X--Ray Spectra from Neutron Stars Accreting at Low Rates

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.Comment: 14 pages (3 postscript figures available on request), PlainTex, submitted to Ap

Spherical Accretion onto Neutron Stars Revisited: Are Hot Solutions Possible ?

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, $10^{9}\div 10^{11}$ 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'' (\`a la Zel'dovich and Shakura) solution is present.Comment: 11 pages (3 postscript files figures available on request), PlainTeX, Ref. SISSA 21/94/

Time--dependent analysis of spherical accretion onto black holes

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.Comment: 14 pages, Plain TeX, 6 postscript figures, replaced version does not include the Blackwell Scientific Publications TeX Macros, to appear in MNRA

Interposition Arthroplasty Versus Hematoma and Distraction for the Treatment of Osteoarthritis of the Trapeziometacarpal Joint

Various surgical techniques were reported with excellent result for the treatment of trapeziometacarpal joint arthritis. However, the best treatment option was not defined yet

A disrupted bulgeless satellite galaxy as counterpart of the ultraluminous X-ray source ESO 243-49 HLX-1

The point-like X-ray source HLX-1, close to the S0 galaxy ESO 243-49, is one the strongest intermediate-mass black hole candidates, but the nature of its counterpart is still puzzling. By means of N-body/smoothed particle hydrodynamics simulations, we investigate the hypothesis that the HLX-1 counterpart is the nucleus of a bulgeless satellite galaxy, which undergoes a minor merger with the S0galaxy.We derived synthetic surface brightness profiles for the simulated counterpart of HLX-1 in six Hubble Space Telescope (HST) filters, ranging from far ultraviolet (FUV) to infrared wavelengths, and we compared them with the observed profiles. Our model matches the emission associated with the HLX-1 counterpart in all considered filters, including the bluer ones, even without requiring the contribution of an irradiated disc. The simulation can also account for an extended FUV emission, which is hinted at by the analysis of the F140LP HST filter. This matching is impossible to achieve by assuming either a bulgy satellite, a young star cluster, or an irradiated disc component

Radiative acceleration and transient, radiation-induced electric fields

The radiative acceleration of particles and the electrostatic potential fields that arise in low density plasmas hit by radiation produced by a transient, compact source are investigated. We calculate the dynamical evolution and asymptotic energy of the charged particles accelerated by the photons and the radiation-induced electric double layer in the full relativistic, Klein-Nishina regime. For fluxes in excess of $10^{27}$ ${\rm erg} {\rm cm}^{-2} {\rm s}^{-1}$, the radiative force on a diluted plasma (n\la 10^{11} cm$^{-3}$) is so strong that electrons are accelerated rapidly to relativistic speeds while ions lag behind owing to their larger inertia. The ions are later effectively accelerated by the strong radiation-induced double layer electric field up to Lorentz factors $\approx 100$, attainable in the case of negligible Compton drag. The asymptotic energies achieved by both ions and electrons are larger by a factor 2--4 with respect to what one could naively expect assuming that the electron-ion assembly is a rigidly coupled system. The regime we investigate may be relevant within the framework of giant flares from soft gamma-repeaters

X-ray study of HLX1: intermediate-mass black hole or foreground neutron star?

We re-assess the XMM-Newton and Swift observations of HLX1, to examine the evidence for its identification as an intermediate-mass black hole. We show that the X-ray spectral and timing properties are equally consistent with an intermediate-mass black hole in a high state, or with a foreground neutron star with a luminosity of about a few times 10^{32} erg/s ~ 10^{-6} L_{Edd}, located at a distance of about 1.5 to 3 kpc. Contrary to previously published results, we find that the X-ray spectral change between the two XMM-Newton observations of 2004 and 2008 (going from power-law dominated to thermal dominated) is not associated with a change in the X-ray luminosity. The thermal component becomes more dominant (and hotter) during the 2009 outburst seen by Swift, but in a way that is consistent with either scenario.Comment: 10 pages, accepted by MNRAS. Corrected LZ's affiliation and updated 1 referenc

The Ultraluminous X-ray Source NGC 1313 X-2 (MS 0317.7-6647) and its Environment

We present new optical and {\it Chandra} observations of the field containing the ultraluminous X-ray source NGC 1313 X-2. On an ESO 3.6 m image, the {\it Chandra} error box embraces a $R=21.6$ point-like object and excludes a previously proposed optical counterpart. The resulting X-ray/optical flux ratio of NGC 1313 X-2 is $\sim 500$. The value of $f_X/f_{opt}$, the X-ray variability history and the spectral distribution derived from a re-analysis of the {\it ROSAT}, {\it ASCA} and {\it XMM} data indicate a luminous X-ray binary in NGC 1313 as a likely explanation for NGC 1313 X-2. If the X-ray soft component observed in the {\it XMM} EPIC spectrum originates from an accretion disk, the inferred mass of the compact remnant is $\approx 100 M_\odot$, making it an intermediate mass black hole. The derived optical luminosity ($L\approx 10^5 L_\odot$) is consistent with that of a $\approx 15-20 M_\odot$ companion. The properties of the environment of NGC 1313 X-2 are briefly discussed.Comment: 12 pages, 6 figures; submitted to The Astrophysical Journa