40 research outputs found
RXTE observations of 4U 1630-47 during the peak of its 1998 outburst
We present an analysis of the RXTE observations of 4U 1630-47 during its
outburst of 1998. The light curve and the spectral evolution of the outburst
were distinctly different from the outbursts of the same source in 1996 and in
1999. Special emphasis of our analysis was on the observations taken during the
initial rise of the flux and during the maximum of the outburst. The maximum of
the outburst was divided into three plateaus, with almost constant flux within
each plateau, and fast jumps between them. The spectral and timing parameters
are stable for each individual plateau, but distinctly different between the
plateaus. The variability detected on the first plateau is of special interest.
During these observations the source exhibits quasi-regular modulations with
period of ~10 - 20 s. Our analysis revealed significant differences in spectral
and temporal behavior of the source at high and low fluxes during this period
of time. The source behavior can be generally explained in the framework of the
two-phase model of the accretion flow, involving a hot inner comptonization
region and surrounding optically thick disk. The variability and spectral
evolution of the source were similar to what was observed earlier for other
X-ray Novae. We show that 4U 1630-47 has a variety of properties which are
typical for Galactic black hole binaries, both transient and persistent. We
argue that this system may be an intermediate case between different groups of
black hole candidates.Comment: 14 pages, 12 figures, submitted to MNRA
On the Integrated Spectrum of the X-ray Binaries and the Origin of Soft X-ray Emission from the Bulge of M31
Using ROSAT PSPC data, we have performed several tests aimed at understanding
the origin of the soft X-ray spectral component detected from the bulge of M31.
We find that a significant soft component in the spectrum of the bulge is
spatially correlated with the unresolved X-ray emission near the core of M31,
which is probably a hot interstellar medium or perhaps a population of multiple
faint sources. For the first time, we extracted the spectrum of this unresolved
emission, by removing point sources dominating the integral spectrum of the
bulge, and found it to be responsible for the most of soft excess. A soft
spectral component is not at all needed to fit the point source spectrum that
remains after subtracting the unresolved emission. The integral spectra of
bright point sources, both inside and outside of the M31 bulge, can be fitted
with a single power-law in the ROSAT band. Our analysis rules out the previous
suggestion that all bulge emission in M31 may be generated by low mass X-ray
binaries (Irwin & Bregman, 1999).Comment: 11 pages incl. 2 figures, 2 tables, accepted to ApJ
Millenium Year X-ray Transients in Andromeda Galaxy
We study three transient X-ray sources, that were bright in the central
region of M31 galaxy in the year 2000. Observations with Chandra and XMM-Newton
allowed us for the first time in the history of X-ray astronomy, to build light
curves of transient sources in M31 suitable for studying their variability on a
time scale of months and, in some periods, weeks. The three sources demonstrate
distinctly different types of X-ray variability and spectral evolution. XMMU
J004234.1+411808 is most likely a black hole candidate based on the similarity
of its X-ray light curve and spectra to typical transient low-mass X-ray
binaries observed in our Galaxy. The outburst of CXO J004242.0+411608 lasted
longer than a year, which makes the source an unusual X-ray transient. The
supersoft transient XMMU J004319.4+ 411759 is probably a classical nova-like
system containing a magnetized, rapidly-spinning white dwarf. We estimate a
total rate of X-ray transient outbursts in the central bulge of M31 to be of
the order ~10 per year. The rate of the hard X-ray transients (~5 1/year) in
the central part of the Andromeda Galaxy appears to be comparable to that of
the central part of our own Galaxy.Comment: ApJ Letters, submitte
Bright X-ray Transients in M31: 2004 July XMM-Newton Observations
We present the results of X-ray observations of four bright transients
sources detected in the July 2004 XMM-Newton observations of the central bulge
of M31. Two X-ray sources, XMMU J004315.5+412440 and XMMU J004144.7+411110,
were discovered for the first time. Two other sources, CXOM31 J004309.9+412332
and CXOM31 J004241.8+411635, were previously detected by Chandra. The
properties of the sources suggest their identification with accreting binary
systems in M31. The X-ray spectra and variability of two sources, XMMU
J004144.7+411110 and CXOM31 J004241.8+411635, are similar to that of the
Galactic black hole transients, making them a good black hole candidates. The
X-ray source XMMU J004315.5+412440 demonstrates a dramatic decline of the X-ray
flux on a time scale of three days, and a remarkable flaring behavior on a
short time scales. The X-ray data on XMMU J004315.5+412440 and CXOM31
J004309.9+412332 suggest that they can be either black hole or neutron star
systems. Combining the results of 2000-2004 XMM observations of M31, we
estimate a total rate of the bright transient outbursts in the central region
of M31 to be 6-12 per year, in agreement with previous studies.Comment: 8 pages, 3 figures, uses emulateapj style. Submitted to Ap
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
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
XMM-Newton discovery of 217 s pulsations in the brightest persistent supersoft X-ray source in M31
We report on the discovery of a periodic modulation in the bright supersoft
X-ray source XMMU J004252.5+411540 detected in the 2000-2004 XMM-Newton
observations of M31. The source exhibits X-ray pulsations with a period P~217.7
s and a quasi-sinusoidal pulse shape and pulsed fraction ~7-11%. We did not
detect statistically significant changes in the pulsation period on the time
scale of 4 years. The X-ray spectra of XMMU J004252.5+411540 are extremely soft
and can be approximated with an absorbed blackbody of temperature 62-77 eV and
a weak power law tail of photon index ~1.7-3.1 in the 0.2-3.0 keV energy band.
The X-ray properties of the source and the absence of an optical/UV counterpart
brighter than 19 mag suggest that it belongs to M31. The estimated bolometric
luminosity of the source varies between ~2e38 and ~8e38 ergs/s at 760 kpc,
depending on the choice of spectral model. The X-ray pulsations and supersoft
spectrum of XMMU J004252.5+411540 imply that it is almost certainly an
accreting white dwarf, steadily burning hydrogen-rich material on its surface.
We interpret X-ray pulsations as a signature of the strong magnetic field of
the rotating white dwarf. Assuming that the X-ray source is powered by disk
accretion, we estimate its surface field strength to be in the range 4e5 G
<B_{0}<8e6 G. XMMU J004252.5+411540 is the second supersoft X-ray source in M31
showing coherent pulsations, after the transient supersoft source XMMU
J004319.4+411758 with 865.5 s pulsation period.Comment: 11 pages, 4 figures, uses emulateapj style. Submitted to Ap
The discovery of 2.78 hour periodic modulation of the X-ray flux from globular cluster source Bo 158 in M31
We report the discovery of periodic intensity dips in the X-ray source XMMU
J004314.1+410724, in the globular cluster Bo158 in M31. The X-ray flux was
modulated by ~83% at a period of 2.78 hr (10017 s) in an XMM-Newton observation
taken 2002 Jan 6-7. The X-ray intensity dips show no energy dependence. We
detected weaker dips with the same period in observations taken 2000 June 25
(XMM-Newton) and 1991 June 26 (ROSAT/PSPC). The amplitude of the modulation has
been found to be anticorrelated with source X-ray flux: it becomes lower when
the source intensity rises. The energy spectrum of Bo158 was stable from
observation to observation, with a characteristic cutoff at ~4-6 keV. The
photo-electric absorption was consistent with the Galactic foreground value. No
significant spectral changes were seen in the course of the dips. If the 2.78
hr cycle is the binary period of Bo158 the system is highly compact, with a
binary separation of ~10e11 cm. The association of the source with a globular
cluster, together with spectral parameters consistent with Galactic neutron
star sources, suggests that X-rays are emitted by an accreting neutron star.
The properties of Bo 158 are somewhat reminiscent of the Galactic X-ray sources
exhibiting a dip-like modulations. We discuss two possible mechanisms
explaining the energy-independent modulation observed in Bo 158: i) the
obscuration of the central source by highly ionized material that scatters
X-rays out of the line of sight; ii) partial covering of an extended source by
an opaque absorber which occults varying fractions of the source.Comment: 10 pages, 4 figures, ApJ, submitted, uses emulateapj styl