Accretion Rates in X--Ray Bursting Sources

We present estimates for the accretion rates in 13 X--ray bursting sources which exhibit photospheric expansion, basing on theoretical models of stationary, radiatively driven winds from neutron stars. The relatively high values obtained, \Mdot_{acc}\magcir 10^{-9} \MS, are in accordance with theoretical limits for unstable helium burning, and, at the same time, almost never exceed the ``dynamical'' limit for stationary accretion, \sim 10 \Mdot_{Edd}. The only exceptions are 1820-30, already known to be a very peculiar object, and 1608-522; there are indications, however, that in both sources, accretion could be non--stationary.Comment: 21 pages, PlainTe

On the Nature of Photospheric Oscillations in Strong X--Ray Bursts

A possible sound origin for the photospheric oscillations in the X--ray bursting sources 1608-522 and 2127+119 is suggested. It is shown that standing sound waves in an expanding spherical envelope can have periods very close to the observed ones. The quite large ratio, $\sim$ 10, of the periods in the two sources is explained in terms of different wave regimes. The relevance of sound oscillations to the observed QPO in type II bursts of the Rapid Burster is also discussed.Comment: 14 pages, PlainTe

Winds from Neutron Stars and Strong Type I X--Ray Bursts

A model for stationary, radiatively driven winds from X--ray bursting neutron stars is presented. General relativistic hydrodynamical and radiative transfer equations are integrated from the neutron star surface outwards, taking into account for helium nuclear burning in the inner, dense, nearly hydrostatic shells. Radiative processes include both bremsstrahlung emission--absorption and Compton scattering; only the frequency--integrated transport is considered here. It is shown that each solution is characterized by just one parameter: the mass loss rate \Mdot, or, equivalently, the envelope mass \Menv. We found that, owing to the effects of Comptonization, steady, supersonic winds can exist only for \Mdot larger than a limiting value \Mdot_{min} \approx\Mdot_{E}. Several models, covering about two decades in mass loss rate, have been computed for given neutron star parameters. We discuss how the sequence of our solutions with decreasing \Menv can be used to follow the time evolution of a strong X--ray burst during the expansion/contraction phase near to the luminosity maximum. The comparison between our numerical results and the observational data of Haberl {\it et al.\/} (1987) for the bursts in 4U/MXB 1820-30 gives an estimate for both the spectral hardening factor and the accretion rate in this source.Comment: 32 pages (10 postsript figures available on request), PlainTe

Modeling the broadband persistent emission of magnetars

In this paper, we discuss our first attempts to model the broadband persistent emission of magnetars within a self consistent, physical scenario. We present the predictions of a synthetic model that we calculated with a new Monte Carlo 3-D radiative code. The basic idea is that soft thermal photons (e.g. emitted by the star surface) can experience resonant cyclotron upscattering by a population of relativistic electrons threated in the twisted magnetosphere. Our code is specifically tailored to work in the ultra-magnetized regime; polarization and QED effects are consistently accounted for, as well different configurations for the magnetosphere. We discuss the predicted spectral properties in the 0.1-1000 keV range, the polarization properties, and we present the model application to a sample of magnetars soft X-ray spectra.Comment: 14 pages, 7 figures, to be published in Advances in Space Research. Proceedings of the conference "Frontieres of Space Astrophysics, Neutron Stars & Gamma Ray Bursts", Cairo/Alexandria, 30 March- 4 April 200

Probing magnetars magnetosphere through X-ray polarization measurements

The study of magnetars is of particular relevance since these objects are the only laboratories where the physics in ultra-strong magnetic fields can be directly tested. Until now, spectroscopic and timing measurements at X-ray energies in soft gamma-repeaters (SGRs) and anomalous X-ray pulsar (AXPs) have been the main source of information about the physical properties of a magnetar and of its magnetosphere. Spectral fitting in the ~ 0.5-10 keV range allowed to validate the "twisted magnetosphere" model, probing the structure of the external field and estimating the density and velocity of the magnetospheric currents. Spectroscopy alone, however, may fail in disambiguating the two key parameters governing magnetospheric scattering (the charge velocity and the twist angle) and is quite insensitive to the source geometry. X-ray polarimetry, on the other hand, can provide a quantum leap in the field by adding two extra observables, the linear polarization degree and the polarization angle. Using the bright AXP 1RXS J170849.0-400910 as a template, we show that phase-resolved polarimetric measurements can unambiguously determine the model parameters, even with a small X-ray polarimetry mission carrying modern photoelectric detectors and existing X-ray optics. We also show that polarimetric measurements can pinpoint vacuum polarization effects and thus provide an indirect evidence for ultra-strong magnetic fields.Comment: 12 pages, 8 figures, accepted for publication in MNRA

General relativistic radiative transfer in hot astrophysical plasmas: a characteristic approach

In this paper we present a characteristic method for solving the transfer equation in differentially moving media in a curved spacetime. The method is completely general, but its capabilities are exploited at best in presence of symmetries, when the existence of conserved quantities allows to derive analytical expressions for the photon trajectories in phase space. In spherically--symmetric, stationary configurations the solution of the transfer problem is reduced to the integration of a single ordinary differential equation along the bi--parametric family of characteristic rays. Accurate expressions for the radiative processes relevant to continuum transfer in a hot astrophysical plasma have been used in evaluating the source term, including relativistic e--p, e--e bremsstrahlung and Compton scattering. A numerical code for the solution of the transfer problem in moving media in a Schwarzschild spacetime has been developed and tested. Some applications, concerning ``hot'' and ``cold'' accretion onto non--rotating black holes as well as static atmospheres around neutron stars, are presented and discussed.Comment: 47 pages plus 9 postscript figures, uuencoded file, PlainTeX. Accepted for publication in the Astrophysical Journa

Hydrodynamics of the Cosmological Quark-Hadron Transition in the Presence of Long-Range Energy and Momentum Transfer

Two previous papers in this series have presented a study of the growth of hadronic bubbles during the cosmological Quark--Hadron transition, treating the material within each phase as a single perfect fluid. Here, we extend the analysis to include the effects of long-range energy and momentum transfer by weakly and electromagnetically interacting particles. After a short review of the formalism adopted, we discuss the numerical strategies used in the computer code which has been constructed in order to solve this system of equations. Results for the growth of single hadronic bubbles are also presented.Comment: 21 pages, Plain Tex, nofigures. uucoded file containing the postcript version of the paper and 9 figures is available on a separate fil

A variable absorption feature in the X-ray spectrum of a magnetar

Soft gamma-ray repeaters (SGRs) and anomalous X-ray pulsars (AXPs) are slowly rotating, isolated neutron stars that sporadically undergo episodes of long-term flux enhancement (outbursts) generally accompanied by the emission of short bursts of hard X-rays. This behaviour can be understood in the magnetar model, according to which these sources are mainly powered by their own magnetic energy. This is supported by the fact that the magnetic fields inferred from several observed properties of AXPs and SGRs are greater than - or at the high end of the range of - those of radio pulsars. In the peculiar case of SGR 0418+5729, a weak dipole magnetic moment is derived from its timing parameters, whereas a strong field has been proposed to reside in the stellar interior and in multipole components on the surface. Here we show that the X-ray spectrum of SGR 0418+5729 has an absorption line, the properties of which depend strongly on the star's rotational phase. This line is interpreted as a proton cyclotron feature and its energy implies a magnetic field ranging from 2E14 gauss to more than 1E15 gauss.Comment: Nature, 500, 312 (including Supplementary Information

The LARASE Spin Model of the two LAGEOS and LARES satellites

Satellite Laser Ranging (SLR) represents a very important technique of the observational space geodesy. In fact, Lunar Laser Ranging, Very Long Baseline Interferometry, Global Navigation Satellite Systems, Doppler Orbitography and Radiopositioning Integrated by Satellite, together with SLR constitute the Global Geodetic Observing System (GGOS). In the context of the GGOS activities, improvements in technology and in modeling will produce advances in Geodesy and Geophysics as well as in General Relativity (GR) measurements. Therefore, these important research fields are not independent, but tightly related to each other. The LARASE (LAser RAnged Satellites Experiment) research program has its main objectives in tests and measurements of Einstein's theory of GR via Precise Orbit Determination (POD) of a set of geodetic satellites. In order to reach such goals by means of very precise measurements of a number of relativistic parameters (and, at the same time, to provide a robust and unassailable error budget of the main systematic effects), we are also reviewing previous models and we are developing new models for the main perturbations (both gravitational and non-gravitational) that act on the orbits of the two LAGEOS and on that of LARES satellites. Within this paper we focus on modeling the spin vector of these satellites. The spin knowledge, both in orientation and rate, is of fundamental importance in order to correctly model the thermal effects acting on the surface of these satellites. These are very important non-gravitational perturbations (NGP) that produce long-term effects on the orbit of the cited satellites, especially for the two LAGEOS, and improvements in their modeling will be very useful both in the field of GR measurements and in those of space geodesy and geophysical applications. Indeed, the current RMS value of the range residuals of the LAGEOS satellites, obtained by the Analysis Centers of the International Laser Ranging Service, is at the level of a few cm since 1992, down to a cm or less during the last years. However, because of the incompleteness in current knowledge of dynamical models, empirical accelerations have been heavily employed to obtain such results. In this context, any step forward in the models developed for the NGP will be useful to reduce the use of empirical accelerations; it also represents an essential prerequisite to reach a sub-mm precision in the RMS of the SLR range residuals and the corresponding benefits in Geophysics and Geodesy, regarding e.g. stations coordinates knowledge, Earth's geocenter and reference frame realization. The paper will focus upon the improvements we obtained with respect on previous models of the spin of the two LAGEOS satellites based on averaged equations for the external torques in the rapid-spin approximation, as well as in a new general model that we developed and based on the solution of the full set of Euler equations