======================================================================== G C N E W S * Newsflash * - The Newsletter for Galactic Center Research - gcnews@aoc.nrao.edu http://www.aoc.nrao.edu/~gcnews ======================================================================== Vol. 21, No. 4 May 23, 2005 Recently submitted papers: -------------------------- 1) Galactic Centre stellar winds and Sgr A* accretion (Cuadra et al., submitted) 2) Evidence for a Weak Galactic Center Magnetic Field from Diffuse Low Frequency Nonthermal Radio Emission (LaRosa et al., ApJL) 3) A Black Hole in the Galactic Center Complex IRS 13E? (Schoedel et al., ApJL) ------------------------------------------------------------------------ Email : jcuadra@MPA-Garching.MPG.DE Title : Galactic Centre stellar winds and Sgr A* accretion Author(s): J. Cuadra, S. Nayakshin, V. Springel, \& T. Di Matteo Paper : submitted to MNRAS EPrint : astro-ph/0505382 Web : http://www.mpa-garching.mpg.de/~jcuadra/Winds/ Abstract: We present a detailed discussion of our new 3D numerical models for the accretion of stellar winds on to \sgra. In our most sophisticated models, we put stellar wind sources on realistic orbits around \sgra, we include recently discovered `slow' winds (v_w 300 km s^-1), and we account for optically thin radiative cooling. We test our approach by first modelling only one phase `fast' stellar winds (v_w 1000 km s^-1). For stellar wind sources fixed in space, the accretion rate is of the order of \dot M 10^-5 \msun yr^-1, fluctuates by lt 10%, and is in a good agreement with previous models. In contrast, \dot M decreases by an order of magnitude for wind sources following circular orbits, and fluctuates by 50%. Then we allow a fraction of stars to produce slow winds. Much of these winds cool radiatively after being shocked, forming cold clumps and filaments immersed into the X-ray emitting gas. We investigate two orbital configurations for the stars in this scenario, an isotropic distribution and two rotating discs with perpendicular orientation. The morphology of cold gas is quite sensitive to the orbital distribution of the stars. In both cases, however, most of the accreted gas is hot, producing a quasi steady `floor' in the accretion rate, of the order of 3* 10^-6 \msun yr^-1, consistent with the values deduced from Chandra observations. The cold gas accretes in intermittent, short but powerful accretion episodes which may give rise to large amplitude variability in the luminosity of \sgra on time scales of tens to hundreds of years. The circularisation radii for the flows are about 10^3 and 10^4 Schwarzschild radii, for the one and two-phase wind simulations, respectively, never forming the quasi-spherical accretion flows suggested in some previous work. Our work suggests that, averaged over time scales of hundreds to thousands of years, the radiative and mechanical luminosity of \sgra may be orders of magnitude higher than it is in its current state. Further improvements of the wind accretion modelling of \sgra will rely on improved observational constraints for the wind velocities, mass loss rates and stellar orbits. ------------------------------------------------------------------------ Email : lazio@exeter.nrl.navy.mil Title : Evidence for a Weak Galactic Center Magnetic Field from Diffuse Low Frequency Nonthermal Radio Emission Author(s): T. N. LaRosa(1), C. L. Brogan(2), S. N. Shore(3), T. J.. Lazio(4), N. E. Kassim(4), M. E. Nord(4,5) Institute: (1) Department of Biological and Physical Sciences, Kennesaw State University, 1000 Chastain Rd., Kennesaw, GA 30144; ted@avatar.kennesaw.edu (2) Institute for Astronomy, 640 North A`ohoku Place, Hilo, HI 96720; cbrogan@ifa.hawaii.edu. (3) Dipartimento di Fisica ``Enrico Fermi'', Universita di Pisa and INFN, Sezione di Pisa, largo B. Pontecorvo 3, I-56127 Pisa Italy; shore@df.unipi.it (4) Remote Sensing Division, Naval Research Laboratory, Washington DC 20375-5351; Joseph.Lazio@nrl.navy.mil; Namir.Kassim@nrl.navy.mil; Michael.Nord@nrl.navy.mil (5) Department of Physics and Astronomy, University of New Mexico, Albuquerque, NM 8713 Paper : ApJL, 2005, in press EPrint : astro-ph/0505244 Abstract: New low frequency 74 and 330 MHz observations of the Galactic center (GC) region reveal the presence of a large-scale (6\arcdeg* 2\arcdeg) diffuse source of nonthermal synchrotron emission. A minimum energy analysis of this emission yields a total energy of ( phi ^4/7f^3/7)* 10^52 ergs and a magnetic field strength of 6( phi /f)^2/7 mu G , (where phi is the proton to electron energy ratio and f is the filling factor of the synchrotron emitting gas). The equipartition particle energy density is 1.2( phi /f)^2/7 \evcm , a value consistent with cosmic ray data. However, the derived magnetic field is several orders of magnitude below the 1 mG field commonly invoked for the GC. The shorter electron radiation lifetimes in such a strong field requires that energy be resupplied to the source at an implausibly high rate. Furthermore, a strong magnetic field implies an abnormally low GC cosmic ray energy density. We conclude that the mean magnetic field in the GC region must be weak, of order 10 mu G (at least on size scales > 125''). ------------------------------------------------------------------------ Email : rainer@ph1.uni-koeln.de Title : A Black Hole in the Galactic Center Complex IRS 13E? Author(s): R. Schoedel(1), A. Eckart(1), C. Iserlohe(1), R. Genzel(2,3), T. Ott(2) Institute: (1) I.Physikalisches Institut, Universitaet zu Koeln, Zuelpicher Str.77, 50937 Koeln, Germany (2) Max-Planck-Institut fuer extraterrestrische Physik, Giessenbachstra\sse, 85748 Garching, Germany (3) Also: Department of Physics, University of California, Berkeley, CA 94720 Paper : ApJL, 2005, in press EPrint : astro-ph/0504474 Abstract: The IRS 13E complex is an unusual concentration of massive, early-type stars at a projected distance of 0.13 pc from the Milky Way's central supermassive black hole Sagittarius A* (Sgr A*). Because of their similar proper motion and their common nature as massive, young stars it has recently been suggested that IRS 13E may be the remnant of a massive stellar cluster containing an intermediate-mass black hole (IMBH) that binds its members gravitationally in the tidal field of Sgr A*. Here, we present an analysis of the proper motions in the IRS 13E environment that combines the currently best available data with a time line of 10 years Based on observations at the Very Large Telescope (VLT) of the European Southern Observatory (ESO) on Paranal in Chile. We find that an IMBH in IRS 13E must have a minimum mass of 10^4 \solm in order to bind the source complex gravitationally. This high mass limit in combination with the absence so far of compelling evidence for a non-thermal radio and X-ray source in IRS 13E make it appear unlikely that an IMBH exists in IRS 13E that is sufficiently massive to bind the system gravitationally. ------------------------------------------------------------------------ (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 ========================================================================