======================================================================== G C N E W S * Newsflash * - The Newsletter for Galactic Center Research - gcnews@aoc.nrao.edu http://www.aoc.nrao.edu/~gcnews ======================================================================== Vol. 25, No. 3 Aug 24, 2006 Recently submitted papers: -------------------------- 1) Stringent Constraint on Galactic Positron Production (Beacom & Yueksel, PRL) 2) Correlation between Flux and Spectral Index during Flares in Sagittarius A* (Bittner et al., ApJ) ------------------------------------------------------------------------ Email : beacom@mps.ohio-state.edu Title : Stringent Constraint on Galactic Positron Production Author(s): John F. Beacom Hasan Yueksel Paper : PRL, Aug 2006, in press EPrint : astro-ph/0512411 Abstract: The intense 0.511 MeV gamma-ray line emission from the Galactic Center observed by INTEGRAL requires a large annihilation rate of nonrelativistic positrons. If these positrons are injected at even mildly relativistic energies, higher-energy gamma rays will also be produced. We calculate the gamma-ray spectrum due to inflight annihilation and compare to the observed diffuse Galactic gamma-ray data. Even in a simplified but conservative treatment, we find that the positron injection energies must be <=sssim 3 MeV, which strongly constrains models for Galactic positron production. ------------------------------------------------------------------------ Email : jon.bittner@gmail.com Title : Correlation between Flux and Spectral Index during Flares in Sagittarius A* Author(s): Jonathan M. Bittner^1, Siming Liu^2, Christopher L. Fryer^2,3 and Vahe Petrosian^4 Institute: (1) Physics Department, Yale University, New Haven, CT 06520-8120 (2) Los Alamos National Laboratory, Los Alamos, NM 87545 (3) Physics Department, The University of Arizona, Tucson, AZ 85721 (4) Center for Space Science and Astrophysics, Department of Physics and Applied Physics, Stanford University, Stanford, CA 94305 Paper : ApJ, submitted (Tue Aug 8 2006) EPrint : astro-ph/0608232 Abstract: Flares in Sagittarius A* are produced by hot plasmas within a few Schwarzschild radii of the supermassive black hole at the Galactic center. The recent detection of a correlation between the spectral index and flux during a near infrared (NIR) flare provides a means to conduct detailed investigations of the plasma heating and radiation processes. We study the evolution of the electron distribution function under the influence of a turbulent magnetic field in a hot collisionless plasma. The magnetic field, presumably generated through instabilities in the accretion flow, can both heat the plasma via resonant wave-particle coupling and cool the electrons via radiation. The electron distribution can generally be approximated as relativistic Maxwellian. To account for the observed correlation, we find that the magnetic field needs to be anti-correlated with the electron ``temperature''. NIR and X-ray light curves are produced for a cooling and a heating phase. The model predicts simultaneous flare activity in the NIR and X-ray bands, which can be compared with observations. These results can be applied to MHD simulations to study the radiative characteristics of collisionless plasmas, especially accretion flows in low-luminosity AGNs. ------------------------------------------------------------------------ (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 ========================================================================