60 research outputs found
An upscattering spectral formation model for the prompt emission of Gamma-Ray Bursts
We propose a model for the spectral formation of Gamma Ray Burst (GRB) prompt
emission, where the phenomenological Band's function is usually applied to
describe the GRB prompt emission. We suggest that the GRB prompt emission is
mainly a result of two upscattering processes. The first process is the
Comptonization of relatively cold soft photons of the star off electrons of a
hot shell of plasma of temperature T_e of the order of 10^{9} K (or kT_e~100
keV) that moves sub-relativistically with the bulk velocity V_b substantially
less than the speed of light c. In this phase, the Comptonization parameter Y
is high and the interaction between a blackbody-like soft seed photon
population and hot electrons leads to formation of a saturated Comptonization
spectrum modified by the sub-relativistic bulk outflow. The second process is
an upscattering of the previously Comptonized spectrum by the plasma outflow
once it becomes relativistic. This process gives rise to the high-energy
power-law component above the peak in the EF(E)-diagram where F(E) is the
energy flux. The latter process can be described by a convolution of the
Comptonized spectrum with a broken-power-law Green function. Possible physical
scenarios for this second upscattering process are discussed. In the framework
of our model, we give an interpretation of the Amati relation between the
intrinsic spectral peak photon energy and radiated energy or luminosity, and we
propose a possible explanation of the GRB temporal variability.Comment: 27 pages, 8 figures, accepted for publication in the Astrophysical
Journa
The Mass-Radius relation for Neutron Stars in gravity
We discuss the Mass -Radius diagram for static neutron star models obtained
by the numerical solution of modified Tolman-Oppenheimer-Volkoff equations in
gravity where the Lagrangians and
are adopted. Unlike the case of the perturbative
approach previously reported, the solutions are constrained by the presence of
an extra degree of freedom, coming from the trace of the field equations. In
particular, the stiffness of the equation of state determines an upper limit on
the central density above which the the positivity condition of
energy-matter tensor trace holds. In the case of
quadratic f(R)-gravity, we find higher masses and radii at lower central
densities with an inversion of the behavior around a pivoting which
depends on the choice of the equation of state. When considering the cubic
corrections, we find solutions converging to the required asymptotic behavior
of flat metric only for . A similar analysis is performed for
considering as the leading parameter. We work
strictly in the Jordan frame in order to consider matter minimally coupled with
respect to geometry. This fact allows us to avoid ambiguities that could emerge
in adopting the Einstein frame.Comment: 10 pages, 6 figures, to appear in Phys. Rev.
The hard X-ray tails in neutron star low mass X-ray binaries: BeppoSAX observations and possible theoretical explanation of the GX 17+2 case
We report results of a new spectral analysis of two BeppoSAX observations of
the Z source GX 17+2. In one of the two observations the source exhibits a
powerlaw-like hard (> 30 keV) X-ray tail which was described in a previous work
by a hybrid Comptonization model. Recent high-energy observations with INTEGRAL
of a sample of Low Mass X-Ray Binaries including both Z and atoll classes have
shown that bulk (dynamical) Comptonization of soft photons can be a possible
alternative mechanism for producing hard X-ray tails in such systems. We start
from the INTEGRAL results and we exploit the broad-band capability of BeppoSAX
to better investigate the physical processes at work. We use GX 17+2 as a
representative case. Moreover, we suggest that weakening (or disappearance) of
the hard X-ray tail can be explained by increasing radiation pressure
originated at the surface of the neutron star (NS). As a result the high
radiation pressure stops the bulk inflow and consequently this radiation
feedback of the NS surface leads to quenching the bulk Comptonization.Comment: 6 pages, 3 figures, Accepted for publication in Ap
A new Comptonization model for low-magnetized accreting neutron stars in low mass X-ray binaries
We developed a new model for the X-ray spectral fitting \xspec package which
takes into account the effects of both thermal and dynamical (i.e. bulk)
Comptonization. The model consists of two components: one is the direct
blackbody-like emission due to seed photons which are not subjected to
effective Compton scattering, while the other one is a convolution of the
Green's function of the energy operator with a blackbody-like seed photon
spectrum. When combined thermal and bulk effects are considered, the analytic
form of the Green's function may be obtained as a solution of the diffusion
Comptonization equation. Using data from the BeppoSAX, INTEGRAL and RXTE
satellites, we test our model on the spectra of a sample of six persistently
low magnetic field bright neutron star Low Mass X-ray Binaries, covering three
different spectral states. Particular attention is given to the transient
powerlaw-like hard X-ray (> 30 keV) tails that we interpret in the framework of
the bulk motion Comptonization process. We show that the values of the best-fit
delta-parameter, which represents the importance of bulk with respect to
thermal Comptonization, can be physically meaningful and can at least
qualitatively describe the physical conditions of the environment in the
innermost part of the system. Moreover, we show that in fitting the thermal
Comptonization spectra to the X-ray spectra of these systems, the best-fit
parameters of our model are in excellent agreement with those of COMPTT, a
broadly used and well established XSPEC model.Comment: 15 pages, 8 figures, accepted for publication in Ap
DPU_ASW Configuration Control Issues
This document is a printed version of the configuration control (CC) of the DPU_ASW. For CC the Jira tool was used, with a repository provided by INFN. The tool provides a web interface at the following link: https://issues.infn.it/jira/browse/EUNIDPUASW/?selectedTab=com.atlassian.jira.jira-projects-plugin:summary-panel, to access INFN credential are need. The webpage as well as this document are organized by sections, corresponding to each release of the DPU_ASW versions. Starting in section 4 are listed the differences with respect to the previous release. The first release under CC is DPU-ASW version v0.0 (delta-CDR).
For each CC entry, an identification code is automatically generated, and a title is assigned e.g. [EUNIDPUASW-72] ICU counter test. Here the entry title (listed in the table of contents) is a hyperlink to the webpage of the corresponding entry in at Jira on-line documentation. Each entry is classified according to its type: New Feature or Bug, and a priority is assigned: Trivial, Minor or Major. The status is specified in the field Resolution: all Done. In the on-line version of the documentation, entries can be sort according any of these criteria; and ancillary attached files quoted in the entries can be downloaded from the on-line version of the document (here are included only plots and pictures).
DPU-ASW versioning is fully documented in RD-8
DPU-ASW Management of the DCU ERROR REG content with possible off-line recovery actions
A new strategy is defined for the management of the errors in the DCU science interface dedicated to the scientific data acquisitio
Test Report of NISP dedicated test activities during the CSL campaign - DCU-SCE Communication issue
A complete analysis of the communication issue in the science interface observed during the NISP CSL test campaig
First detection of X-ray polarization from the accreting neutron star 4U 1820-303
This paper reports the first detection of polarization in the X-rays for
atoll-source 4U 1820-303, obtained with the Imaging X-ray Polarimetry Explorer
(IXPE) at 99.999% confidence level (CL). Simultaneous polarimetric measurements
were also performed in the radio with the Australia Telescope Compact Array
(ATCA). The IXPE observations of 4U 1820-303 were coordinated with Swift-XRT,
NICER, and NuSTAR aiming to obtain an accurate X-ray spectral model covering a
broad energy interval. The source shows a significant polarization above 4 keV,
with a polarization degree of 2.0(0.5)% and a polarization angle of -55(7) deg
in the 4-7 keV energy range, and a polarization degree of 10(2)% and a
polarization angle of -67(7) deg in the 7-8 keV energy bin. This polarization
also shows a clear energy trend with polarization degree increasing with energy
and a hint for a position-angle change of about 90 deg at 96% CL around 4 keV.
The spectro-polarimetric fit indicates that the accretion disk is polarized
orthogonally to the hard spectral component, which is presumably produced in
the boundary/spreading layer. We do not detect linear polarization from the
radio counterpart, with a 99.97% upper limit of 50% at 7.25 GHz
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