arXiv subjects
Publications and source records attributed to Roberto Turolla.
We briefly review selected results in astrophysics of neutron stars (NSs) obtained during the last two years, focusing on isolated radioquiet objects. We discuss in some details population synthesis of close-by isolated NSs, spectra of INSs, detection of spectral features in these sources (including cyclotron features), recent results on velocity distribution of NSs and accretion onto INSs.
In a cool neutron star (T < 1.e6 K) endowed with a rather high magnetic field (B > 1.e13 G), a phase transition may occur in the outermost layers. As a consequence the neutron star becomes `bare', i.e. no gaseous atmosphere sits on the top of the crust. The surface of a cooling, bare neutron star does not necessarily emit a blackbody spectrum because the emissivity is strongly suppressed at energies below the electron plasma frequency, omega_p. Since omega_p ~ 1 keV under the conditions typical of the denseelectron gas in the condensate, the emission from a T ~ 100 eV bare neutron star will be substantially depressed with respect to that of a perfect Planckian radiator at most energies. Here we present a detailed analysis of the emission properties of a bare neutron star. In particular, we derive the surface emissivity for a Fe composition in a range of magnetic fields and temperatures representative of cooling isolated neutron stars, like RX J1856.5-3754. We find that the emitted spectrum is strongly dependent on the electron conductivity in the solid surface layers. In the cold electron gas approximation (no electron-lattice interactions), the spectrum turns out to be a featureless depressed blackbody in the 0.1-2 keV band with a steeper low-energy distribution. When damping effects due to collisions between electrons and the ion lattice (mainly due to electron-phonon interactions) are accounted for, the spectrum is more depressed depending on the magnetic field strength. Details of the emitted spectrum are found, however, to be strongly dependent of the assumed treatment of the transition from the external vacuum to the metallic surface. The implications of out results to RX J1856.5-3754 and other isolated neutron stars are discussed.
We report evidence of cyclotron resonance features from the Soft Gamma Repeater SGR 1806-20 in outburst, detected with the Rossi X-ray Timing Explorer in the spectrum of a long, complex precursor that preceded a strong burst. The features consist of a narrow 5.0 keV absorption line with modulation near its second and third harmonics (at 11.2 keV and 17.5 keV respectively). The line features are transient and are detected in the harder part of the precursor. The 5.0 keV feature is strong, with an equivalent width of ~ 500 eV and a narrow width of less than 0.4 keV. Interpreting the features as electron cyclotron lines in the context of accretion models leads to a large mass-radius ratio (M/R > 0.3 M_sun/km) that is inconsistent with neutron stars or that requires a low (5-7)x10^{11} G magnetic field that is unlikely for SGRs. The line widths are also narrow compared with those of electron cyclotron resonances observed so far in X-ray pulsars. In the magnetar picture, the features are plausibly explained as ion cyclotron resonances in an ultra-strong magnetic field that have recently been predicted from magnetar candidates. In this view, the 5.0 keV feature is consistent with a proton cyclotron fundamental whose energy and width are close to model predictions. The line energy would correspond to a surface magnetic field of 1.0x10^{15} G for SGR 1806-20, in good agreement with that inferred from the spin-down measure in the source.
The effects of bulk motion comptonization on the spectral formation in a converging flow onto a black hole are investigated. The problem is tackled by means of both a fully relativistic, angle-dependent transfer code and a semi-analytical, diffusion-approximation method. We find that a power-law high-energy tail is a ubiquitous feature in converging flows and that the two approaches produce consistent results at large enough accretion rates, when photon diffusion holds. Our semi-analytical approach is based on an expansion in eigenfunctions of the diffusion equation. Contrary to previous investigations based on the same method we find that, although the power-law tail at really large energies is always dominated by the flatter spectral mode, the slope of the hard X-ray portion of the spectrum is dictated by the second mode and it approaches Gamma=3 at large accretion rate, irrespective of the model parameters. The photon index in the tail is found to be largely independent on the spatial distribution of soft seed photons when the accretion rate is either quite low (< 5 in Eddington units) or sufficiently high (> 10). On the other hand, the spatial distribution of source photons controls the photon index at intermediate accretion rates, when Gamma switches from the first to the second mode. Our analysis confirms that a hard tail with photon index Gamma <3 is produced by the up-scattering of primary photons onto infalling electrons if the central object is a black hole.
Thermal instability driven by radiation pressure might be relevant for intrinsically bright accreting sources. The most promising candidate where this instability seems to be at work is one of the two known galactic superluminal sources, GRS 1915+105 (Belloni et al. 1997). In spite of being of relevance, this scenario has not yet been confirmed by proper time-dependent modelling. Non-linear time-dependent calculations performed by Szuszkiewicz and Miller (1998) show that thermally unstable discs undergo limit-cycle behaviour with successive evacuation and refilling of the central parts of the disc. This evolution is very similar to the one proposed by Belloni et al. (1997) in their phenomenological model. Further investigations are needed to confirm the thermal instability being operational in this source. First of all the spectra emitted from the disc during its evolution should be calculated and compared with observations. Here such spectra are computed assuming local blackbody emission from the best studied transonic disc model.
In some models of $γ$-ray bursts super-strong electric fields ($E\sim 10^{14} {\rm statvolt cm}^{-1}$) have been surmized. Such large fields may provide copious pair production through vacuum polarization. Here we examine various astrophysical scenarios where huge electric field can be present. It is shown that when the conditions for quantistic vacuum breakdown are met, pair creation through conventional processes is likely to be always more important. Although of great physical interest, quantum pair production seems therefore of little astrophysical relevance.
As many as $10^9$ neutron stars populate the Galaxy, but only $\approx 10^3$ are directly observed as pulsars or as accreting sources in X-ray binaries. In principle also the accretion of the interstellar medium may make isolated neutron stars shine, and their weak luminosity could be detected in soft X-rays. Recent ROSAT observations have convincingly shown that neutron stars accreting from the interstellar medium are extremely rare, if observed at all, in contrast with earlier theoretical predictions. Until now two possible explanations for their elusiveness have been proposed: their velocity distribution may peak at $\sim 200-400 {\rm km s}^{-1}$, as inferred from pulsar statistics, and this would severely choke accretion; the magnetic field may decay on timescales $\sim 10^8-10^9$ yr, preventing a large fraction of neutron stars from entering the accretor stage. The search for accreting neutron stars has produced up to now a handful of promising candidates. While little doubt is left that these objects are indeed isolated neutron stars, the nature of their emission is still controversial. In particular accreting objects can be confused with much younger, cooling neutron stars. However, a combination of observations and theoretical modeling may help in discriminating between the two classes.
Results are presented from a time--dependent, numerical investigation of spherical accretion onto black holes, within the framework of relativistic radiation hydrodynamics. We have studied the stability of self--consistent, stationary solutions of black hole accretion with respect to thermal and radiative perturbations and also the non--linear evolution of unstable, high temperature models, heated by the hard radiation produced by the accretion flow itself in the inner region near to the horizon. In some cases, a hydrodynamic shock forms at around $10^3$--$10^4$ Schwarzschild radii, where Compton heating exceeds radiative cooling. The calculations were made using a suitably designed radiation hydrodynamics code, in which radiative transfer is handled by means of the PSTF moment formalism and which contains an original treatment of the radiation temperature equation.
The contribution of weakly--magnetized ($B\sim 10^9$ G) neutron stars accreting the interstellar medium to the diffuse X--ray emission observed in the Galactic Center is investigated. It is shown that, under rather conservative assumptions about the neutron stars and gas distributions, the accretion luminosity can account for a sizable fraction, possibly most, of the detected X--ray flux in the 2.5--7 keV band. In particular, model results are compared with {\it Granat\/} data and show a general agreement in both the flux energy and radial distributions.
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.
Old isolated accreting neutron stars may show up among unidentified soft X--ray sources detected by the ROSAT All Sky Survey. We argue that the chances of identification are greater for ONSs located in the closest overdense regions of the solar neighbourhood. In particular, we consider the neutral hydrogen wall in the second Galactic quadrant ($15^\circ < l < 150^\circ$) which shrinks the estimated contour of the Local Bubble to $\approx 16-30$ pc. Due to their vicinity, we expect $\sim 10$ ONSs to be detectable at a relatively high flux limit ($\sim 0.1$ counts/s) in the 0.2--2.4 keV band. This implies that about 5 \% of unidentified sources above this threshold could be ONSs. No optical counterpart is present, but EUV emission from these objects could be detected using EUVE Lex filter at the highest sensitivity limits.
The issue of the observability of Old Isolated Neutron Stars (ONSs) accreting from the interstellar gas, is reconsidered using the spectra presented in Zampieri {\it et al.\/} (1995). In particular, we focus our attention on the overall soft X--ray emission of ONSs, which may provide a substantial contribution to the X--ray background in the 0.5--2 keV band. It is found that if accretion is channeled into polar caps by a magnetic field of $\sim 10^9$ G, up to $\sim$ $12-25 \%$ of the unresolved soft excess observed at high latitudes in the X--ray background can be explained in terms of emission from accreting isolated ONSs. However, this would imply the detection with ROSAT of $\sim 10$ sources deg$^{-2}$ above a threshold of $\sim 10^{-14} \, {\rm erg\, cm^{-2} s^{-1}}$. A brief reconsideration of the observability of isolated ONSs as individual X--ray sources is also presented.
The spectral properties of X--ray radiation produced in a static atmosphere around a neutron star accreting at very low rates are investigated. Previous results by Alme \& Wilson (1973) are extended to the range $10^{-7}\leq L/L_{Edd}\leq 10^{-3}$ to include the typical luminosities, $L\sim 10^{31}-10^{32} \ {\rm ergs\, s^{-1}}$, expected from isolated neutron stars accreting the interstellar medium. The emergent spectra show an overall hardening with respect to the blackbody at the neutron star effective temperature in addition to a significant excess over the Wien tail. The relevance of present results in connection with the observability of low--luminosity X--ray sources is briefly discussed.
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.
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.
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.
Stationary, spherical accretion onto an unmagnetized neutron star is here reconsidered on the wake of the seminal paper by Zel'dovich \& Shakura (1969). It is found that new ``hot'' solutions may exist for a wide range of luminosities. These solutions are characterized by a high temperature, $10^{9}÷10^{11}$ K, and arise from a stationary equilibrium model where the dominant radiative mechanisms are multiple Compton scattering and bremsstrahlung emission. For low luminosities, $\mincir 10^{-2} \ L_{E}$, only the ``cold'' (à la Zel'dovich and Shakura) solution is present.
We investigate the effects of dynamical Comptonization on the emergent radiation spectrum produced by near critical accretion onto a neutron star. The flow dynamics and the transfer of radiation are self--consistently solved in the case of a spherically symmetric, ``cold'', pure scattering flow, including general relativity. A sequence of models, each characterized by the value of the total observed luminosity, was obtained assuming that the spectrum at the star surface is black body in shape. It is found that when the luminosity approaches the Eddington limit dynamical effects become important shifting the spectrum to the blue and producing a power--law, high--energy tail. The relevance of these results in connection with the observed spectral properties of LMXBs, and of Cyg X--2 in particular, are discussed.