======================================================================== 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. 9 Nov 30, 2006 Recently submitted papers: -------------------------- 1) An Unambiguous Detection of Faraday Rotation in Sagittarius A* (Marrone et al., ApJL) 2) Discovery of Variable Iron Fluorescence from Reflection Nebulae in the Galactic Center (Muno et al., ApJL) 3) The Stellar Mass Spectrum in Warm and Dusty Gas: Deviations from Salpeter in the Galactic Centre and in Circum-Nuclear Starburst Regions (Klessen et al., MNRAS) 4) A polarised infrared flare from Sagittarius A* and the signatures of orbiting plasma hotspots (Trippe et al., MNRAS) ------------------------------------------------------------------------ Email : dmarrone@cfa.harvard.edu Title : An Unambiguous Detection of Faraday Rotation in Sagittarius A* Author(s): Daniel P. Marrone(1,2) James M. Moran(1) Jun-Hui Zhao(1) Ramprasad Rao(3) Institute: (1) Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, (2) Current Address: Jansky Fellow, NRAO, University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637 (3) Institute of Astronomy and Astrophysics, Academia Sinica, P.O. Box 23-141, Taipei 10617, Taiwan. Paper : ApJL, 2006, accepted EPrint : astro-ph/0611791 Abstract: The millimeter/submillimeter wavelength polarization of Sgr A* is known to be variable in both magnitude and position angle on time scales down to a few hours. The unstable polarization has prevented measurements made at different frequencies and different epochs from yielding convincing measurements of Faraday rotation in this source. Here we present observations made with the Submillimeter Array polarimeter at 227 and 343 GHz with sufficient sensitivity to determine the rotation measure at each band without comparing position angles measured at separate epochs. We find the 10-epoch mean rotation measure to be (-5.6+/-0.7)*10^5 rad m^-2; the measurements are consistent with a constant value. We conservatively assign a 3 sigma upper limit of 2*10^5 rad m^-2 to rotation measure changes, which limits accretion rate fluctuations to 25%. This rotation measure detection limits the accretion rate to less than 2*10^-7 M_\sun yr^-1 if the magnetic field is near equipartition, ordered, and largely radial, while a lower limit of 2*10^-9 M_\sun yr^-1 holds even for a sub-equipartition, disordered, or toroidal field. The mean intrinsic position angle is 167 DEG+/-7 DEG and we detect variations of 31^+18_-9 degrees. These variations must originate in the submillimeter photosphere, rather than arising from rotation measure changes. ------------------------------------------------------------------------ Email : mmuno@srl.caltech.edu Title : Discovery of Variable Iron Fluorescence from Reflection Nebulae in the Galactic Center Author(s): M. P. Muno(1), F. K. Baganoff(2), W. N. Brandt(3), S. Park(3) \& M. R. Morris(4) Institute: (1) Space Radiation Laboratory, California Institute of Technology (2) Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology (3) Department of Astronomy and Astrophysics, The Pennsylvania State University (4) Department of Physics and Astronomy, University of California Paper : ApJL Submitted EPrint : astro-ph/0611651 Abstract: Based on three years of deep observations of the Galactic center with the Chandra X-ray Observatory, we report the discovery of changes in the intensities and morphologies of two hard X-ray nebulosities. The nebulosities are dominated by fluorescent iron emission, and are coincident with molecular clouds. The morphological changes are manifest on parsec scales, which requires that these iron features are scattered X-rays from a 2 or 3-year-long outburst of a point source (either \sgrastar or an X-ray binary) with a luminosity of at least 10^37 erg s^-1. The variability precludes the hypotheses that these nebulae either are produced by keV electrons bombarding molecular clouds, or are iron-rich ejecta from supernovae. Moreover, the morphologies of the reflection nebulae implies that the dense regions of the clouds are filamentary, with widths of 0.3 pc and lengths of 2 pc. ------------------------------------------------------------------------ Email : rklessen@ita.uni-heidelberg.de Title : The Stellar Mass Spectrum in Warm and Dusty Gas: Deviations from Salpeter in the Galactic Centre and in Circum-Nuclear Starburst Regions Author(s): Ralf S. Klessen^1,2, Marco Spaans^3, Anne-Katharina Jappsen^2,4 Institute: 1) Zentrum fuer Astronomie der Universitaet Heidelberg, Institut fuer Theoretische Astrophysik 2) Astrophysikalisches Institut Potsdam 3) Kapteyn Astronomical Institute, 4) Canadian Institute for Theoretical Astrophysics Paper : MNRAS, in press EPrint : astro-ph/0610557 Web : http://www.blackwell-synergy.com/doi/abs/10.1111/j.1745-3933. 2006.00258.x Abstract: Understanding the origin of stellar masses is a key problem in astrophysics. In the solar neighborhood, the mass distribution of stars follows a seemingly universal pattern. In the centre of the Milky Way, however, there are indications for strong deviations and the same may be true for the nuclei of distant starburst galaxies. Here we present the first numerical hydrodynamical calculations of stars formed in a molecular region with chemical and thermodynamic properties similar to those of warm and dusty circum-nuclear starburst regions. The resulting IMF is top-heavy with a peak at 15 M_o, a sharp turn-down below 7 M_o and a power-law decline at high masses. We find a natural explanation for our results in terms of the temperature dependence of the Jeans mass, with collapse occuring at a temperature of 100 K and an H_2 density of a few * 10^5 cm^-3, and discuss possible implications for galaxy formation and evolution. ------------------------------------------------------------------------ Email : trippe@mpe.mpg.de Title : A polarised infrared flare from Sagittarius A* and the signatures of orbiting plasma hotspots Author(s): S. Trippe,^1, T. Paumard,^1, T. Ott,^1, S. Gillessen,^1 F. Eisenhauer,^1 F. Martins^1 and R. Genzel^1,2 Institute: 1) Max-Planck-Institut fuer extraterrestrische Physik, Postfach 1312, D-85741 Garching, Germany, 2) Department of Physics, University of California, CA 94720, Berkeley, USA Paper : MNRAS, Nov 2006, in press EPrint : astro-ph/0611737 Abstract: In this article we summarise and discuss the infrared, radio, and X-ray emission from the supermassive black hole in the Galactic Centre, SgrA*. We include new results from near-infrared polarimetric imaging observations obtained on May 31st, 2006. In that night, a strong flare in K_s band (2.08 micron ) reaching top fluxes of 16 mJy could be observed. This flare was highly polarised (up to 40 %) and showed clear sub-structure on a time scale of 15 minutes, including a swing in the polarisation angle of about 70 degrees. For the first time we were able to observe both polarised flux and short-time variability, with high significance in the same flare event. This result adds decisive information to the puzzle of the SgrA* activity. The observed polarisation angle during the flare peak is the same as observed in two events in 2004 and 2005. Our observations strongly support the dynamical emission model of a decaying plasma hotspot orbiting SgrA* on a relativistic orbit. The observed polarisation parameters and their variability with time might allow to constrain the orientation of accretion disc and spin axis with respect to the Galaxy. ------------------------------------------------------------------------ (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 ========================================================================