2,674 research outputs found
Numerical solution of the radiative transfer equation: X-ray spectral formation from cylindrical accretion onto a magnetized neutron star
Predicting the emerging X-ray spectra in several astrophysical objects is of
great importance, in particular when the observational data are compared with
theoretical models. To this aim, we have developed an algorithm solving the
radiative transfer equation in the Fokker-Planck approximation when both
thermal and bulk Comptonization take place. The algorithm is essentially a
relaxation method, where stable solutions are obtained when the system has
reached its steady-state equilibrium. We obtained the solution of the radiative
transfer equation in the two-dimensional domain defined by the photon energy E
and optical depth of the system tau using finite-differences for the partial
derivatives, and imposing specific boundary conditions for the solutions. We
treated the case of cylindrical accretion onto a magnetized neutron star. We
considered a blackbody seed spectrum of photons with exponential distribution
across the accretion column and for an accretion where the velocity reaches its
maximum at the stellar surface and at the top of the accretion column,
respectively. In both cases higher values of the electron temperature and of
the optical depth tau produce flatter and harder spectra. Other parameters
contributing to the spectral formation are the steepness of the vertical
velocity profile, the albedo at the star surface, and the radius of the
accretion column. The latter parameter modifies the emerging spectra in a
specular way for the two assumed accretion profiles. The algorithm has been
implemented in the XSPEC package for X-ray spectral fitting and is specifically
dedicated to the physical framework of accretion at the polar cap of a neutron
star with a high magnetic field (> 10^{12} G), which is expected to be typical
of accreting systems such as X-ray pulsars and supergiant fast X-ray
transients.Comment: 13 pages, 20 figures, accepted for publication in A&
Wide band observations of the X-ray burster GS 1826-238
GS 1826-238 is a well-studied X-ray bursting neutron star in a low mass
binary system. Thermal Comptonisation by a hot electron cloud is a widely
accepted mechanism accounting for its high energy emission, while the nature of
most of its soft X-ray output is not completely understood. A further low
energy component is typically needed to model the observed spectra: pure
blackbody and Comptonisation-modified blackbody radiation by a lower
temperature (a few keV) electron plasma were suggested to explain the low
energy data. We studied the steady emission of GS 1826-238 by means of broad
band (X to soft Gamma-rays) measurements obtained by the INTEGRAL observatory
in 2003 and 2006. The newly developed, up-to-date Comptonisation model CompTB
is applied for the first time to study effectively the low-hard state
variability of a low-luminosity neutron star in a low-mass X-ray binary system.
We confirm that the 3-200 keV emission of \GS is characterised by
Comptonisation of soft seed photons by a hot electron plasma. A single spectral
component is sufficient to model the observed spectra. At lower energies, no
direct blackbody emission is observed and there is no need to postulate a low
temperature Compton region. Compared to the 2003 measurements, the plasma
temperature decreased from 20 to 14 keV in 2006, together with the seed photons
temperature. The source intensity was also found to be 30% lower in 2006,
whilst the average recurrence frequency of the X-ray bursts significantly
increased. Possible explanations for this apparent deviation from the typical
limit-cycle behaviour of this burster are discussed.Comment: 6 pages, 2 figures. Accepted for publication in A&
On the stability of the thermal Comptonization index in neutron star low-mass X-ray binaries in their different spectral states
Most of the spectra of neutron star low mass X-ray binaries (NS LMXBs), being
them persistent or transient, are characterized by the presence of a strong
thermal Comptonization bump, thought to originate in the transition layer (TL)
between the accretion disk and the NS surface. The observable quantities which
characterize this component dominating the emission below 30 keV, are the
spectral index alpha and the rollover energy, both related to the electron
temperature and optical depth of the plasma. Starting from observational
results on a sample of NS LMXBs in different spectral states, we formulate the
problem of X-ray spectral formation in the TL of these sources. We predict a
stability of the thermal Comptonization spectral index in different spectral
states if the energy release in the TL is much higher than the intercepted flux
coming from the accretion disk. We use an equation for the energy balance and
the radiative transfer diffusion equation for a slab geometry in the TL, to
derive a formula for the thermal Comptonization index alpha. We show that in
this approximation the TL electron temperature kTe and optical depth tau_0 can
be written as a function of the energy flux from the disk intercepted by the
corona (TL) and that in the corona itself Qdisk/Qcor, in turn leading to a
relation alpha=f(Qdisk/Qcor), with alpha ~ 1 when Qdisk/Qcor <<1. We show that
the observed spectral index alpha for the sample of sources here considered
lies in a belt around 1 +/- 0.2 a part for the case of GX 354--0. Comparing our
theoretical predictions with observations, we claim that this result, which is
consistent with the condition Qdisk/Qcor <<1, can give us constraints on the
accretion geometry of these systems, an issue that seems difficult to be solved
using only the spectral analysis method.Comment: 7 pages, 3 figures, accepted for publication in A&
Ivermectin Treatment of a Traveler Who Returned from Peru with Cutaneous Gnathostomiasis
We describe a 21-year-old patient who experienced a relapse of cutaneous gnathostomiasis after receiving initial treatment with albendazole and who had a successful outcome after receiving a short course of ivermectin for the relapse. This is the first reported case of gnathostomiasis acquired by a human in Per
Comptonization in Ultra-Strong Magnetic Fields: Numerical Solution to the Radiative Transfer Problem
We consider the radiative transfer problem in a plane-parallel slab of thermal electrons in the presence of an ultra-strong magnetic field (B approximately greater than B(sub c) approx. = 4.4 x 10(exp 13) G). Under these conditions, the magnetic field behaves like a birefringent medium for the propagating photons, and the electromagnetic radiation is split into two polarization modes, ordinary and extraordinary, that have different cross-sections. When the optical depth of the slab is large, the ordinary-mode photons are strongly Comptonized and the photon field is dominated by an isotropic component. Aims. The radiative transfer problem in strong magnetic fields presents many mathematical issues and analytical or numerical solutions can be obtained only under some given approximations. We investigate this problem both from the analytical and numerical point of view, provide a test of the previous analytical estimates, and extend these results with numerical techniques. Methods. We consider here the case of low temperature black-body photons propagating in a sub-relativistic temperature plasma, which allows us to deal with a semi-Fokker-Planck approximation of the radiative transfer equation. The problem can then be treated with the variable separation method, and we use a numerical technique to find solutions to the eigenvalue problem in the case of a singular kernel of the space operator. The singularity of the space kernel is the result of the strong angular dependence of the electron cross-section in the presence of a strong magnetic field. Results. We provide the numerical solution obtained for eigenvalues and eigenfunctions of the space operator, and the emerging Comptonization spectrum of the ordinary-mode photons for any eigenvalue of the space equation and for energies significantly lesser than the cyclotron energy, which is on the order of MeV for the intensity of the magnetic field here considered. Conclusions. We derived the specific intensity of the ordinary photons, under the approximation of large angle and large optical depth. These assumptions allow the equation to be treated using a diffusion-like approximation
The X-ray spectrum of the bursting atoll source 4U~1728-34 observed with INTEGRAL
We present for the first time a study of the 3-200 keV broad band spectra of
the bursting atoll source 4U 1728-34 (GX 354-0) along its hardness intensity
diagram. The analysis was done using the INTEGRAL public and Galactic Center
deep exposure data ranging from February 2003 to October 2004. The spectra are
well described by a thermal Comptonization model with an electron temperature
from 35 keV to 3 keV and Thomson optical depth, tau_T, from 0.5 to 5 in a slab
geometry. The source undergoes a transition from an intermediate/hard to a soft
state where the source luminosity increases from 2 to 12% of Eddington. We have
also detected 36 type I X-ray bursts two of which show photospheric radius
expansion. The energetic bursts with photospheric radius expansion occurred at
an inferred low mass accretion rate per unit area of \dot m ~ 1.7x10E3 g/cm2/s,
while the others at a higher one between 2.4x10E3 - 9.4x10E3 g/cm2/s. For
4U1728-34 the bursts' total fluence, and the bursts' peak flux are
anti-correlated with the mass accretion rate. The type I X-ray bursts involve
pure helium burning either during the hard state, or during the soft state of
the source.Comment: 11 pages, 7 figures, and 2 tables. Accepted for publication in A&
The X-ray spectral evolution of Cyg X-2 in the framework of bulk Comptonization
We used the newly developed thermal plus bulk Comptonization model comptb to
investigate the spectral evolution of the neutron star LMXB Cyg X-2 along its
Z-track. We selected a single source in order to trace in a quantitative way
the evolution of the physical parameters of the model. We analyzed archival
broad-band BeppoSAX spectra of Cyg X-2. Five broad-band spectra have been newly
extracted according to the source position in the Z-track described in the
colour-colour and colour-intensity diagrams. We have fitted the spectra of the
source with two comptb components. The first one, with bulk parameter delta=0,
dominates the overall source broad-band spectrum and its origin is related to
thermal upscattering (Comptonization) of cold seed photons off warm electrons
in high-opacity enviroment. We attribute the origin of these seed photons to
the part of the disk which illuminates the outer coronal region (transition
layer) located between the accretion disk itself and the neutron star surface.
This thermal component is roughly constant with time and with inferred mass
accretion rate. The second comptb model describes the overall Comptonization
(thermal plus bulk, delta > 0) of hotter seed photons which come from both the
inner transition layer and from the neutron star surface. The appearance of
this component in the colour-colour or hardness-intensity diagram is more
pronounced in the horizontal branch and is progressively disappearing towards
the normal branch, where a pure blackbody spectrum is observed. The spectral
evolution of Cyg X-2 is studied and interpreted in terms of changes in the
innermost environmental conditions of the system, leading to a variable
thermal-bulk Comptonization efficiency.Comment: 10 pages, 7 figures, accepted for publication in A&
Spectral evolution of bright NS LMXBs with INTEGRAL: an application of the thermal plus bulk Comptonization model
The aim of this work is to investigate in a physical and quantitative way the
spectral evolution of bright Neutron Star Low-Mass X-ray Binaries (NS LMXBs),
with special regard to the transient hard X-ray tails. We analyzed INTEGRAL
data for five sources (GX 5-1, GX 349+2, GX 13+1, GX 3+1, GX 9+1) and built
broad-band X-ray spectra from JEM-X1 and IBIS/ISGRI data. For each source,
X-ray spectra from different states were fitted with the recently proposed
model compTB. The spectra have been fit with a two-compTB model. In all cases
the first compTB describes the dominant part of the spectrum that we interpret
as thermal Comptonization of soft seed photons (< 1 keV), likely from the
accretion disk, by a 3-5 keV corona. In all cases, this component does not
evolve much in terms of Comptonization efficiency, with the system converging
to thermal equilibrium for increasing accretion rate. The second compTB varies
more dramatically spanning from bulk plus thermal Comptonization of blackbody
seed photons to the blackbody emission alone. These seed photons (R < 12 km,
kT_s > 1 keV), likely from the neutron star and the innermost part of the
system, the Transition Layer, are Comptonized by matter in a converging flow.
The presence and nature of this second compTB component (be it a pure blackbody
or Comptonized) are related to the inner local accretion rate which can
influence the transient behaviour of the hard tail: high values of accretion
rates correspond to an efficient Bulk Comptonization process (bulk parameter
delta > 0) while even higher values of accretion rates suppress the
Comptonization, resulting in simple blackbody emission (delta=0).Comment: 12 pages, 10 figures, accepted for publication in A&
Monte Carlo Tree Search Planning for continuous action and state space
Sequential decision-making in real-world environments is an important problem of artificial intelligence and robotics. In the last decade reinforcement learning has provided effective solutions in small and simulated environments but it has also shown some limits on large and real-world domains characterized by continuous state and action spaces. In this work, we aim to evaluate some state-of-the-art algorithms based on Monte Carlo Tree Search planning in continuous state/action spaces and propose a first version of a new algorithm based on action widening. Algorithms are evaluated on a synthetic domain in which the agent aims to control a car through a narrow curve for reaching the goal in the shortest possible time and avoiding the car going off the road. We show that the proposed method outperforms the state-of-the-art techniques
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