======================================================================== 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. 9 Jun 11, 2009 Recently submitted papers: -------------------------- 1) Simultaneous H.E.S.S. and Chandra observations of Sgr A* during an X-ray flare (Aharonian et al., A&A) 2) Influence of a stellar cusp on the dynamics of young stellar discs and the origin of the S-stars in the Galactic Centre (Loeckmann et al., MNRAS) 3) On the number of young stellar discs in the Galactic Centre (Loeckmann & Baumgardt, MNRAS) ------------------------------------------------------------------------ Email : j.a.hinton@leeds.ac.uk Title : Simultaneous H.E.S.S. and Chandra observations of Sgr A* during an X-ray flare Author(s): F. Aharonian and 154 other authors Paper : A&A, 492, L25 Abstract: The rapidly varying ( 10 minute timescale) non-thermal X-ray emission observed from Sgr A* implies that particle acceleration is occuring close to the event horizon of the supermassive black hole. The TeV gamma -ray source HESS J1745-290 is coincident with Sgr A* and may be closely related to its X-ray emission. Simultaneous X-ray and TeV observations are required to elucidate the relationship between these objects. We report on joint H.E.S.S./Chandra observations performed in July 2005, during which an X-ray flare was detected. Despite a factor of 9 increase in the X-ray flux of Sgr A*, no evidence is found for an increase in the TeV gamma -ray flux from this region. We find that an increase in the gamma -ray flux of a factor of 2 or greater can be excluded at a confidence level of 99%. This finding disfavours scenarios in which the keV and TeV emission are associated with the same population of accelerated particles and in which the bulk of the gamma -ray emission is produced within 10^14 cm ( 100 R_S) of the supermassive black hole. ------------------------------------------------------------------------ Email : uloeck@astro.uni-bonn.de Title : Influence of a stellar cusp on the dynamics of young stellar discs and the origin of the S-stars in the Galactic Centre Author(s): Ulf Loeckmann, Holger Baumgardt, and Pavel Kroupa Paper : MNRAS, 2009, in press EPrint : 0906.0574 Abstract: Observations of the Galactic Centre show evidence of one or two disc-like structures of very young stars orbiting the central super-massive black hole within a distance of a few 0.1 pc. A number of analyses have been carried out to investigate the dynamical behaviour and consequences of these discs, including disc thickness and eccentricity growth as well as mutual interaction and warping. However, most of these studies have neglected the influence of the stellar cusp surrounding the black hole, which is believed to be 1-2 orders of magnitude more massive than the disc(s). By means of N-body integrations using our BHINT code, we study the impact of stellar cusps of different compositions. We find that although the presence of a cusp does have an important effect on the evolution of an otherwise isolated flat disc, its influence on the evolution of disc thickness and warping is rather mild in a two-disc configuration. However, we show that the creation of highly eccentric orbits strongly depends on the graininess of the cusp (i.e. the mean and maximum stellar masses): While Chang (2008) recently found that full cycles of Kozai resonance are prevented by the presence of an analytic cusp, we show that relaxation processes play an important role in such highly dense regions and support short-term resonances. We thus find that young disc stars on initially circular orbits can achieve high eccentricities by resonant effects also in the presence of a cusp of stellar remnants, yielding a mechanism to create S-stars and hyper-velocity stars. Furthermore, we discuss the underlying initial mass function (IMF) of the young stellar discs and find no definite evidence for a non-canonical IMF. ------------------------------------------------------------------------ Email : uloeck@astro.uni-bonn.de Title : On the number of young stellar discs in the Galactic Centre Author(s): Ulf Loeckmann and Holger Baumgardt Paper : MNRAS, 394, 1841-1846, 2009 EPrint : 0901.1660 Web : http://www3.interscience.wiley.com/cgi-bin/fulltext/122245841 /HTMLSTART Abstract: Observations of the Galactic Centre show evidence of disc-like structures of very young stars orbiting the central super-massive black hole within a distance of a few 0.1 pc. While it is widely accepted that about half of the stars form a relatively flat disc rotating clockwise on the sky, there is a substantial ongoing debate on whether there is a second, counter-clockwise disc of stars. By means of N-body simulations using our BHINT code, we show that two highly inclined stellar discs with the observed properties cannot be recognised as two flat circular discs after 5 Myr of mutual interaction. Instead, our calculations predict a significant warping of the two discs, which we show to be apparent among the structures observed in the Galactic Centre. While the high eccentricities of the observed counter-clockwise orbits suggest an eccentric origin of this system, we show the eccentricity distribution in the inner part of the more massive clockwise disc to be perfectly consistent with an initially circular disc in which stellar eccentricities increase due to both non-resonant and resonant relaxation. We conclude that the relevant question to ask is therefore not whether there are two discs of young stars, but whether there were two such discs to begin with. ------------------------------------------------------------------------ (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 ========================================================================