======================================================================== 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. 14, No. 13 Feb 18, 2002 Recently submitted papers: -------------------------- 1) Polarimetric Imaging of the Massive Black Hole at the Galactic Center (Bromley et al., ApJ) 2) Measuring the Black Hole Spin in Sgr A* (Melia et al., ApJ) ------------------------------------------------------------------------ Email : liusm@blackhole.physics.arizona.edu Title : Polarimetric Imaging of the Massive Black Hole at the Galactic Center Author(s): Benjamin C. Bromley,^1 Fulvio Melia,^2,3,4 and Siming Liu^2 Institute: (1) Department of Physics, University of Utah, 201 JFB, Salt Lake City, UT 84112 (2) Physics Department, The University of Arizona, Tucson, AZ 85721 (3) Steward Observatory, The University of Arizona, Tucson, AZ 85721 (4) Sir Thomas Lyle Fellow and Miegunyah Fellow. Paper : ApJ, 555, L83, 2001 EPrint : astro-ph/0106180 Abstract: The radio source Sgr A* in the Galactic center emits a polarized spectrum at millimeter and sub-millimeter wavelengths that is strongly suggestive of relativistic disk accretion onto a massive black hole. We use the well-constrained mass of Sgr A* and a magnetohydrodynamic model of the accretion flow to match both the total flux and polarization from this object. Our results demonstrate explicitly that the shift in the position angle of the polarization vector, seen at wavelengths near the peak of the mm to sub-mm emission from this source, is a signal of relativistic accretion flow in a strong gravitational field. We provide maps of the polarized emission to illustrate how the images of polarized intensity from the vicinity of the black hole would appear in upcoming observations with very long baseline radio interferometers (VLBI). Our results suggest that near-term VLBI observations will be able to directly image the polarized Keplerian portion of the flow near the horizon of the black hole. ------------------------------------------------------------------------ Email : liusm@blackhole.physics.arizona.edu Title : Measuring the Black Hole Spin in Sgr A* Author(s): Fulvio Melia^1,2,3, Benjamin C. Bromley^4, Siming Liu(2), and Christopher K. Walker(3) Institute: (1) Sir Thomas Lyle Fellow and Miegunyah Fellow. (2) Physics Department, The University of Arizona, Tucson, AZ 85721 (3) Steward Observatory, The University of Arizona, Tucson, AZ 85721 (4) Department of Physics, University of Utah, 201 JFB, Salt Lake City, UT 84112 Paper : ApJ, 554, L37, 2001 EPrint : astro-ph/0105188 Abstract: The polarized mm/sub-mm radiation from Sgr A* is apparently produced by a Keplerian structure whose peak emission occurs within several Schwarzschild radii (r_S\equiv 2GM/c^2) of the black hole. The Chandra X-ray counterpart, if confirmed, is presumably the self-Comptonized component from this region. In this paper, we suggest that sub-mm timing observations could yield a signal corresponding to the period P_0 of the marginally stable orbit, and therefore point directly to the black hole's spin a. Sgr A*'s mass is now known to be (2.6+/- 0.2)* 10^6 M_o (an unusually accurate value for supermassive black hole candidates), for which 2.7 min < P_0 < 36 min, depending on the value of a and whether the Keplerian flow is prograde or retrograde. A Schwarzschild black hole (a=0) should have P_0 20 min. The identification of the orbital frequency with the innermost stable circular orbit is made feasible by the transition from optically thick to thin emission at sub-mm wavelengths. With stratification in the emitter, the peak of the sub-mm bump in Sgr A*'s spectrum is thus produced at the smallest radius. We caution, however, that theoretical uncertainties in the structure of the emission region may still produce some ambiguity in the timing signal. Given that Sgr A*'s flux at nu 1 mm is several Jy, these periods should lie within the temporal-resolving capability of sub-mm telescopes using bolometric detectors. A determination of P_0 should provide not only a value of a, but it should also define the angular momentum vector of the orbiting gas in relation to the black hole's spin axis. By analogy with low-mass X-ray binaries and Galactic black hole candidates, Sgr A* may also display quasi-periodic oscillations, which can reveal additional features in the geometry of the accreting gas. In addition, since the X-ray flux detected by Chandra appears to be the self-Comptonized mm to sub-mm component, these temporal fluctuations may also be evident in the X-ray signal. ------------------------------------------------------------------------ (Older versions of the Newsflash can be found at the gcnews web-page) ======================================================================== Edited by Angela Cotera Heino Falcke & Sera Markoff (cotera@as.arizona.edu) (hfalcke,smarkoff@mpifr-bonn.mpg.de) - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - For Abstract submission please send the (La)Tex file of your paper to gcnews@mpifr-bonn.mpg.de ========================================================================