Subject: GCFLASH - Vol. 5, No. 1 (Feb 20, 1997) ======================================================================== G C N E W S * Newsflash * - The Newsletter for Galactic Center Research - gcnews@astro.umd.edu http://www.astro.umd.edu/~gcnews ======================================================================== Vol. 5, No. 1 Feb 20, 1997 Since the fast expansion in the beginning where we crossed the 100 subsribers mark farily quickly it took a while until we approached the next 100, but thanks to Michael Linqvist (Onsala) we have now reached the 200 mark! That is quite a healthy size for a Newsletter of this kind and ensures that your abstracts will receive a wide distribution. Two more abstracts have been received and they are included in this Newsflash: more interesting findings from KAO observations of the GC and a theoretical paper about gamma-ray emission from the GC (Sgr A*) -- the focus of the last newsletter. Recently submitted papers: -------------------------- 1) DUST COMPOSITION, ENERGETICS, AND MORPHOLOGY OF THE GALACTIC CENTER (Chan et al., ApJ) 2) Gamma-ray Emission From Advection-Dominated Accretion Flows Around Black Holes: Application to the Galactic Center (Mahadevan et al., ApJ) ------------------------------------------------------------------------ Email : kwc@ssa1.arc.nasa.gov Title : DUST COMPOSITION, ENERGETICS, AND MORPHOLOGY OF THE GALACTIC CENTER Author(s): Kin-Wing Chan(1), Harvey Moseley(2), Sean Casey(2), Patrick Harrington(3), Eli Dwek(4), Robert Loewenstein(5), Frank Varosi(6), William Glaccum(7) Institute: (1) NASA/Ames Research Center, MS 245-6, Moffett Field, CA 94035-1000 (2) NASA/Goddard Space Flight Center, Code 685, Greenbelt, MD 20771 (3) Department of Astronomy, University of Maryland, College Park, MD 20742 (4) NASA/Goddard Space Flight Center, Code 685, Greenbelt, MD 20771 (5) Yerkes Observatory, William Bay, Wisconsin, WI 53191 (6) NASA/Goddard Space Flight Center, Code 685, Greenbelt, MD 20771 (7) Applied Research Corp., 8201 Corporate Dr., Landover, MD 20785 Paper : ApJ (1997), in press Abstract: 16 - 45 micron spectra of several regions within the central 80" of the Galaxy have been obtained at 20" resolution using the Goddard Cryogenic Grating Spectrometer on the Kuiper Airborne Observatory. A broad band of excess emission extending from 24 to 45 micron is present in the spectra at positions covering the "tongue" and the inner edge of the circumnuclear disk. A similar dust emission feature has been observed in some carbon-rich evolved stars and in a nitrogen-rich evolved massive star. The observations reported here are the first detection of this dust emission feature in the interstellar medium. We find that the dust properties in the "tongue" and the inner edge of the circumnuclear disk are different from the dust in the cavity which shows no evidence for the 30 micron feature in its spectrum. The relation between the tongue feature and the inner disk has been a subject of considerable debate, with suggestions that the tongue material is either falling in or has been expelled from the Galactic Center. Our observations suggest that if there is a physical connection between the two regions, then the tongue material is more likely falling in towards the Galactic Center. Two kinds of dust components and corresponding heating sources are argued to be responsible for the dust emission in the Galactic Center: (1) the hot dust component (140 - 300 K) heated by IRS sources which are distributed throughout the cavity; and (2) the warm dust component (60 - 90 K) heated by a He I emission line star cluster which give rise to a local interstellar radiation field that heats the dust in the central several parsecs. We find a lower limit of ~ 1 x 10^6 L_o for the He I emission line star cluster. Finally, we find that the line of sight extinction across the cavity is not uniform. It is diminished and roughly constant in the northeastern side of the cavity, but increases to the southwestern side. Based on this result, together with the past study by Zylka et al. (1995) who found that there is a cold dust component along the line of sight to the central 30" of the Galaxy, we propose that there is a cool dust envelope with non-uniform distribution covering the cavity of the Galactic Center. ------------------------------------------------------------------------ Email : rohan@cfata5.harvard.edu Title : Gamma-ray Emission From Advection-Dominated Accretion Flows Around Black Holes: Application to the Galactic Center Author(s): Rohan Mahadevan, Ramesh Narayan, and Julian Krolik Institute: (1) Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA 02138 and (2) Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218 Paper : submitted to ApJ Abstract: We calculate the flux and spectrum of gamma-rays emitted by a two-temperature advection-dominated accretion flow (ADAF) around a black hole. The gamma-rays are from the decay of neutral pions produced through proton-proton collisions. We discuss both thermal and power-law distributions of proton energies and show that the gamma-ray spectra in the two cases are very different. We apply the calculations to the gamma-ray source, 2EG J1746-2852, detected by EGRET from the direction of the Galactic Center. We show that the flux and spectrum of this source are consistent with emission from an ADAF around the supermassive accreting black hole Sgr A*. The model uses accretion parameters within the range made likely by other considerations, and requires a power-law energy distribution of protons. If this model is correct, it provides evidence for the presence of a two temperature plasma in Sgr A*, and predicts gamma-ray fluxes from other accreting black holes which could be observed with more sensitive detectors. ------------------------------------------------------------------------ (Older versions of the Newsflash can be found at the gcnews web-page) ======================================================================== Edited by Angela Cotera Heino Falcke (cotera@ipac.caltech.edu) (hfalcke@astro.umd.edu) - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - For Abstract submission please send the (La)Tex file of your paper to gcnews@astro.umd.edu ========================================================================