2,792 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&
A cylindrical GEM detector for BES III
BESIII is a particle physics experiment located at the Institute of
High-Energy Physics (BEPC-II) e+e- collider at IHEP in Beijing. The Italian
collaboration is leading the effort for the development of a cylindrical GEM
(CGEM) detector with analog readout to upgrade the current inner drift chamber
that is suffering early ageing due to the increase of the machine luminosity.
Within the CGEM project, this work aims to perform full detector simulation for
the optimisation of the tracker geometry and its operational parameters. The
goal is achieved by means of three different, but well connected, studies: a
background estimation, a simulation of the detection elements and the data
analysis of a beam test. I participated to the construction of the cathode
electrode, that was produced in Ferrara, helping during the assembling and
manufacturing procedures. For the background studies and the detector
simulation I took care both of the framework development and of the data
analysis. Finally, I participated to installation and data taking of the beam
test at CERN, and I was involved in the production of the reconstruction and
analysis software. During and after the beam test I participated to the
processing and analysis of the data
Decentralised Coordination of Low-Power Embedded Devices Using the Max-Sum Algorithm
This paper considers the problem of performing decentralised coordination of low-power embedded devices (as is required within many environmental sensing and surveillance applications). Specifically, we address the generic problem of maximising social welfare within a group of interacting agents. We propose a novel representation of the problem, as a cyclic bipartite factor graph, composed of variable and function nodes (representing the agentsâ states and utilities respectively). We show that such representation allows us to use an extension of the max-sum algorithm to generate approximate solutions to this global optimisation problem through local decentralised message passing. We empirically evaluate this approach on a canonical coordination problem (graph colouring), and benchmark it against state of the art approximate and complete algorithms (DSA and DPOP). We show that our approach is robust to lossy communication, that it generates solutions closer to those of DPOP than DSA is able to, and that it does so with a communication cost (in terms of total messages size) that scales very well with the number of agents in the system (compared to the exponential increase of DPOP). Finally, we describe a hardware implementation of our algorithm operating on low-power Chipcon CC2431 System-on-Chip sensor nodes
Resource-Aware Junction Trees for Efficient Multi-Agent Coordination
In this paper we address efficient decentralised coordination of cooperative multi-agent systems by taking into account the actual computation and communication capabilities of the agents. We consider coordination problems that can be framed as Distributed Constraint Optimisation Problems, and as such, are suitable to be deployed on large scale multi-agent systems such as sensor networks or multiple unmanned aerial vehicles. Specifically, we focus on techniques that exploit structural independence among agentsâ actions to provide optimal solutions to the coordination problem, and, in particular, we use the Generalized Distributive Law (GDL) algorithm. In this settings, we propose a novel resource aware heuristic to build junction trees and to schedule GDL computations across the agents. Our goal is to minimise the total running time of the coordination process, rather than the theoretical complexity of the computation, by explicitly considering the computation and communication capabilities of agents. We evaluate our proposed approach against DPOP, RDPI and a centralized solver on a number of benchmark coordination problems, and show that our approach is able to provide optimal solutions for DCOPs faster than previous approaches. Specifically, in the settings considered, when resources are scarce our approach is up to three times faster than DPOP (which proved to be the best among the competitors in our settings)
A new model for the X-ray continuum of the magnetized accreting pulsars
Accreting highly magnetized pulsars in binary systems are among the brightest
X-ray emitters in our Galaxy. Although a number of high statistical quality
broad-band (0.1-100 keV) X-ray observations are available, the spectral energy
distribution of these sources is usually investigated by adopting pure
phenomenological models, rather than models linked to the physics of accretion.
In this paper, a detailed spectral study of the X-ray emission recorded from
the high-mass X-ray binary pulsars Cen X-3, 4U 0115+63, and Her X-1 is carried
out by using BeppoSAX and joined Suzaku+NuStar data, together with an advanced
version of the compmag model. The latter provides a physical description of the
high energy emission from accreting pulsars, including the thermal and bulk
Comptonization of cyclotron and bremsstrahlung seed photons along the neutron
star accretion column. The compmag model is based on an iterative method for
solving second-order partial differential equations, whose convergence
algorithm has been improved and consolidated during the preparation of this
paper. Our analysis shows that the broad-band X-ray continuum of all considered
sources can be self-consistently described by the compmag model. The cyclotron
absorption features, not included in the model, can be accounted for by using
Gaussian components. From the fits of the compmag model to the data we inferred
the physical properties of the accretion columns in all sources, finding values
reasonably close to those theoretically expected according to our current
understanding of accretion in highly magnetized neutron stars. The updated
version of the compmag model has been tailored to the physical processes that
are known to occur in the columns of highly magnetized accreting neutron stars
and it can thus provide a better understanding of the high energy radiation
from these sources.Comment: 19 pages, 10 figures, accepted for publication in A&
RX J0440.9+4431: a persistent Be/X-ray binary in outburst
The persistent Be/X-ray binary RX J0440.9+4431 flared in 2010 and 2011 and
has been followed by various X-ray facilities Swift, RXTE, XMM-Newton, and
INTEGRAL. We studied the source timing and spectral properties as a function of
its X-ray luminosity to investigate the transition from normal to flaring
activity and the dynamical properties of the system. We have determined the
orbital period from the long-term Swift/BAT light curve, but our determinations
of the spin period are not precise enough to constrain any orbital solution.
The source spectrum can always be described by a bulk-motion Comptonization
model of black body seed photons attenuated by a moderate photoelectric
absorption. At the highest luminosity, we measured a curvature of the spectrum,
which we attribute to a significant contribution of the radiation pressure in
the accretion process. This allows us to estimate that the transition from a
bulk-motion-dominated flow to a radiatively dominated one happens at a
luminosity of ~2e36 erg/s. The luminosity dependency of the size of the black
body emission region is found to be . This
suggests that either matter accreting onto the neutron star hosted in RX
J0440.9+4431 penetrates through closed magnetic field lines at the border of
the compact object magnetosphere or that the structure of the neutron star
magnetic field is more complicated than a simple dipole close to the surfaceComment: Accepted for publication by 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&
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