PARSIFAL (PARametrized SImulation) is a software tool that can reproduce the complete response of both triple-GEM and micro-RWELL-based trackers. It takes into account the involved physical processes ...by their simple parametrization and thus in a very fast way. Existing software such as GARFIELD++ is robust and reliable, but CPU time-consuming. The implementation of PARSIFAL was driven by the necessity to reduce the processing time, without losing the precision of a complete simulation. A series of parameters, that can be extracted from the GARFIELD++ simulation, are set as input to PARSIFAL, which then runs independently from GARFIELD++. PARSIFAL can simulate samples with high statistics much faster, taking into account the various steps (ionization, diffusion, multiplication, signal induction, and electronics) from the simple sampling from parameterized distributions. In the case of the micro-RWELL MPGD, the effect of the high resistivity layer on the charge spread on the anode was introduced, following M.S. Dixit and A. Rankin’s treatment. PARSIFAL was used to simulate triple-GEM chambers and the results were tuned to match experimental data from testbeams. In this case, the adopted electronics was APV-25 readout by SRS system, which has been simulated in the code. The same procedure was later applied to micro-RWELL chambers, readout by the TIGER ASIC and the GEMROC system. These new electronics were added to PARSIFAL code, and the simulated-to-real data was tuned. A presentation of the full code will be given in this contribution, setting the focus on the latest implementations and a first comparison with experimental data from micro-RWELL.
Context. X-ray binaries hosting a compact object have been among the main targets of the Imaging X-ray Polarimetry Explorer (IXPE) since its launch, due to their high brightness in the 2–8 keV energy ...band. The spectropolarimetric analysis performed so far has proved to be of great importance in providing constraints on the accretion geometry of these systems. However, the data statistics is not enough to unambiguously disentangle the contribution of the single components to the net observed polarimetric signal. Aims. In this work, we aim to present a model for computing the polarization degree and polarization angle of the boundary layer around weakly magnetized neutron stars in low-mass X-ray binaries in the soft state. The main motivation is to provide strong theoretical support to data interpretation of observations performed by IXPE or future satellites for X-ray polarimetry. Methods. The results were obtained by modeling the boundary layer as an equatorial belt around the compact object and locally approximating it as a plane-parallel scattering atmosphere, for which the associated radiative transfer equation for polarized radiation in the Thomson limit was solved. The polarimetric quantities were then transformed from the comoving frame to the observer frame using the numerical methods formerly developed for X-ray pulsars. Results. For typical values of the optical depth and electron temperature of the boundary layer of these systems in a soft state, the polarization degree was less then 0.5%, while the polarization angle was rotated by ≲5° with respect to the neutron star spin axis due to special and general relativistic effects for fast rotation, the amount progressively decreasing for lower spin frequencies. The derived quantities can be used to remove degeneracy when multicomponent spectropolarimetry is performed.
ABSTRACT
X-ray polarimetry missions like Imaging X-ray Polarimetry Explorer will be able to measure for the first time the polarization properties of accreting, weakly magnetized neutron stars in ...low-mass X-ray binaries. In this work, we present simulations of the expected X-ray polarized signal, including the coronal emission for different geometries of the corona itself, i.e. a slab above the accretion disc and a spherical shell around the neutron star. The simulations are performed with the fully relativistic Monte Carlo code monk capable of computing the X-ray polarization degree and angle for various physical input parameters of the neutron star, disc, and corona. Different coronal geometries result in significantly different X-ray polarization properties, which can therefore be used to constrain the geometry of the systems.
The modified Lane-Emden equation for stellar hydrostatic equilibrium in f(R)-gravity is numerically solved by using an iterative procedure. Such an integro-differential equation can be obtained in ...the weak field limit approximation of f(R)-gravity by considering a suitable polytropic equation of state in the modified Poisson equation. The approach allows, in principle, to deal with still unexplored self-gravitating systems that could account for exotic stellar structures that escape standard stellar theory. PUBLICATION ABSTRACT
Context. 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. This requires ...developing numerical routines for the solution of the radiative equation according to the expected physical conditions of the systems under study. Aims. 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. Methods. 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 τ 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. Results. 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 τ 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. Conclusions. 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 (≳ 1012 G). This latter case is expected to be typical of accreting systems such as X-ray pulsars and supergiant fast X-ray transients.
Thanks to IXPE, the X-ray spectro-polarimeter launched at the end of 2021, X-ray polarimetry has finally become an extraordinary tool in investigating the physics of accretion in low mass X-ray ...binaries. Similarly to what happened with gravitational waves, X-ray polarimetry would play a new complementary but at the same time fundamental role in the high-energy astrophysical domain. We summarize here the first 1.5 yr results on accreting low-mass X-ray binaries obtained by a huge IXPE observation campaign coordinated with the principal X-ray and
-ray telescopes. Then we compare these results with the theoretical prediction highlighting the unexpected results.
We report the results of simultaneous multiwavelength observations of the X-ray transient source SWIFT J1753.5-0127 performed with INTEGRAL, RXTE, NTT, REM, and VLA on 2005 August 10-12. The source, ...which underwent an X-ray outburst since 2005 May 30, was observed during the INTEGRAL Target of Opportunity program dedicated to new X-ray novae located in the Galactic halo. Broadband spectra and fast timing variability properties of SWIFT J1753.5-0127 are analyzed together with the optical, near-infrared, and radio data. We show that the source was significantly detected up to 600 keV with Comptonization parameters and timing properties typical of the so-called low/hard state of black hole candidates. We build a spectral energy distribution and show that SWIFT J1753.5-0127 does not follow the usual radio/X-ray correlation of X-ray binaries in the low/hard state. We give estimates of distance and mass. We conclude that SWIFT J1753.5-0127 belongs to the X-ray nova class and that it is likely a black hole candidate transient source of the Galactic halo that remained in the low/hard state during its main outburst. We discuss our results in the context of Comptonization and jet models.
Context. Most of the spectra of neutron star low-mass X-ray binaries (NS LMXBs), whether they are persistent or transient, are characterized by the presence of a strong thermal Comptonization bump, ...which is thought to originate in the transition layer (TL) between the accretion disk and the NS surface. The observable quantities that characterize this component, which is dominating the emission below 30 keV, are the spectral index α and the rollover energy, both related to the electron temperature and optical depth of the plasma. Aims. 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. Methods. 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 α. We show that in this approximation the TL electron temperature kTe and optical depth τ0 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. Because the spectral index α depends on kTe and τ0, this in turn leads to a relation α = f(Qdisk/Qcor), with α ~1 when Qdisk/Qcor ≪ 1. Results. We show that the observed spectral index α for the sample of sources here considered lies in a belt around 1 ± 0.2 apart 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 with only the spectral analysis method.
We report results of a systematic study of the broadband (2-2000 keV) time-resolved prompt emission spectra of a sample of gamma-ray bursts (GRBs) detected with both Wide Field Cameras (WFCs) on ...board the BeppoSAX satellite and the Burst and Transient Source Experiment (BATSE) on board the Compton Gamma Ray Observatory. The main goal of this paper is to test spectral models of the GRB prompt emission that have recently been proposed. In particular, we test a recent photospheric model proposed, i.e., blackbody plus power law, the addition of a blackbody emission to the Band function in the cases in which this function does not fit the data, and a recent Comptonization model. By considering the few spectra for which the simple Band function does not provide a fully acceptable fit to the data, we find a statistically significant better fit by adding a blackbody to this function only in one case. We confirm earlier results found fitting the BATSE spectra alone with a blackbody plus power law. Instead, when the BATSE GRB spectra are joined to those obtained with WFCs (2-28 keV), this model becomes unacceptable in most time intervals in which we subdivide the GRB light curves. We find instead that the Comptonization model is always acceptable, even in the few cases in which the Band function is inconsistent with the data. We discuss the implications of these results.
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