and spectral evolution of the prototype
dwarf nova system SS Cygni using RXTE data and AAVSO observations. A series of
pointed RXTE/PCA observations allow us to trace the evolution of the X-ray
spectrum of SS Cygni in unprecedented detail, while 6 years of optical AAVSO
and RXTE/ASM light curves show long-term patterns. Employing a technique in
which we stack the X-ray flux over multiple outbursts, phased according to the
optical light curve, we investigate the outburst morphology. We find that the
3-12 keV X-ray flux is suppressed during optical outbursts, a behavior seen
previously, but only in a handful of cycles. The several outbursts of SS Cygni
observed with the more sensitive RXTE/PCA also show a depression of the X-rays
during optical outburst. We quantify the time lags between the optical and
X-ray outbursts, and the timescales of the X-ray recovery from outburst. The
optical light curve of SS Cygni exhibits brief anomalous outbursts. During
these events the hard X-rays and optical flux increase together. The long-term
data suggest that the X-rays decline between outburst. Our results are in
general agreement with modified disk instability models (DIM), which invoke a
two-component accretion flow consisting of a cool optically thick accretion
disk truncated at an inner radius, and a quasi-spherical hot corona-like flow
extending to the surface of the white dwarf. We discuss our results in the
framework of one such model, involving the evaporation of the inner part of the
optically thick accretion disk, proposed by Meyer & Meyer-Hofmeister (1994).Comment: 24 pages, 8 figures, 2 tables, accepted for publication in Ap
An Unusual X-ray Burst from the Globular Cluster M28
We report the discovery of an unusual X-ray burst from the direction of the
Globular Cluster M28 using data acquired with the ASCA Observatory. The burst
was recorded by all four ASCA telescopes and displays a fast (~ 70 ms) rise
followed by an exponential decay (t = 7.5 s) and a steady afterglow which lasts
between 800 - 3250 s. The image of the burst is consistent with an ASCA point
source and is centered on quiescent X-ray emission from the core of M28. The
burst temporal profile is similar to Type-I bursts emitted by accreting neutron
stars of low mass X-ray binaries (LMXB). We argue that the burst arises from an
LMXB that is located in the core of M28. The burst is unique in two ways: it is
intrinsically sub-luminous, ~ 0.02 L_Edd and more importantly, originates from
a source whose quiescent luminosity is fainter than that of the known cluster
bursters by three orders of magnitude. We suggest that this burst is from a
highly magnetized neutron star accreting at a low rate. These accreting systems
may account for the mysterious low luminosity X-ray sources in globular
clusters.Comment: 13 pages with 3 eps figures, LATEX, aastex, psfig. To appear in the
Astrophysical Journal Letter
Warping of Accretion Disks with Magnetically Driven Outflows: A Possible Origin for Jet Precession
Current theoretical models for the outflows/jets from AGN, X-ray binaries and
young stellar objects involve large-scale magnetic fields threading an
underlying accretion disk. We suggest that such a disk is subjected to warping
instability and retrograde precession driven by magnetic torques associated
with the outflow. The growth timescale for the disk warp and the precession
period are of order the radial infall time of the disk. These effects may be
relevant to jet precession and other variabilities observed in many systems.Comment: 4 pages with 2 figures. ApJL in pres
Fast X-ray Transients and Their Connection to Gamma-Ray Bursts
Fast X-ray transients (FXTs) with timescales from seconds to hours have been
seen by numerous space instruments. We have assembled archival data from
Ariel-5, HEAO-1 (A-1 and A-2), WATCH, ROSAT, and Einstein to produce a global
fluence-frequency relationship for these events. Fitting the log N-log S
distribution over several orders of magnitude to simple power law we find a
slope of -1.0. The sources of FXTs are undoubtedly heterogeneous, the -1 power
law is an approximate result of the summation of these multiple sources. Two
major contributions come from gamma-ray bursts and stellar flares.
Extrapolating from the BATSE catalog of GRBs, we find that the fraction of
X-ray flashes that can be the X-ray counterparts of gamma-ray bursts is a
function of fluence. Certainly most FXTs are not counterparts of standard
gamma-ray bursts. The fraction of FXTs from non-GRB sources, such as magnetic
stars, is greatest for the faintest FXTs. Our understanding of the FXT
phenomenon remains limited and would greatly benefit from a large, homogeneous
data set, which requires a wide-field, sensitive instrument.Comment: 36 pages, 8 figure
Changes in the long term intensity variations in Cyg X-2 and LMC X-3
We report the detection of changes in the long-term intensity variations in
two X-ray binaries, Cyg X-2 and LMC X-3. In this work, we have used the
long-term light curves obtained with the All-Sky Monitors (ASMs) of the Rossi
X-Ray Timing Explorer (RXTE), Ginga, Ariel 5, and Vela 5B and the scanning
modulation collimator of HEAO 1. It is found that in the light curves of both
the sources, obtained with these instruments at various times over the last 30
years, more than one periodic or quasi-periodic component is always present.
The multiple prominent peaks in the periodograms have frequencies unrelated to
each other. In Cyg X-2, RXTE-ASM data show strong peaks at 40.4 and 68.8 days,
and Ginga-ASM data show strong peaks at 53.7 and 61.3 days. Multiple peaks are
also observed in LMC X-3. The various strong peaks in the periodograms of LMC
X-3 appear at 104, 169, and 216 days (observed with RXTE-ASM) and 105, 214, and
328 days (observed with Ginga-ASM). The present results, when compared with the
earlier observations of periodicities in these two systems, demonstrate the
absence of any stable long period. The 78 day periodicity detected earlier in
Cyg X-2 was probably due to the short time base in the RXTE data that were
used, and the periodicity of 198 days in LMC X-3 was due to a relatively short
duration of observation with HEAO 1.Comment: 11 pages, 7 postscript figures include
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