======================================================================== G C N E W S * Newsflash * - The Newsletter for Galactic Center Research - gcnews@aoc.nrao.edu http://www.aoc.nrao.edu/~gcnews ======================================================================== Vol. 23, No. 1 Dec 14, 2005 Recently submitted papers: -------------------------- 1) The Nature of the Faint Chandra X-ray Sources in the Galactic Centre (Ruiter et al., ApJL) 2) Interferometric Measurements of Variable 340 GHz Linear Polarization in Sagittarius A* (Marrone et al., ApJ) 3) The Observed Galactic Annihilation Line: Possible Signature of Accreting Small Mass Black Holes in the Galactic Center (Titarchuk and Chardonnet, ApJ) 4) Variations in the spectral slope of Sgr A* during a NIR flare (Gillessen et al., ApjL) 5) A Possible Rossby Wave Instability Origin for the Flares in Sagittarius A* (Tagger and Melia et al., ApJL) ------------------------------------------------------------------------ Email : aruiter@nmsu.edu Title : The Nature of the Faint Chandra X-ray Sources in the Galactic Centre Author(s): Ashley J. Ruiter (1), Krzysztof Belczynski (1,2), and Thomas E. Harrison (1) Institute: (1)New Mexico State University, Dept. of Astronomy, 1320 Frenger Mall, Las Cruces, NM 88003 (2) Tombaugh Fellow Paper : ApJL submitted EPrint : astro-ph/0511813 Web : http://arxiv.org/abs/astro-ph/0511813 Abstract: Recent Chandra observations have revealed a large population of faint X-ray point sources in the Galactic Centre. The observed population consists of 2000 faint sources in the luminosity range 10^31-10^33 erg s^-1. The majority of these sources (70%) are described by hard spectra, while the rest are rather soft. The nature of these sources still remains unknown. Belczynski \& Taam (2004) demonstrated that X-ray binaries with neutron star or black hole accretors may account for most of the soft sources, but are not numerous enough in order to account for the observed number and X-ray properties of faint hard sources. Both wind-fed systems and quiescent Roche lobe overflow transients were tested as potential source candidates. Muno et al. (2004) proposed that intermediate polars (subclass of magnetic cataclysmic variables) may be able to explain the faint hard population. Since an observational test of this hypothesis is not currently feasible due to (i) the large extinction toward the Galactic Centre, and (ii) low luminosity of intermediate polar donors (K-M dwarfs), we propose a theoretical test. A full population synthesis calculation of the Galactic Centre region has been carried out. Our results indicate that the numbers and X-ray luminosities of intermediate polars are consistent with the observed faint hard Galactic Centre population. We discuss the properties of the intermediate polar population, and suggest future tests for the hypothesis. For example, the derived slope of the X-ray luminosity function from our synthetic population of 0.8 could be compared with the observed slope, once one has been obtained from observations. ------------------------------------------------------------------------ Email : dmarrone@cfa.harvard.edu Title : Interferometric Measurements of Variable 340 GHz Linear Polarization in Sagittarius A* Author(s): Daniel P. Marrone, James M. Moran, Jun-Hui Zhao, Ramprasad Rao Institute: (1) Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 Paper : ApJ, accepted EPrint : astro-ph/0511653 Abstract: Using the Submillimeter Array, we have made the first high angular resolution measurements of the linear polarization of Sagittarius A* at submillimeter wavelengths, and the first detection of intra-day variability in its linear polarization. We detected linear polarization at 340 GHz (880 um) at several epochs. At the typical resolution of 1.''4*2.''2, the expected contamination from the surrounding (partially polarized) dust emission is negligible. We found that both the polarization fraction and position angle are variable, with the polarization fraction dropping from 8.5% to 2.3% over three days. This is the first significant measurement of variability in the linear polarization fraction in this source. We also found variability in the polarization and total intensity within single nights, although the relationship between the two is not clear from these data. The simultaneous 332 and 342 GHz position angles are the same, setting a one-sigma rotation measure (RM) upper limit of 7*10^5 rad/m^2. From position angle variations and comparison of ``quiescent'' position angles observed here and at 230 GHz we infer that the RM is a few*10^5 rad/m^2, a factor of a few below our direct detection limit. A generalized model of the RM produced in the accretion flow suggests that the accretion rate at small radii must be low, below 10^-6-10^-7 Mo/yr depending on the radial density and temperature profiles, but in all cases below the gas capture rate inferred from X-ray observations. ------------------------------------------------------------------------ Email : lev.titarchuk@nrl.navy.mil Title : The Observed Galactic Annihilation Line: Possible Signature of Accreting Small Mass Black Holes in the Galactic Center Author(s): Lev Titarchuk (GMU/NRL/GSFC) and Pascal Chardonnet (LAPTH/LAPP) Paper : ApJ (2006), Vol. 639 (March 10, 2006), in press Abstract: Various balloon and satellite observatories have revealed what appears to be an extended source of 0.511 MeV annihilation radiation with flux of 10^-3 photons cm^-2 s^-1 centered on the Galactic Center. Positrons from radioactive products of stellar explosions can account for a significant fraction of the emission. We discuss an additional source for this emission: namely e^+e^- pairs produced when X-rays generated from the 2.6x10^6 M solar mass Galactic Center Black Hole interact with 10 MeV temperature blackbody emission from 10^17 g black holes within 10^14-15 cm of the center. The number of such Small Mass Black Holes (SMMBHs) can account for the production of the 10^42 e^+ s^-1 that produces the observed annihilation in the inner Galaxy when transport effects are taken into account. We consider the possibility for confirming the presence of these SMMBHs in the Galactic Center region with future generations of gamma-ray instruments if a blackbody like emission of 10 MeV temperature would be detected by them. Small Mass Black Hole can be a potential candidate for dark (invisible) matter halo. ------------------------------------------------------------------------ Email : ste@mpe.mpg.de Title : Variations in the spectral slope of Sgr A* during a NIR flare Author(s): S. Gillessen^1, F. Eisenhauer^1, E. Quataert^2, R. Genzel^1, 3, T. Paumard^1, S. Trippe^1, T. Ott^1, R. Abuter^1, A. Eckart^4, P. O. Lagage^5, M. D. Lehnert^1, L. J. Tacconi^1, F. Martins^1 Institute: (1) Max-Planck-Institut fuer Extraterrestrische Physik, 85748 Garching, Germany (2) Astronomy Department, University of California, Berkeley, CA 94720, USA (3) Physics Department, University of California, Berkeley, CA 94720, USA (4) Physikalisches Institut, Universitaet zu Koeln, Zuelpicher Str. 77, 50937 Koeln, Germany (5) UMR 7158, CEA-CNRS-Universite Paris 7, DSM/DAPNIA/Service d'Astrophysique, CEA/Saclay, France Paper : ApjL, submitted EPrint : astro-ph/0511302 Web : http://www.mpe.mpg.de/~ste/ms_gillessen.pdf Abstract: We have observed a bright flare of Sgr A* in the near infrared with the adaptive optics assisted integral field spectrometer SINFONI This work is based on observations collected at the European Southern Observatory, Paranal, Chile. Program ID: 075.B-0547(B). The observed spectrum is featureless and can be described by a power law. We show for the first time that the spectral index is correlated with the instantaneous flux and that both flux and spectral index experience significant changes within less than one hour. We argue that the near infrared flares from Sgr A* are due to synchrotron emission of transiently heated electrons, the emission being affected by orbital dynamics and synchrotron cooling, both acting on timescales of 20 minutes. The synchrotron cooling process may account for the observed variation in spectral index, which in turn requires a magnetic field 30 G, consistent with the equipartition field in a hot accretion flow with an accretion rate of 10^-8 M_o/yr. ------------------------------------------------------------------------ Email : tagger@cea.fr Title : A Possible Rossby Wave Instability Origin for the Flares in Sagittarius A* Author(s): Michel Tagger ^1, Fulvio Melia^2^3 Institute: (1) Service d'Astrophysique, (UMR AstroParticules et Cosmologie), CEA Saclay, 91191 Gif-sur-Yvette, France (2) Departments of Physics and Astronomy, The University of Arizona,, Tucson AZ 85721, USA (3) Sir Thomas Lyle Fellow and Miegunyah Fellow. Paper : Ap.J. Letters, accepted EPrint : astro-ph/0511520 Abstract: In recent years, near-IR and X-ray flares have been detected from the Galaxy's central radio point source, Sagittarius A* (Sgr A*), believed to be a 3 * 10^6 M_o supermassive black hole. In some cases, the transient emission appears to be modulated with a (quasi-)period of 17-20 minutes. The implied 3 r_S size of the emitter (where r_S 2GM/c^2 is the Schwarzschild radius) points to an instability-- possibly induced by accretion--near the Marginally Stable Orbit (MSO) of a slowly spinning object. But Sgr A* is not accreting via a large, `standard' disk; instead, the low density environment surrounding it apparently feeds the black hole with low angular momentum clumps of plasma that circularize within 10-300 r_S and merge onto a compact, hot disk. In this Letter, we follow the evolution of the disk following such an event, and show that a Rossby wave instability, particularly in its magnetohydrodynamic form, grows rapidly and produces a period of enhanced accretion lasting several hours. Both the amplitude of this response, and its duration, match the observed flare characteristics rather well. ------------------------------------------------------------------------ (Older versions of the Newsflash can be found at the gcnews web-page) ======================================================================== Edited by Sera Markoff, Loránt Sjouwerman, Joseph Lazio, Cornelia Lang, Rainer Schödel, Robin Herrnstein - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - For Abstract submission please follow the instructions which are at http://www.aoc.nrao.edu/~gcnews/home/submission.shtml ========================================================================