======================================================================== G C N E W S * Newsflash * - The Newsletter for Galactic Center Research - gcnews@mpifr-bonn.mpg.de http://www.mpifr-bonn.mpg.de/gcnews ======================================================================== Vol. 11, No. 12 Apr 27, 2000 Recently submitted papers: -------------------------- 1) The Population of Faint Transients in the Galactic Centre (King, MNRAS) 2) Constraining the Accretion Rate Onto Sagittarius A* Using Linear Polarization (Quataert & Gruzinov, ApJ) ------------------------------------------------------------------------ Email : ark@star.le.ac.uk Title : The Population of Faint Transients in the Galactic Centre Author(s): A.R. King^1 Institute: (1) Astronomy Group, University of Leicester, Leicester LE1 7RH, U.K Paper : MNRAS, in press EPrint : astro-ph/0004277 Abstract: BeppoSAX has detected a population of faint transient X-ray sources in the Galactic Centre. I show that a simple irradiated-disc picture gives a consistent fit to the properties of this population, and that it probably consists of low-mass X-ray binaries which have evolved beyond their minimum orbital periods 80 min. Since all post-minimum systems are transient, and neutron-star LMXBs are more common than black-hole LMXBs in the Galaxy, the majority of these systems should contain neutron stars, as observed. This picture predicts that the Galactic Centre transients should have orbital periods in the range 80 - 120 minutes, and that most of them should repeat in the next few years. In this case, the total number of post-minimum transients in the Galaxy would be considerably smaller than the usual estimates of its total LMXB population. I discuss possible reasons for this. ------------------------------------------------------------------------ Email : eliot@ias.edu Title : Constraining the Accretion Rate Onto Sagittarius A* Using Linear Polarization Author(s): Eliot Quataert and Andrei Gruzinov Institute: Institute for Advanced Study, School of Natural Sciences, Einstein Drive, Princeton, NJ 08540; eliot@ias.edu, andrei@ias.edu Paper : ApJ, submitted EPrint : astro-ph/0004286 Abstract: Two possible explanations for the low luminosity of the supermassive black hole at the center of our galaxy are (1) an accretion rate of order the canonical Bondi value, but a very low radiative efficiency for the accreting gas or (2) an accretion rate much less than the Bondi rate. Both models can explain the broad-band spectrum of the Galactic Center. We show that they can be distinguished using the linear polarization of synchrotron radiation. Accretion at the Bondi rate predicts no linear polarization at any frequency due to Faraday depolarization. Low accretion rate models, on the other hand, have much lower gas densities and magnetic field strengths close to the black hole; polarization is therefore observable at high frequencies. If confirmed, a recent detection of linear polarization from Sgr A^* at >150 GHz argues for an accretion rate 10^-8 M_o/yr, much less than the Bondi rate. This test can be applied to other low-luminosity galactic nuclei. ------------------------------------------------------------------------ (Older versions of the Newsflash can be found at the gcnews web-page) ======================================================================== Edited by Angela Cotera Heino Falcke (cotera@ipac.caltech.edu) (hfalcke@mpifr-bonn.mpg.de) - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - For Abstract submission please send the (La)Tex file of your paper to gcnews@mpifr-bonn.mpg.de ========================================================================