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Weekly Seminars for August 2009 Print E-mail

Wednesday 26th 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 gravitational-wave 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 quasi-stationary, and far away from it, where they behave as electromagnetic waves. While a number of factors, such as the type of mode, the magnetic-field 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 general-relativistic 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 f-mode oscillations; Finally, electromagnetic losses represents the most efficient mechanism for the damping of g-mode oscillations. Second, we present here general-relativistic analysis of the production of a force-free magnetosphere around oscillating stars. We give a derivation of the general relativistic Maxwell equations for small-amplitude arbitrary oscillations of a non-rotating 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 27th 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 X-ray spectral and timing properties observed from a number of Galactic Black Hole (BH) binaries during hard-soft state spectral evolution. We analyze 17 transition episodes from 11 BH sources observed with Rossi X-ray Timing Explorer RXTE}. Our scaling technique for BH mass determination uses a correlation between spectral index and quasi-periodic oscillation (QPO) frequency. In addition, we use a correlation between index and the normalization of the disk "seed" component to cross-check the BH mass determination and estimate the distance to the source. While the index-QPO correlations for two given sources contain information on the ratio of the BH masses in those sources, the index-normalization correlations depend on the ratio of the BH masses and the distance square ratio. In fact, the index-normalization correlation also discloses the index-mass 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 J1655-40 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 X-1, GX 339-4, 4U 1543-47, XTE J1550-564, XTE J1650-500, H 1743-322, XTE J1859-226, GRS 1915+105, SS 433, Cyg X-3 Good agreement of our results for sources with known values of BH masses and distance provides an independent verification for our scaling technique.

 
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