======================================================================== G C N E W S * Newsflash * - The Newsletter for Galactic Center Research - gcnews@mpifr-bonn.mpg.de http://www.mpifr-bonn.mpg.de/gcnews ======================================================================== Vol. 14, No. 12 Feb 13, 2002 Recently submitted papers: -------------------------- 1) An Accretion-Induced X-ray Flare in Sgr A* (Liu & Melia, ApJL) 2) New Constraints on the Nature of Radio Emission in Sagittarius A* (Liu & Melia, ApJL) ------------------------------------------------------------------------ Email : liusm@blackhole.physics.arizona.edu Title : An Accretion-Induced X-ray Flare in Sgr A* Author(s): Siming Liu(1) and Fulvio Melia^1,2,3 Institute: (1) Physics Department, The University of Arizona, Tucson, AZ 85721 (2) Steward Observatory, The University of Arizona, Tucson, AZ 85721 (3) Sir Thomas Lyle Fellow and Miegunyah Fellow. Paper : ApJ Letters, Feb (2002) EPrint : astro-ph/0110564 Abstract: The recent detection of a three-hour X-ray flare from Sgr A* by Chandra provides very strong evidence for a compact emitting region near this supermassive black hole at the Galactic center. Sgr A*'s mm/sub-mm spectrum and linear polarimetric properties, and its quiescent-state X-ray flux density, are consistent with a model in which low angular momentum gas captured at large radii circularizes to form a hot, magnetized Keplerian flow within tens of Schwarzschild radii of the black hole's event horizon. In Sgr A*'s quiescent state, the X-ray emission appears to be produced by self-Comptonization (SSC) of the mm/sub-mm synchrotron photons emitted in this region. In this paper, we show that the prominent X-ray flare seen in Sgr A* may be due to a sudden enhancement of accretion through the circularized flow. Depending on whether the associated response of the anomalous viscosity is to increase or decrease in tandem with this additional injection of mass, the X-ray photons during the outburst may be produced either via thermal bremsstrahlung (if the viscosity decreases), or via SSC (if the viscosity increases). However, the latter predicts a softer X-ray spectrum than was seen by Chandra, so it appears that a bremsstrahlung origin for the X-ray outburst is favored. A strong correlation is expected between the mm/sub-mm and X-ray fluxes when the flare X-rays are produced by SSC, while the correlated variability is strongest between the sub-mm/far-IR and X-rays when bremsstrahlung emission is dominant during the flare. In addition, we show that future coordinated multi-wavelength observations planned for the 2002 and 2003 cycles may be able to distinguish between the accretion and jet scenarios. ------------------------------------------------------------------------ Email : liusm@blackhole.physics.arizona.edu Title : New Constraints on the Nature of Radio Emission in Sagittarius A* Author(s): Siming Liu(1) and Fulvio Melia^1,2,3 Institute: (1) Physics Department, The University of Arizona, Tucson, AZ 85721 (2) Steward Observatory, The University of Arizona, Tucson, AZ 85721 (3) Sir Thomas Lyle Fellow and Miegunyah Fellow. Paper : ApJ Letters, 561, L77, (2001) EPrint : astro-ph/0110563 Abstract: The mm to sub-mm spectrum of Sgr A* at the Galactic center, as well as its polarization characteristics, are consistent with the inner 10 Schwarzschild radii of a tight Keplerian emitting region of hot, magnetized, orbiting gas. This plasma may also be the source (through self-Comptonization) of the X-rays detected by Chandra. It has long been suspected that the circularization region between the quasi-spherical infall at large radii, and this inner zone, is responsible for producing the rest of Sgr A*'s spectrum. In this paper, we report the results of a detailed study of this region, with several important conclusions that will be highly relevant to upcoming coordinated multi-wavelength observations. First, the combination of existing cm and X-ray data preclude the possibility of producing the observed strong 1.36 GHz radio flux via thermal synchrotron within a bounded flow. If Sgr A*'s radio spectrum is produced by accreting gas, it appears that a non-thermal particle distribution is a necessity. This may not be surprising, given that the energy associated with the radial motion is probably dissipated by shocks before the gas circularizes, which can produce the required power-law distribution. Second, if this is the correct picture for how Sgr A*'s spectrum is produced, it appears that the Chandra-detected X-rays may originate either from self-Comptonization in the inner Keplerian region, or from optically-thin nonthermal synchrotron emission in the much larger, circularization zone, extending up to 500 Schwarzschild radii or more. This is a question that should be answered by upcoming broadband observations, since the mm-bump and X-rays are strongly correlated in the former case, whereas the X-rays are strongly correlated to the cm-radio flux in the latter. In addition, X-rays produced in the circularized gas could show periodic or quasi-periodic variations, but not those produced via nonthermal synchrotron emission much farther out. ------------------------------------------------------------------------ (Older versions of the Newsflash can be found at the gcnews web-page) ======================================================================== Edited by Angela Cotera Heino Falcke & Sera Markoff (cotera@as.arizona.edu) (hfalcke,smarkoff@mpifr-bonn.mpg.de) - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - For Abstract submission please send the (La)Tex file of your paper to gcnews@mpifr-bonn.mpg.de ========================================================================