392 research outputs found
Relation between the X-ray and Optical Luminosities in Binary Systems with Accreting Nonmagnetic White Dwarfs
We investigate the relation between the optical (g-band) and X-ray (0.5-10
keV) luminosities of accreting nonmagnetic white dwarfs. According to the
present-day counts of the populations of star systems in our Galaxy, these
systems have the highest space density among the close binary systems with
white dwarfs. We show that the dependence of the optical luminosity of
accreting white dwarfs on their X-ray luminosity forms a fairly narrow
one-parameter curve. The typical half-width of this curve does not exceed
0.2-0.3 dex in optical and X-ray luminosities, which is essentially consistent
with the amplitude of the aperiodic flux variability for these objects. At
X-ray luminosities Lx~1e32 erg/sec or lower, the optical g-band luminosity of
the accretion flow is shown to be related to its X-ray luminosity by a factor
~2-3. At even lower X-ray luminosities (Lx~1e30 erg/sec), the contribution from
the photosphere of the white dwarf begins to dominate in the optical spectrum
of the binary system and its optical brightness does not drop below Mg~13-14.
Using the latter fact, we show that in current and planned X-ray sky surveys,
the family of accreting nonmagnetic white dwarfs can be completely identified
to the distance determined by the sensitivity of an optical sky survey in this
region. For the Sloan Digital Sky Survey (SDSS) with a limiting sensitivity
m_g~22.5, this distance is ~400-600 pcComment: 7 pages, 3 figures, published in Astronomy Letter
Models of neutron star atmospheres enriched with nuclear burning ashes
Low-mass X-ray binaries hosting neutron stars (NS) exhibit thermonuclear
(type-I) X-ray bursts, which are powered by unstable nuclear burning of helium
and/or hydrogen into heavier elements deep in the NS "ocean". In some cases the
burning ashes may rise from the burning depths up to the NS photosphere by
convection, leading to the appearance of the metal absorption edges in the
spectra, which then force the emergent X-ray burst spectra to shift toward
lower energies. These effects may have a substantial impact on the color
correction factor and the dilution factor , the parameters of the
diluted blackbody model that is commonly used
to describe the emergent spectra from NSs. The aim of this paper is to quantify
how much the metal enrichment can change these factors. We have developed a new
NS atmosphere modeling code, which has a few important improvements compared to
our previous code required by inclusion of the metals. The opacities and the
internal partition functions (used in the ionization fraction calculations) are
now taken into account for all atomic species. In addition, the code is now
parallelized to counter the increased computational load. We compute a detailed
grid of atmosphere models with different exotic chemical compositions that
mimic the presence of the burning ashes. From the emerging model spectra we
compute the color correction factors and the dilution factors that
can then be compared to the observations. We find that the metals may change
by up to about 40%, which is enough to explain the scatter seen in the
blackbody radius measurements. The presented models open up the possibility for
determining NS mass and radii more accurately, and may also act as a tool to
probe the nuclear burning mechanisms of X-ray bursts.Comment: 14 pages, 7 figures, to be published in A&
High efficiency of soft X-ray radiation reprocessing in supersoft X-ray sources due to multiple scattering
Detailed analysis of the lightcurve of CAL 87 clearly has shown that the high
optical luminosity comes from the accretion disc rim and can only be explained
by a severe thickening of the disc rim near the location where the accretion
stream impinges. This area is irradiated by the X-rays where it faces the white
dwarf. Only if the reprocessing rate of X-rays to optical light is high a
luminosity as high as observed can be understood. But a recent detailed study
of the soft X-ray radiation reprocessing in supersoft X-ray sources has shown
that the efficiency is not high enough. We here propose a solution for this
problem. As already discussed in the earlier lightcurve analysis the impact of
the accretion stream at the outer disc rim produces a ``spray'', consisting of
a large number of individual gas blobs imbedded in a surrounding corona. For
the high mass flow rate this constitutes an optically thick vertically extended
screen at the rim of the accretion disc. We analyse the optical properties of
this irradiated spray and find that the multiple scattering between these gas
blobs leads to an effective reprocessing of soft X-rays to optical light as
required by the observations.Comment: 7 pages, 6 figures, accepted for publication in A&
Neutron star mass and radius measurements from atmospheric model fits to X-ray burst cooling tail spectra
Observations of thermonuclear X-ray bursts from accreting neutron stars (NSs)
in low-mass X-ray binary systems can be used to constrain NS masses and radii.
Most previous work of this type has set these constraints using Planck function
fits as a proxy: both the models and the data are fit with diluted blackbody
functions to yield normalizations and temperatures which are then compared
against each other. Here, for the first time, we fit atmosphere models of X-ray
bursting NSs directly to the observed spectra. We present a hierarchical
Bayesian fitting framework that uses state-of-the-art X-ray bursting NS
atmosphere models with realistic opacities and relativistic exact Compton
scattering kernels as a model for the surface emission. We test our approach
against synthetic data, and find that for data that are well-described by our
model we can obtain robust radius, mass, distance, and composition
measurements. We then apply our technique to Rossi X-ray Timing Explorer
observations of five hard-state X-ray bursts from 4U 1702-429. Our joint fit to
all five bursts shows that the theoretical atmosphere models describe the data
well but there are still some unmodeled features in the spectrum corresponding
to a relative error of 1-5% of the energy flux. After marginalizing over this
intrinsic scatter, we find that at 68% credibility the circumferential radius
of the NS in 4U 1702-429 is R = 12.4+-0.4 km, the gravitational mass is
M=1.9+-0.3 Msun, the distance is 5.1 < D/kpc < 6.2, and the hydrogen mass
fraction is X < 0.09.Comment: 15 pages, 11 figures, submitted to A&
Burst-induced coronal cooling in GS 1826-24 The clock wagging its tail
Type I X-ray bursts in GS 1826-24, and in several other systems, may induce
cooling of the hot inner accretion flow that surrounds the bursting neutron
star. Given that GS 1826-24 remained persistently in the hard state over the
period 2003-2008 and presented regular bursting properties, we stacked the
spectra of the X-ray bursts detected by INTEGRAL (JEM-X and ISGRI) and
XMM-Newton (RGS) during that period to study the effect of the burst photons on
the properties of the Comptonizing medium. The extended energy range provided
by these instruments allows the simultaneous observation of the burst and
persistent emission spectra. We detect an overall change in the shape of the
persistent emission spectrum in response to the burst photon shower. For the
first time, we observe simultaneously a drop in the hard X-ray emission,
together with a soft X-ray excess with respect to the burst blackbody emission.
The hard X-ray drop can be explained by burst-induced coronal cooling, while
the bulk of the soft X-ray excess can be described by fitting the burst
emission with an atmosphere model, instead of a simple blackbody model.
Traditionally, the persistent emission was assumed to be invariant during X-ray
bursts, and more recently to change only in normalization but not in spectral
shape; the observed change in the persistent emission level during X-ray bursts
may thus trigger the revision of existing neutron star mass-radius constraints,
as the derived values rely on the assumption that the persistent emission does
not change during X-ray bursts. The traditional burst fitting technique leads
to up to a 10% overestimation of the bolometric burst flux in GS 1826-24, which
significantly hampers the comparisons of the KEPLER and MESA model against this
'textbook burster'
Painlev\'e Transcendent Describes Quantum Correlation Function of the XXZ Antiferromagnet away from the free-fermion point
We consider quantum correlation functions of the antiferromagnetic
spin- Heisenberg XXZ spin chain in a magnetic field. We show that
for a magnetic field close to the critical field (for the critical
magnetic field the ground state is ferromagnetic) certain correlation functions
can be expressed in terms of the solution of the Painlev\'e V transcendent.
This establishes a relation between solutions of Painlev\'e differential
equations and quantum correlation functions in models of {\sl interacting}
fermions. Painlev\'e transcendents were known to describe correlation functions
in models with free fermionic spectra.Comment: 10 pages, LaTeX2
A New Low-Noise Avalanche Photodiode With Micro-Pixel Structure
A new design of the avalanche photodiodes with an array of micro-pixel p-n-junctions was developed on base of metal-oxide-silicon
structure. The thermal oxide layer of 1000A thickness contains tunnel oxide regions with about 25A thickness. The device exhibits a noise
factor ~ 4 at a high multiplication factor (M~10000). A high space uniformity of sensitivity was found for gain of M~ 1000
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