Wednesday 26^{th} August 2009, 5 pm.
ICRANet, Pescara  Seminars Room
Speaker: 
Prof. B.J. Ahmedov (Institute of Nuclear Physics & Ulugh Beg Astronomical Institute, Tashkent, Uzbekistan) 
Title: GENERAL RELATIVISTIC ELECTROMAGNETIC FIELDS AND MAGNETOSPHERE OF OSCILLATING MAGNETIZED STARS
Abstract: An important issue in the astroseismology of compact and magnetized stars is the determination of the dissipation mechanism which is most efficient in damping the oscillations. In a linear regime for low multipolarity modes these mechanisms are confined to either gravitationalwave or electromagnetic losses. First, we here consider in detail the latter and compute the energy losses in the form of Poynting fluxes, Joule heating and Ohmic dissipation in a relativistic magnetized spherical star in vacuum. In particular, we provide analytical expressions for the electric and magnetic fields produced by the most common modes of oscillation both in the vicinity of the star, where they are quasistationary, and far away from it, where they behave as electromagnetic waves. While a number of factors, such as the type of mode, the magneticfield strength and the compactness of the star, concur in determining what is the main damping mechanism of the oscillations, the following results are generically true for a typical neutron star with a dipolar magnetic field: Firstly, the generalrelativistic corrections to the electromagnetic fields lead to a damping timescale due to electromagnetic losses which is at least one order of magnitude smaller than its Newtonian counterpart; Secondly, the emission of gravitational waves represents the most efficient mechanism for the damping of p and fmode oscillations; Finally, electromagnetic losses represents the most efficient mechanism for the damping of gmode oscillations. Second, we present here generalrelativistic analysis of the production of a forcefree magnetosphere around oscillating stars. We give a derivation of the general relativistic Maxwell equations for smallamplitude arbitrary oscillations of a nonrotating neutron star with a generic magnetic field and show that these can be solved analytically under the assumption of low current density in the magnetosphere. We apply our formalism to toroidal oscillations of a neutron star with a dipole magnetic field and find that the low current density approximation is valid for at least half of the oscillation modes, similarly to the Newtonian case. Using an improved formula for the determination of the last closed field line, we calculate the energy losses resulting from toroidal stellar oscillations for all of the modes for which the size of the polar cap is small. We find that general relativistic effects lead to shrinking of the size of the polar cap and an increase in the energy density of the outflowing plasma. These effects act in opposite directions but the net result is that the energy loss from the neutron star is significantly smaller than suggested by the Newtonian treatment.
Thursday 27^{th} August 2009, 5 pm.
ICRANet, Pescara  Seminars Room
Speaker: 
Prof. L. Titarchuck (University of Ferrara) 
Title: Index and mass accretion rate saturation in Black Hole (BH) Candidates binaries. BH mass determination
Abstract: We present a study of correlations between Xray spectral and timing properties observed from a number of Galactic Black Hole (BH) binaries during hardsoft state spectral evolution. We analyze 17 transition episodes from 11 BH sources observed with Rossi Xray Timing Explorer RXTE}. Our scaling technique for BH mass determination uses a correlation between spectral index and quasiperiodic oscillation (QPO) frequency. In addition, we use a correlation between index and the normalization of the disk "seed" component to crosscheck the BH mass determination and estimate the distance to the source. While the indexQPO correlations for two given sources contain information on the ratio of the BH masses in those sources, the indexnormalization correlations depend on the ratio of the BH masses and the distance square ratio. In fact, the indexnormalization correlation also discloses the indexmass accretion rate saturation effect given that the normalization of disk ``seed'' photon supply is proportional to the disk mass accretion rate. We present arguments that this observationally established index saturation effect is a signature of the bulk motion (converging) flow onto black hole which was early predicted by the dynamical Comptonization theory. We use GRO J165540 as a primary reference source for which the BH mass, distance and inclination angle are evaluated by dynamical measurements with unprecedented precision among other Galactic BH sources. We apply our scaling technique to determine BH masses and distances for Cygnus X1, GX 3394, 4U 154347, XTE J1550564, XTE J1650500, H 1743322, XTE J1859226, GRS 1915+105, SS 433, Cyg X3 Good agreement of our results for sources with known values of BH masses and distance provides an independent verification for our scaling technique.
