======================================================================== G C N E W S * Newsflash * - The Newsletter for Galactic Center Research - gcnews@aoc.nrao.edu http://www.aoc.nrao.edu/~gcnews ======================================================================== Vol. 20, No. 1 Dec 16, 2004 Recently submitted papers: -------------------------- 1) TeV Emission from the Galactic Center Black-Hole Plerion (Atoyan & Dermer, ApJL) 2) A disk in the Galactic Center in the past? (Meyer-Hofmeister, Meyer & Liu, GrowingBlackHoles) 3) Variable Linear Polarization from Sagittarius A*: Evidence for a Hot Turbulent Accretion Flow (Bower et al., ApJL) 4) Sgr A* in context: daily flares as a probe of the fundamental X-ray emission process in accreting black holes (Markoff, ApJL) 5) A lambda 20cm Survey of the Galactic Center Region I: Detection of Numerous Linear Filaments (Yusef-Zadeh et al., ApJS) ------------------------------------------------------------------------ Email : atoyan@CRM.UMontreal.CA Title : TeV Emission from the Galactic Center Black-Hole Plerion Author(s): Armen Atoyan(1) and Charles D. Dermer(2) Institute: (1) CRM, Universite de Montreal, C.P. 6128, Montreal, Canada H3C 3J7; atoyan@crm.umontreal.ca (2) E O. Hulburt Center for Space Research, Code 7653, Naval Research Laboratory,, Washington, DC 20375-5352; dermer@gamma.nrl.navy.mil Paper : ApJL, Dec 2004, in press EPrint : astro-ph/0410243 Abstract: The HESS collaboration recently reported highly significant detection of TeV gamma -rays coincident with Sgr A^* \citepHESS. In the context of other Galactic Center (GC) observations, this points to the following scenario: In the extreme advection-dominated accretion flow (ADAF) regime of the GC black hole (BH), synchrotron radio/sub-mm emission of 100 MeV electrons emanates from an inefficiently radiating turbulent magnetized corona within 20 R_S (Schwarzschild radii) of the GCBH. These electrons are accelerated through second-order Fermi processes by MHD turbulence, as suggested by Liu et al. (2004). Closer to the innermost stable orbit of the ADAF, instabilities and shocks within the flow inject power-law electrons through first-order Fermi acceleration to make synchrotron X-ray flares observed with Chandra, XMM, and INTEGRAL. A subrelativistic MHD wind subtending a 1 sr cone with power > 10^37 erg s^-1 is driven by the ADAF from the vicinity of the GCBH. As in pulsar powered plerions, electrons are accelerated at the wind termination shock, at > 10^16.5 cm from the GCBH, and Compton-scatter the ADAF and the far infra-red (FIR) dust radiation to TeV energies. The synchrotron radiation of these electrons forms the quiescent X-ray source resolved by Chandra. The radio counterpart of this TeV/X-ray plerion, formed when the injected electrons cool on timescales of > 10^4 yrs, could explain the origin of nonthermal radio emission in the pc-scale bar of the radio nebula Sgr A West. ------------------------------------------------------------------------ Email : emm@MPA-Garching.MPG.DE Title : A disk in the Galactic Center in the past? Author(s): Emmi Meyer-Hofmeister, Friedrich Meyer and Bifang Liu Institute: Max-Planck-Institut fuer Astrophysik, Garching, Germany Paper : GrowingBlackHoles, in press EPrint : astro-ph/0410071 Abstract: We raise the question whether in the past a disk could have existed in our Galactic Center which has disappeared now. Our model for the interaction of a cool disk and a hot corona above (Liu et al. 2004) allows to estimate an upper limit for the mass that might have been present in a putative accretion disk after a last star forming event, but would now have evaporated by coronal action. ------------------------------------------------------------------------ Email : gbower@astron.Berkeley.EDU Title : Variable Linear Polarization from Sagittarius A*: Evidence for a Hot Turbulent Accretion Flow Author(s): Geoffrey C. Bower(1), Heino Falcke(2), Melvyn C.H. Wright(1), Donald C. Backer(1) Institute: (1)Astronomy Department \& Radio Astronomy Laboratory, Berkeley (2)Radio Observatory Westerbork, The Netherlands Paper : ApJL 2004 in press EPrint : astro-ph/0411551 Abstract: We report the discovery of variability in the linear polarization from the Galactic Center black hole source, Sagittarius A*. New polarimetry obtained with the Berkeley-Illinois-Maryland Association array at a wavelength of 1.3 mm shows a position angle that differs by 28 +/- 5 degrees from observations 6 months prior and then remains stable for 15 months. This difference may be due to a change in the source emission region on a scale of 10 Schwarzschild radii or due to a change of 3* 10^5\rdm in the rotation measure. We consider a change in the source physics unlikely, however, since we see no corresponding change in the total intensity or polarized intensity fraction. On the other hand, turbulence in the accretion region at a radius 10 to 1000 R_s could readily account for the magnitude and time scale of the position angle change. ------------------------------------------------------------------------ Email : sera@space.mit.edu Title : Sgr A* in context: daily flares as a probe of the fundamental X-ray emission process in accreting black holes Author(s): Sera Markoff Institute: Massachusetts Institute of Technology, Center for Space Research, Rm. NE80-6035, Cambridge, MA 02139 Paper : ApJL, in press EPrint : astro-ph/0412140 Abstract: Our central Galactic supermassive black hole, Sgr A*, exists mostly in a very stable, extremely low-luminosity ( 10^-9 L_Edd), thermal quiescent state, which is interrupted roughly daily by a brief, nonthermal X-ray flare. Because they are not accompanied by significant changes in the radio wavelengths, the flares make Sgr A* unusual in the context of black holes accreting at slightly higher rates. Those sources display a radio/X-ray luminosity correlation whose normalization scales with central mass, and that holds over orders of magnitude in accretion power. There is significant scatter in this correlation, due in part to measurement uncertainties and intrinsic variability. By studying the correlation in sources bracketing Sgr A* in radio luminosity and whose physical parameters are well measured, we can derive a statistical measure of this local scatter. We find that Sgr A* in quiescence and the lower intensity flares fall well below the correlation in X-ray luminosity. The brightest flares are consistent within the scatter, which may indicate an upper bound on the X-ray luminosity. This trend is suggestive of a state transition at the extreme low end of accretion activity, only above which the radio/X-ray correlation is tracked. This scenario is easily testable because it must fulfill three unique observational predictions: 1) As long as Sgr A* remains at its current radio luminosity, no X-ray flare will be seen which statistically exceeds the prediction of the correlation, 2) no source already on the correlation will be seen to flare in the X-rays similar to Sgr A* (i.e., without corresponding increases in the radio luminosity), and 3) sources below a critical accretion rate or luminosity will show similar flares as Sgr A*, on timescales appropriate to their masses. ------------------------------------------------------------------------ Email : zadeh@northwestern.edu Title : A lambda 20cm Survey of the Galactic Center Region I: Detection of Numerous Linear Filaments Author(s): F. Yusef-Zadeh(1) J. Hewitt(2) W. Cotton(3) Institute: (1) Department of Physics and Astronomy, Northwestern University, Evanston, Il. 60208 (zadeh@northwestern.edu) (2) Department of Physics and Astronomy, Northwestern University, Evanston, Il. 60208 (j-hewitt@northwestern.edu) (3) National Radio Astronomy Observatory, Charlottesville, VA 22903 (bcotton@nrao.edu) Paper : ApJS, December 2004 EPrint : astro-ph/0409292 Abstract: This is a first in a series of papers presenting a sensitive lambda 20cm VLA continuum survey of the Galactic center region using new and archival data based on multi-configuration observations taken with relatively uniform uv coverage. The high dynamic range images cover the regions within -2^o < l < 5^o and -40' < b < 40' with a spatial resolution of 30'' and 10''. The wide field imaging technique is used to construct a low-resolution mosaic of 40 overlapping pointings. The mosaic image includes the Effelsburg observations filling the low spatial frequency uv data. We also present high resolution images of twenty three overlapping fields using DnC and CnB array configurations. These high-resolution images are sensitive to both compact and extended continuum features with a wide range of angular scales with rms noise of 0.2 mJy beam^-1 in the outer parts of the Galactic center region. The survey has resulted in a catalog of 345 discrete sources as well as 140 images revealing structural details of HII regions, SNRs, pulsar wind nebulae and more than 80 linear filaments distributed toward the complex region of the Galactic center. These observations show the evidence for an order of magnitude increase in the number of faint linear filaments with typical lengths of few arcminutes. Many of the filaments show morphological characteristics similar to the Galactic center nonthermal radio filaments (NRFs). The linear filaments are not isolated but are generally clustered in star forming regions where prominent NRFs had been detected previously. The extensions of many of these linear filaments appear to terminate at either a compact source or a resolved shell-like thermal source. A relationship between the filaments, the compact and extended thermal sources as well as a lack of preferred orientation for many RFs should constrain models that are proposed to explain the origin of nonthermal radio filaments in the Galactic center. ------------------------------------------------------------------------ (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 ========================================================================