======================================================================== G C N E W S * Newsflash * - The Newsletter for Galactic Center Research - gcnews@aoc.nrao.edu http://www.aoc.nrao.edu/~gcnews ======================================================================== Vol. 24, No. 2 Mar 23, 2006 Recently submitted papers: -------------------------- 1) Complex small-scale structure in the infrared extinction towards the Galactic Centre (Gosling et al., ApJL) 2) Challenges in Detecting Gamma-Rays From Dark Matter Annihilations in the Galactic Center (Hooper, astro-ph/0603540) 3) Stellar remnants in galactic nuclei: mass segregation (Freitag et al., ApJ) 4) Sgr A* as probe of the theory of supermassive compact objects without event horizon (Verozub, Astron.) ------------------------------------------------------------------------ Email : ajgosling@gmail.com Title : Complex small-scale structure in the infrared extinction towards the Galactic Centre Author(s): Andrew J. Gosling, Katherine M. Blundell, Reba Bandyopadhyay Institute: (1) University of Oxford, Department of Physics, Keble Road, Oxford, OX1 3RH, U.K. Paper : ApJL accepted, Feb 2006 EPrint : astro-ph/0602410 Abstract: A high level of complex structure, or ``granularity'', has been observed in the distribution of infrared-obscuring material towards the Galactic Centre (GC), with a characteristic scale of 5'' - 15'', corresponding to 0.2 - 0.6 pc at a GC distance of 8.5 kpc. This structure has been observed in ISAAC images which have a resolution of 0.6'', significantly higher than that of previous studies of the GC. We have discovered granularity throughout the GC survey region, which covers an area of 1.6^o * 0.8^o in longitude and latitude respectively (300 pc * 120 pc at 8.5 kpc) centred on Sgr A*. This granularity is variable over the whole region, with some areas exhibiting highly structured extinction in one or more wavebands and other areas displaying no structure and a uniform stellar distribution in all wavebands. The granularity does not appear to correspond to longitude, latitude or radial distance from Sgr A*. We find that regions exhibiting high granularity are strongly associated with high stellar reddening. ------------------------------------------------------------------------ Email : dhooper@fnal.gov Title : Challenges in Detecting Gamma-Rays From Dark Matter Annihilations in the Galactic Center Author(s): Gabrijela Zaharijas and Dan Hooper Institute: (1) Fermi National Accelerator Laboratory, Particle Astrophysics Center, Batavia, IL 60510 Paper : astro-ph/0603540 EPrint : astro-ph/0603540 Abstract: Atmospheric Cerenkov Telescopes, including HESS and MAGIC, have detected a spectrum of gamma-rays from the galactic center region which extends from 200 GeV or lower, to at least 10 TeV. Although the source of this radiation is not yet known, the spectrum appears to behave as a simple power-law, which is not the expectation for gamma-rays generated through the annihilation of dark matter particles. If instead we conclude that the source of these gamma-rays is astrophysical in origin, this spectrum will constitute a background for future dark matter searches using gamma-rays from this region. In this paper we study how this background will affect the prospects for experiments such as GLAST to detect dark matter in the galactic center. We find that only a narrow range of dark matter annihilation rates are potentially observable by GLAST given this newly discovered background and considering current constraints from EGRET and HESS. We also find that a detection of line emission, while not completely ruled out, is only possible for a very narrow range of dark matter models and halo profiles. ------------------------------------------------------------------------ Email : marc.freitag@gmail.com Title : Stellar remnants in galactic nuclei: mass segregation Author(s): Marc Freitag(1,2), Pau Amaro-Seoane(3), Vassiliki Kalogera(1) Institute: (1) Department of Physics and Astronomy, Northwestern University, Evanston, IL 60208, USA (2) Institute of Astronomy, University of Cambridge, Madingley Road, CB3 0HA Cambridge, UK (3) Max Planck Intitut fuer Gravitationsphysik (Albert-Einstein-Institut), D-14476 Potsdam, Germany Paper : ApJ, Mar 2006, submitted EPrint : astro-ph/0603280 Abstract: The study of how stars distribute themselves around a massive black hole (MBH) in the center of a galaxy is an important prerequisite for the understanding of many galactic-center processes. These include the observed overabundance of point X-ray sources at the Galactic center, the prediction of rates and characteristics of tidal disruptions of extended stars by the MBH and of inspirals of compact stars into the MBH, the latter being events of high importance for the future space borne gravitational wave interferometer LISA. In relatively small galactic nuclei, hosting MBHs with masses in the range 10^5-10^7 M_o, the single most important dynamical process is 2-body relaxation. It induces the formation of a steep density cusp around the MBH and strong mass segregation, as more massive stars lose energy to lighter ones and drift to the central regions. Using a spherical stellar dynamical Monte-Carlo code, we simulate the long-term relaxational evolution of galactic nucleus models with a spectrum of stellar masses. Our focus is the concentration of stellar black holes to the immediate vicinity of the MBH. We quantify this mass segregation for a variety of galactic nucleus models and discuss its astrophysical implications. Special attention is given to models developed to match the conditions in the Milky Way nucleus; we examine the presence of compact objects in connection to recent high-resolution X-ray observations. ------------------------------------------------------------------------ Email : verozub@t-online.de Title : Sgr A* as probe of the theory of supermassive compact objects without event horizon Author(s): L. Verozub Institute: Kharkov National University, Kharkov, Ukraine EPrint : astro-ph/0601662 Abstract: In the present paper some consequences of the hypothesis that the supermassive compact object in the Galaxy centre relates to a class of objects without event horizon are examined. The possibility of the existence of such objects was substantiated by the author earlier. It is shown that accretion of a surrounding gas can cause nuclear combustion in the surface layer which, as a result of comptonization of the superincumbent hotter layer, may give a contribution to the observed Sgr A* radiation in the range 10^15 \div 10^20 Hz. It is found a contribution of the possible proper magnetic moment of the object to the observed synchrotron radiation on the basis of Boltzmann's equation for photons which takes into account the influence of gravity to their motion and frequency. We arrive at the conclusion that the hypothesis of the existence in the Galaxy centre of the object with such extraordinary gravitational properties at least does not contradict observations. ------------------------------------------------------------------------ (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 ========================================================================