Silvia Zane

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 � L=L Edd � 10 −3 to include the typical luminosities, L � 10 31 10 32 ergss −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.

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.

Old Isolated Accreting Neutron Stars: The Diffuse X-Ray Emission from the Galactic Center

The contribution of weakly magnetized (B ~ 109 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 Granat data and show a general agreement in both the flux energy and radial distributions.

Dynamical Comptonization in spherical flows: black hole accretion and stellar winds

The transport of photons in steady, spherical, scattering flows is investigated. The moment equations are solved analytically for accretion onto a Schwarzschild black hole, taking into full account relativistic effects. We show that the emergent radiation spectrum is a power law at high frequencies with a spectral index smaller (harder spectrum) than in the non--relativistic case. Radiative transfer in an expanding envelope is also analyzed. We find that adiabatic expansion produces a drift of injected monochromatic photons towards lower frequencies and the formation of a power--law, low--energy tail with spectral index

On Electrostatic Positron Acceleration in the Accretion Flow onto Neutron Stars

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End points of stellar evoltution

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XMM-Newton reveals magnetars magnetospheric plasma densities

As first shown by Shvartsman (1970), a neutron star accreting close to the Eddington limit must acquire a positive charge in order for electrons and protons to move at the same speed. The resulting electrostatic field may contribute to accelerating positrons produced near the star surface in conjunction with the radiative force. We reconsider the balance between energy gains and losses, including inverse Compton (IC), bremsstrahlung, and nonradiative scatterings. It is found that, even accounting for IC losses only, the maximum positron energy never exceeds ≈ 400 keV. The electrostatic field alone may produce energies of ≈ 50 keV at most. We also show that Coulomb collisions and annihilation with accreting electrons severely limit the number of positrons that escape to infinity.