======================================================================== G C N E W S * Newsflash * - The Newsletter for Galactic Center Research - gcnews@aoc.nrao.edu http://www.aoc.nrao.edu/~gcnews ======================================================================== Vol. 29, No. 14 Sep 18, 2009 Recently submitted papers: -------------------------- 1) Spectrum and variability of the Galactic Center VHE gamma -ray source HESS J1745-290 (Aharonian et al., A&A) 2) The Distribution of Stars and Stellar Remnants at the Galactic Center (Merritt, ApJ) 3) On the Origin of the 511 keV Emission in the Galactic Centre (Bandyopadhyay et al., MNRAS) 4) A Trigonometric Parallax of Sgr B2 (Reid et al., submitted) 5) Strong mass segregation around a massive black hole (Alexander & Hopman, ApJ) ------------------------------------------------------------------------ Email : matthieu.vivier@cea.fr Title : Spectrum and variability of the Galactic Center VHE gamma -ray source HESS J1745-290 Author(s): F. Aharonian et al. Paper : A&A, September 2009, in press Web : http://arxiv.org/abs/0906.1247 Abstract: A detailed study of the spectrum and variability of the source HESS J1745-290 in the Galactic Center (GC) region using new data from the H.E.S.S. array of Cherenkov telescopes is presented. Flaring activity and quasi periodic oscillations (QPO) of HESS J1745-290 are investigated. The image analysis is performed with a combination of a semi-analytical shower model and the statistical moment-based Hillas technique. The spectrum and lightcurves of HESS J1745-290 are derived with a likelihood method based on a spectral shape hypothesis. Rayleigh tests and Fourier analysis of the H.E.S.S. GC signal are used to study the periodicity of the source. With a three-fold increase in statistics compared to previous work, a deviation from a simple power law spectrum is detected for the first time. The measured energy spectrum over the three years 2004, 2005 and 2006 of data taking is compatible with both a power law spectrum with an exponential cut-off and a broken power law spectrum. The curvature of the energy spectrum is likely to be intrinsic to the photon source, as opposed to effects of interstellar absorption. The power law spectrum with an exponential cut-off is characterized by a photon index of 2.10 +/- 0.04_\mathrmstat +/- 0.10_\mathrmsyst and a cut-off energy at 15.7 +/- 3.4_\mathrmstat +/- 2.5_\mathrmsyst TeV. The broken power law spectrum exhibits spectral indices of 2.02 +/- 0.08_\mathrmstat +/- 0.10_\mathrmsyst and 2.63 +/- 0.14_\mathrmstat +/- 0.10_\mathrmsyst with a break energy at 2.57 +/- 0.19_\mathrmstat +/- 0.44_\mathrmsyst TeV. No significant flux variation is found. Increases in the gamma -ray flux of HESS J1745-290 by at least a factor of two would be required for a 3 sigma detection of a flare with time scales of an hour. Investigation of possible QPO activity at periods claimed to be detected in X-rays does not show any periodicities in the H.E.S.S. signal. ------------------------------------------------------------------------ Email : merritt@astro.rit.edu Title : The Distribution of Stars and Stellar Remnants at the Galactic Center Author(s): David Merritt Institute: (1) Department of Physics and Center for Computational Relativity and Gravitation, Rochester Institute of Technology, Rochester, NY 14623, USA Paper : ApJ, submitted Abstract: Motivated by recent observations that suggest a low density of old stars around the Milky Way supermassive black hole, evolutionary models for the nuclear star cluster are considered that postulate a parsec-scale core as initial conditions. Gravitational encounters cause the core to shrink; a core of initial radius 1-1.5 pc evolves to a size of 0.5 pc after 10 Gyr, roughly the size of the observed core. The absence of a Bahcall-Wolf cusp is naturally explained. In these models, the time for a 10\msun black hole to spiral in to the Galactic center from an initial distance of 5 pc can be much greater than 10 Gyr. Assuming that the stellar black holes had the same phase-space distribution initially as the stars, their density after 5-10 Gyr is predicted to rise very steeply going into the stellar core, but to remain substantially below the densities inferred from steady-state models that include a steep density cusp in the stars. The implications of these models are discussed for the rates of gravitational-wave inspiral events and of other physical processes that depend on a high density of stars or stellar mass black holes near \sgr. ------------------------------------------------------------------------ Email : reba@astro.ufl.edu Title : On the Origin of the 511 keV Emission in the Galactic Centre Author(s): Reba M. Bandyopadhyay^1, Joseph Silk^2, James E. Taylor^3 and Thomas J. Maccarone^4 Institute: (1) Department of Astronomy, 211 Bryant Space Science Centre, Gainesville, FL 32611-2055 USA (2) Department of Astrophysics, Denys Wilkinson Building, 1 Keble Road, Oxford OX1 3RH, UK (3) Department of Physics \& Astronomy, 200 University Avenue West, Waterloo, Ontario, N2L 3G1 Canada (4) School of Physics \& Astronomy, University of Southampton, Highfield, Southampton SO17 1BJ, UK Paper : MNRAS, 2009, vol. 392 EPrint : 0810.3674 Abstract: Diffuse 511 keV line emission, from the annihilation of cold positrons, has been observed in the direction of the Galactic Centre for more than 30 years. The latest high-resolution maps of this emission produced by the SPI instrument on INTEGRAL suggest at least one component of the emission is spatially coincident with the distribution of 70 luminous, low-mass X-ray binaries detected in the soft gamma-ray band. The X-ray band, however, is generally a more sensitive probe of X-ray binary populations. Recent X-ray surveys of the Galactic Centre have discovered a much larger population (>4000) of faint, hard X-ray point sources. We investigate the possibility that the positrons observed in the direction of the Galactic Centre originate in pair-dominated jets generated by this population of fainter accretion-powered X-ray binaries. We also consider briefly whether such sources could account for unexplained diffuse emission associated with the Galactic Centre in the microwave (the WMAP `haze') and at other wavelengths. Finally, we point out several unresolved problems in associating Galactic Centre 511 keV emission with the brightest X-ray binaries. ------------------------------------------------------------------------ Email : reid@cfa.harvard.edu Title : A Trigonometric Parallax of Sgr B2 Author(s): M. J. Reid(1), K. M. Menten(2), X. W. Zheng(3), A. Brunthaler(2), Y. Xu(4) Institute: (1) Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA (2) Max-Planck-Institut fuer Radioastronomie, Auf dem Huegel 69, 53121 Bonn, Germany (3) Department of Astronomy, Nanjing University Nanjing 210093, China (4) Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008, China Paper : submitted to ApJ EPrint : 0908.3637 Abstract: We have measured the positions of H2O masers in Sgr B, a massive star forming region in the Galactic center, relative to an extragalactic radio source with the Very Long Baseline Array. The positions measured at 12 epochs over a time span of one year yield the trigonometric parallax of Sgr B and hence a distance to the Galactic center of Ro=7.9^+0.8_-0.7 kpc. The proper motion of Sgr B relative to Sgr A suggests that Sgr B is 0.13 kpc nearer than the Galactic center, assuming a low-eccentricity Galactic orbit. ------------------------------------------------------------------------ Email : tal.alexander@weizmann.ac.il Title : Strong mass segregation around a massive black hole Author(s): Tal Alexander, Clovis Hopman Institute: (1) Faculty of Physics, Weizmann Institute of Science, P.O. Box 26, Rehovot 76100, Israel (2) Leiden University, Leiden Observatory, P.O. box 9513, NL-2300 RA Leiden, The Netherlands Paper : ApJ, June 2009, 697, 1861 Web : http://adsabs.harvard.edu/abs/2009ApJ...697.1861A Abstract: We show that the mass-segregation solution for the steady state distribution of stars around a massive black hole (MBH) has two branches: the well-known weak segregation solution \citepbah+77, and a strong segregation solution, which is analyzed here for the first time. The nature of the solution depends on the heavy-to-light stellar mass ratio M_H/M_L and on the unbound population number ratio N_H/N_L, through the relaxational coupling parameter \Delta = 4N_HM_H^2<=ft/<=ft[N_LM_L^2(3 + M_H/M_L)\right]\ri ght.. When the heavy stars are relatively common (\Delta \gg 1), they scatter frequently on each other. This efficient self-coupling leads to weak mass segregation, where the stars form n \propto r^- alpha _M mass-dependent cusps near the MBH, with indices alpha _H = 7/4 for the heavy stars and 3/2 < alpha _L < 7/4 for the light stars (i.e. \max( alpha _H - alpha _L) 1/4). However, when the heavy stars are relatively rare (\Delta \ll 1), they scatter mostly on light stars, sink to the center by dynamical friction and settle into a much steeper cusp with 2 <=sssim alpha _H <=sssim 11/4, while the light stars form a 3/2 < alpha _L < 7/4 cusp, resulting in strong segregation (i.e. \max( alpha _H - alpha _L) 1). We show that the present-day mass function of evolved stellar populations with a universal initial mass function (coeval or continuously star forming) separate into two distinct mass scales, 1 \Mo of main sequence and compact dwarfs, and 10 \Mo of stellar black holes (SBHs), and have \Delta < 0.1. We conclude that it is likely that many relaxed galactic nuclei are strongly segregated. We review indications of strong segregation in observations of the Galactic Center and in results of numeric simulations, and briefly list possible implications of a very high central concentration of SBHs around a MBH. ------------------------------------------------------------------------ (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, Masaaki Sakano, Feng Yuan - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - For Abstract submission please follow the instructions which are at http://www.aoc.nrao.edu/~gcnews/home/submission.shtml ========================================================================