======================================================================== G C N E W S * Newsflash * - The Newsletter for Galactic Center Research - gcnews@aoc.nrao.edu http://www.aoc.nrao.edu/~gcnews ======================================================================== Vol. 27, No. 1 May 9, 2007 Please note that there is a new book on Sagittarius A*: "The Galactic Supermassive Black Hole" by Fulvio Melia Princeton University Press (2007) You can check it out at http://press.princeton.edu/titles/8453.html or http://www.amazon.com/Galactic-Supermassive-Black-Hole/dp/0691131295 Recently submitted papers: -------------------------- 1) Variable accretion and emission from the stellar winds in the Galactic centre (Cuadra et al., MNRAS) 2) General Relativistic Flux Modulations from Disk Instabilities in Sagittarius A* (Falanga et al., ApJ) ------------------------------------------------------------------------ Email : jcuadra@cervantes.colorado.edu Title : Variable accretion and emission from the stellar winds in the Galactic centre Author(s): Jorge Cuadra, Sergei Nayakshin, Fabrice Martins Paper : MNRAS, submitted Web : http://www.arxiv.org/ps/0705.0769 Abstract: We present numerical simulations of stellar wind dynamics in the central parsec of the Galactic centre. We are particularly interested in accretion of gas on to Sgr A*, the super-massive black hole. Unlike our previous efforts, here we use the state of the art observational data on orbits and wind properties of individual wind-producing stars. Since wind velocities were revised upwards and non-zero eccentricities were considered, our new simulations show fewer clumps of cold gas and no conspicuous disc-like structure. The accretion flow circularisation radius is roughly unchaged, 5000 Schwarzschild radii. The accretion rate is dominated by a few close `slow wind stars' (v_w <= 750 km/sec ), and is consistent with the Bondi estimate, but variable on time-scales of tens to hundreds of years. This variability is due to the stochastic in-fall of cold clumps, as in earlier simulations, and to the eccentric orbits of stars. The present models fail to explain the higher luminosity of Sgr A* a few hundred years ago implied by Integral observations, but we argue that the accretion of a cold clump with a small impact parameter could have caused it. Finally, we show the possibility of constraining the total mass-loss rate of the `slow wind stars' using near infra-red observations of gas in the central few arcseconds. ------------------------------------------------------------------------ Email : mfalanga@cea.fr Title : General Relativistic Flux Modulations from Disk Instabilities in Sagittarius A* Author(s): Maurizio Falanga(1,2) Fulvio Melia(3,4) Michel Tagger(1,5) Andrea Goldwurm(1,5) and Guillaume Belanger(6) Institute: (1) CEA Saclay, DSM/DAPNIA/Service d'Astrophysique, 91191 Gif-sur-Yvette, France (2) AIM - Unite Mixte de Recherche CEA - CNRS - Universite Paris 7 - UMR no 7158, France (3) Physics Department and Steward Observatory, The University of Arizona, Tucson, AZ 85721 (4) Sir Thomas Lyle Fellow and Miegunyah Fellow. (5) Universite Paris Diderot-Paris 7 et Observatoire de Paris, Laboratoire APC, Paris, France (6) ESA/ESAC, Apartado 50727, 28080 Madrid, Spain. Paper : ApJ, accepted EPrint : astro-ph/0705.0238 Abstract: Near-IR and X-ray flares have been detected from the supermassive black hole Sgr A* at the center of our Galaxy with a (quasi)-period of 17-20 minutes, suggesting an emission region only a few Schwarzschild radii above the event horizon. The latest X-ray flare, detected with XMM-Newton, is notable for its detailed lightcurve, yielding not only the highest quality period thus far, but also important structure reflecting the geometry of the emitting region. Recent MHD simulations of Sgr A*'s disk have demonstrated the growth of a Rossby wave instability, that enhances the accretion rate for several hours, possibly accounting for the observed flares. In this Letter, we carry out ray-tracing calculations in a Schwarzschild metric to determine as accurately as possible the lightcurve produced by general relativistic effects during such a disruption. We find that the Rossby wave induced spiral pattern in the disk is an excellent fit to the data, implying a disk inclination angle of 77^o. Note, however, that if this association is correct, the observed period is not due to the underlying Keplerian motion but, rather, to the pattern speed. The favorable comparison between the observed and simulated lightcurves provides important additional evidence that the flares are produced in Sgr A*'s inner disk. ------------------------------------------------------------------------ (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 ========================================================================