======================================================================== G C N E W S * Newsflash * - The Newsletter for Galactic Center Research - gcnews@aoc.nrao.edu http://www.aoc.nrao.edu/~gcnews ======================================================================== Vol. 28, No. 12 Oct 9, 2008 Recently submitted papers: -------------------------- 1) High accretion rates in magnetised Keplerian discs mediated by a Parker instability driven dynamo (Levin, A&A) 2) Simulation of Cosmic Ray propagation in the Galactic Centre Ridge in Accordance with Observed VHE gamma -ray Emission (STAVROS et al., ICRC2007) 3) A near-IR variability study of the Galactic black hole: a red noise source with no detected periodicity (Do et al., ApJ) 4) Kinematics of the old stellar population at the Galactic Center (Gerhard et al., A&A) 5) Using Millimeter VLBI to Constrain RIAF Models of Sagittarius A* (Fish et al., ApJL) ------------------------------------------------------------------------ Email : ajohan@mail.strw.leidenuniv.nl Title : High accretion rates in magnetised Keplerian discs mediated by a Parker instability driven dynamo Author(s): Anders Johansen(1) and Yuri Levin(1) Institute: (1) Leiden Observatory, Leiden University, P.O. Box 9513, 2300 RA Leiden, The Netherlands Paper : A&A, accepted EPrint : 0808.3579 Web : http://arxiv.org/abs/0808.3579 Abstract: Hydromagnetic stresses in accretion discs have been the subject of intense theoretical research over the past one and a half decades. Most of the disc simulations have assumed a small initial magnetic field and studied the turbulence that arises from the magnetorotational instability. However, gaseous discs in galactic nuclei and in some binary systems are likely to have significant initial magnetisation. Motivated by this, we performed ideal magnetohydrodynamic simulations of strongly magnetised, vertically stratified discs in a Keplerian potential. Our initial equilibrium configuration, which has an azimuthal magnetic field in equipartion with thermal pressure, is unstable to the Parker instability. This leads to the expelling of magnetic field arcs, anchored in the midplane of the disc, to around five scale heights from the midplane. Transition to turbulence happens primarily through magnetorotational instability in the resulting vertical fields, although magnetorotational shear instability in the unperturbed azimuthal field plays a significant role as well, especially in the midplane where buoyancy is weak. High magnetic and hydrodynamical stresses arise, yielding an effective alpha -value of around 0.1 in our highest resolution run. Azimuthal magnetic field expelled by magnetic buoyancy from the disc is continuously replenished by the stretching of a radial field created as gas parcels slide in the linear gravity field along inclined magnetic field lines. This dynamo process, where the bending of field lines by the Parker instability leads to re-creation of the azimuthal field, implies that highly magnetised discs are astrophysically viable and that they have high accretion rates. ------------------------------------------------------------------------ Email : sdimis@phys.uoa.gr Title : Simulation of Cosmic Ray propagation in the Galactic Centre Ridge in Accordance with Observed VHE gamma -ray Emission Author(s): DIMITRAKOUDIS STAVROS^1, MASTICHIADIS APOSTOLOS^1, GERANIOS ATHANASIOS^2 Institute: (1) University of Athens, Physics Department, Section of Astrophysics, Astronomy and Mechanics, Panepis-timioupoli 15771, Greece (2) University of Athens, Nuclear and Particle Physics Department, Panepistimioupoli 15771, Greece Paper : ICRC2007 proceedings EPrint : 0807.4448 Abstract: Diffuse VHE gamma radiation from the Galactic Centre ridge observed by the H.E.S.S. telescope has been convincingly linked with the propagation of recently accelerated cosmic rays that interact with molecular hydrogen clouds during their diffusion. Through a series of time-dependent simulations of that diffusion for different propagation parameters we have obtained the most probable values of the diffusion coefficient for the Galactic Centre region. Assuming that the diffusion coefficient is of the form \kappa(E) = \kappa_0(E/E_0)^ delta , then for different optimal combinations of \kappa_0 and delta its value is obtained for cosmic rays originating from a central point (possibly Sgr A East) 10 kyr ago. ------------------------------------------------------------------------ Email : tdo@astro.ucla.edu Title : A near-IR variability study of the Galactic black hole: a red noise source with no detected periodicity Author(s): T. Do, A. M. Ghez, M. R. Morris, S. Yelda, L. Meyer, J. R. Lu S. D. Hornstein K. Matthews Institute: (1) Physics and Astronomy Department, University of California, Los Angeles, CA 90095-1547 (2) Center for Astrophysics and Space Astronomy, Department of Astrophysical and Planetary Sciences, University of Colorado, Boulder, CO 80309 (3) California Institute of Technology, Pasadena, CA Paper : ApJ, in press EPrint : 0810.0446 Abstract: We present the results of near-infrared (2 and 3 micron ) monitoring of Sgr A*-IR with 1 min time sampling using the natural and laser guide star adaptive optics (LGS AO) system at the Keck II telescope. Sgr A*-IR was observed continuously for up to three hours on each of seven nights, between 2005 July and 2007 August. Sgr A*-IR is detected at all times and is continuously variable, with a median observed 2 micron flux density of 0.192 mJy, corresponding to 16.3 magnitude at K^'. These observations allow us to investigate Nyquist sampled periods ranging from about 2 minutes to an hour. Using Monte Carlo simulations, we find that the variability of Sgr A* in this data set is consistent with models based on correlated noise with power spectra having frequency dependent power law slopes between 2.0 to 3.0, consistent with those reported for AGN light curves. Of particular interest are periods of 20 min, corresponding to a quasi-periodic signal claimed based upon previous near-infrared observations and interpreted as the orbit of a `hot spot' at or near the last stable orbit of a spinning black hole. We find no significant periodicity at any time scale probed in these new observations for periodic signals. This study is sensitive to periodic signals with amplitudes greater than 20% of the maximum amplitude of the underlying red noise component for light curves with duration greater than 2 hours at a 98% confidence limit. ------------------------------------------------------------------------ Email : trippe@mpe.mpg.de Title : Kinematics of the old stellar population at the Galactic Center Author(s): S. Trippe (1) and S. Gillessen (1) and O.E. Gerhard (1) and H. Bartko (1) and T.K. Fritz (1) and H.L. Maness (3) and F. Eisenhauer (1) and F. Martins (2) and T. Ott (1) and K. Dodds-Eden (1) and R. Genzel (1,3) Institute: (1) Max-Planck-Institut fuer extraterrestrische Physik, Giessenbachstras se 1, D-85748 Garching, Germany and GRAAL-CNRS, Universite Montpellier II, Pl ace Eugene Bataillon, F-34090 Montpellier, France and University of Californi a, 94720 Berkeley, California, USA Paper : A&A, Oct 2008, in press EPrint : 0810.1040 Abstract: We aim at a detailed description of the kinematic properties of the old (several Gyrs) late-type CO absorption star population among the Galactic Center (GC) cl uster stars. This cluster is composed of a central supermassive black hole (Sgr A*) and a self-gravitating system of stars. Understanding its kinematics thus of fers the opportunity to understand the dynamical interaction between a central p oint mass and the surrounding stars in general, especially in view of understand ing other galactic nuclei. We apply AO assisted near-infrared imaging and integral field spectroscopy using the instruments NAOS/CONICA and SINFONI at the VLT. We obtain proper motions fo r 5445 stars, 3D velocities for 664 stars, and acceleration limits (in the sky p lane) for 750 stars. Global kinematic properties are analyzed using velocity and velocity dispersion distributions, phase-space maps, two-point correlation func tions, and the Jeans equation. We detect for the first time significant cluster rotation in the sense of the ge neral Galactic rotation in proper motions. Out of the 3D velocity dispersion, we derive an improved statistical parallax for the GC of R_0=8.07+/-0.32_sta t+/-0.13_sys kpc. The distribution of 3D stellar speeds can be app roximated by local Maxwellian distributions. Kinematic modelling provides de projected 3D kinematic parameters, including the mass profile of the cluster. We find an upper limit of 4% for the amplitude of fluctuations in the phase-space distribution of the cluster stars compared to a uniform, spherical model cluste r. Using upper limits on accelerations, we constrain the minimum line-of-sight d istances from the plane of Sgr A* of five stars located within the innermost few (projected) arcsec. The stars within 0.7'' radius from the star group IRS13E do not co-move with this group, making it unlikely that IRS13E is the core of a su bstantial star cluster. Overall, the GC late-type cluster is well described as a uniform, isotropic, rotating, dynamically relaxed, phase-mixed system. ------------------------------------------------------------------------ Email : vfish@franklin.haystack.mit.edu Title : Using Millimeter VLBI to Constrain RIAF Models of Sagittarius A* Author(s): Vincent L. Fish, Avery E. Broderick, Sheperd S. Doeleman, \& Abraham Loeb Paper : ApJL, submitted EPrint : 0809.4489 Abstract: The recent detection of Sagittarius A* at lambda = 1.3 mm on a baseline from Hawaii to Arizona demonstrates that millimeter wavelength very long baseline interferometry (VLBI) can now spatially resolve emission from the innermost accretion flow of the Galactic center region. Here, we investigate the ability of future millimeter VLBI arrays to constrain the spin and inclination of the putative black hole and the orientation of the accretion disk major axis within the context of radiatively inefficient accretion flow (RIAF) models. We examine the range of baseline visibility and closure amplitudes predicted by RIAF models to identify critical telescopes for determining the parameters of the Sgr A* black hole and accretion disk system. We find that baseline lengths near 3 gigalambda have the greatest power to distinguish amongst RIAF model parameters, and that it will be important to include either the Large Millimeter Telescope or a Chilean telescope in future VLBI arrays. Many RIAF models predict detectable fluxes on baselines between the continental United States and even a single 12 m-class dish in Chile. The extra information provided from closure amplitudes by a four-antenna array enhances the ability of VLBI to discriminate amongst models. ------------------------------------------------------------------------ (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 ========================================================================