======================================================================== G C N E W S * Newsflash * - The Newsletter for Galactic Center Research - gcnews@aoc.nrao.edu http://www.aoc.nrao.edu/~gcnews ======================================================================== Vol. 22, No. 4 Sep 9, 2005 Recently submitted papers: -------------------------- 1) The First Laser Guide Star Adaptive Optics Observations of the Galactic Center: Sgr A*'s Infrared Color and the Extended Red Emission in its Vicinity (Ghez et al., ApJ) 2) Structure and Dynamics at the Central 10 Parsecs of the Galaxy (Lee, PhD) 3) The XMM-Newton/Chandra monitoring campaign on the Galactic center region (Wijnands et al., A&A) 4) Probing Post-Newtonian Gravity near the Galactic Black Hole with Stellar Doppler Measurements (Zucker et al., ApJL) ------------------------------------------------------------------------ Email : ghez@astro.UCLA.EDU Title : The First Laser Guide Star Adaptive Optics Observations of the Galactic Center: Sgr A*'s Infrared Color and the Extended Red Emission in its Vicinity Author(s): A. M. Ghez^1,2, S. D. Hornstein^1, J.Lu^1, A. Bouchez^3, D. Le Mignant^3, M. A. van Dam^3, P. Wizinowich^3, K. Matthews^4, M. Morris^1, E. E. Becklin^1, R. D. Campbell^3, J. C. Y. Chin^3, S. K. Hartman^3, E. M. Johansson^3, R. E. Lafon^3, P. J. Stomski^3, D. M. Summers^3 Institute: (1) Division of Astronomy and Astrophysics, UCLA, Los Angeles, CA 90095-1547 (2) Institute of Geophysics and Planetary Physics, UCLA, Los Angeles, CA 90095-1565 (3) W. M. Keck Observatory, 65-1120 Mamalahoa Hwy, Kamuela, HI 96743 (4) Caltech Optical Observatories, California Institute of Technology, MS 320-47, Pasadena, CA 91125 Paper : ApJ, 2005 Dec 20, in press EPrint : astro-ph/0508664 Abstract: We present the first Laser Guide Star Adaptive Optics (LGS-AO) observations of the Galactic center. LGS-AO has dramatically improved the quality and robustness with which high angular resolution infrared images of the Galactic center can be obtained with the W. M. Keck II 10-meter telescope. Specifically, Strehl ratios of 0.7 and 0.3 at L'[3.8 micron ] and K'[2.1 micron ], respectively, are achieved in these LGS-AO images; these are at least a factor of two higher and a factor of four to five more stable against atmospheric fluctuations than the Strehl ratios delivered thus far with the Keck Natural Guide Star AO system on the Galactic center. Furthermore, these observations are the first that cover a large area (76'' * 76'') surrounding the central black hole at diffraction-limited resolution for an 8-10 meter class telescope. During our observations, the infrared counterpart to the central supermassive black hole, Sgr A*-IR, showed significant infrared intensity variations, with observed L' magnitudes ranging from 12.6 to 14.5 mag and a decrease in flux density of a factor of two over an 8 minute interval. The faintest end of our L' detections, 1.3 mJy (dereddened), is the lowest level of emission yet observed for this source by a factor of 3. No significant variation in the location of SgrA*-IR is detected as a function of either wavelength or intensity. Previous claims of such positional variations are easily attributable to a nearby (0.'' 09 or 720 AU, projected), extended, very red source, which we suggest arises from a locally heated dust feature. Near a peak in its intensity, we obtained the first measurement of SgrA*-IR's K'-L' color; its K'-L' of 3.0 +/- 0.2 mag (observed) or 1.4 +/- 0.2 (dereddened) corresponds to an intrinsic spectral index of alpha -0.5 +/- 0.3 for F_ nu nu ^ alpha . This is significantly bluer than other recent infrared measurements from the literature, which suggest alpha = -4 +/- 1. Because our measurement was taken at a time when Sgr A* was 6 times brighter in the infrared than the other measurements, we posit that the spectral index of the emission arising from the vicinity of our Galaxy's central black hole may depend on the strength of the flare, with stronger flares giving rise to a higher fraction of high energy electrons in the emitting region. ------------------------------------------------------------------------ Email : leesh@kasi.re.kr Title : Structure and Dynamics at the Central 10 Parsecs of the Galaxy Author(s): Sungho Lee Institute: (1) Korea Astronomy and Space Science Institute, Daejeon 305-348, Korea Paper : PhD thesis Web : http://www.kasi.re.kr/~leesh/pub/thesis/leesh0726.pdf Abstract: The Galactic center influences the current nature as well as the formation, the evolution and the future fate of the Milky Way. The Galactic nucleus stands for the galactic nuclei of other galaxies and provides an opportunity to study the environment around a super-massive black hole (SMBH) at high spatial resolution. The central 10 pc of the Galaxy, the Sgr A region, contains several principal components; the SMBH candidate (Sgr A*), the Central cluster, the circum-nuclear disk (CND), Sgr A West, a powerful supernova-like remnant (Sgr A East), and surrounding molecular clouds. Developing a consistent picture of the interactions between these components will improve our understanding of the Galaxy and the nature of galactic nuclei in general. Previous studies on the spatial and dynamical relationships between the various objects are mostly based on indirect and qualitative evidence and leave many unsolved questions, which need more robust evidence. Molecular hydrogen (H_2) emission has been used as an excellent tracer and diagnostic for interactions between dense molecular clouds and hot, powerful sources. We observed the H_2 1-0 S(1) (lambda = 2.1218 micron ) and H_2 2-1 S(1) (lambda = 2.2477 micron ) emission line spectra from the interaction regions between Sgr A East, the CND, and the surrounding molecular clouds. Using the long-slit Cooled Grating Spectrometer 4 (CGS4) with an echelle grating at the 3.8 m United Kingdom Infrared Telescope (UKIRT) on Mauna Kea, we scanned 56 positions in the interaction regions. We reduced 2-D spectral images using IRAF and analyzed a 3-D data cube using MIRIAD. The data cube has the H_2 information both in space (with a resolution of 2'') and in velocity (with a resolution of 18 km s^-1). The H_2 1-0 S(1) data cube was directly compared with the NH_3(3,3) data cube from McGary, Coil, \& Ho (2001, ApJ, 559, 326) to investigate the gas kinematics. Based on the H_2 1-0 S(1) and 2-1 S(1) line intensities and gas kinematics, we concluded that the H_2 excitation can be explained by two mechanisms; a combination of fluorescence and C-shocks in very strong magnetic fields, or a mixture of slow C-shocks and fast J-shocks. We estimated shock velocities ( 100 km s^-1) of Sgr A East by comparing H_2 line profiles with those of NH_3. From the distribution of the shocked H_2 emission, we determined the interacting boundary of Sgr A East in projection as an ellipse with the center at 1.5 pc offset from Sgr A* and the dimension of 10.8 pc * 7.6 pc. We also determined the positional relationship between Sgr A East and the molecular clouds along the line-of-sight and suggested a revised model for the 3-D structure of the central 10 pc. From the estimated shock velocities, we deduced the initial explosion energy (0.2-4 * 10^53 ergs) of Sgr A East. This extremely large energy excludes the hypothesis of a single, typical, supernova (SN) for the origin of Sgr A East. We examined other hypotheses (tidal disruption of a star by the SMBH, multiple supernovae, and a hypernova) and we concluded that a hypernova (collapsar or microquasar) is the most probable origin of Sgr A East. Based on the energy, we investigated the influences of the Sgr A East-like explosions (hypernovae) and normal SNe on the mass inflow to the Galactic nucleus. We suggest a scenario that the continuous mass inflow into the Galactic nucleus makes it active by igniting the SMBH or stimulating a starburst every 10^8 yr, but each active phase continues only < 10^7 yr since a large number of SNe resulting from newly born massive stars cease the mass supply soon. The Galactic nucleus is likely to spend only about 1/10 of its life in active. As for the recent history of the central 10 pc, the mass inflow restarted several 10^6 yr ago after a quiescent phase for 10^8 yr. In its usual schedule, the Galactic nucleus would continue its activity for a few 10^6 yr more from now before a huge number of SNe occur. However, the active phase was unexpectedly ceased 10^4 yr ago, by Sgr A East. ------------------------------------------------------------------------ Email : rudy@science.uva.nl Title : The XMM-Newton/Chandra monitoring campaign on the Galactic center region Author(s): R. Wijnands(1), J.J.M. in 't Zand(2), M. Rupen(3, 4), T. Maccarone(5), J. Homan(6), R. Cornelisse(5), R. Fender(5, 1), J. Grindlay(7), M. van der Klis(1), E. Kuulkers(8), C.B. Markwardt(4, 9), J.C.A. Miller-Jones(1), Q.D. Wang(10) Institute: (1) Astronomical Institute ``Anton Pannekoek'', University of Amsterdam, Kruislaan 403, 1098 SJ, The Netherlands (2) SRON National Institute for Space Research, Sorbonnelaan 2, 3584 CA, Utrecht, The Netherlands (3) National Radio Astronomy Observatory, 1003 Lopezville Road, Socorro, NM 87801, USA (4) NASA, Goddard Space Flight Center, Greenbelt, MD 20711, USA (5) School of Physics and Astronomy, University of Southampton, Southampton SO17 1BJ, UK (6) Center for Space Research, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA (7) Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA (8) ISOC, ESA/ESAC, Urb. Villafranca del Castillo, P.O. Box 50727, 28080 Madrid, Spain (9) Department of Astronomy, University of Maryland, College Park, MD 20742, USA (10) Astronomy Department, University of Massachusetts, Amherst, MA 01003, USA Paper : A&A, submitted EPrint : astro-ph/0508648 Abstract: We present the first results of our X-ray monitoring campaign on a 1.7 square degree region centered on Sgr A* using the X-ray satellites XMM-Newton and Chandra. The purpose of this campaign is to monitor the X-ray behavior (below 10 keV) of X-ray sources (both persistent and transient) which are too faint to be detected by monitoring instruments aboard satellites currently in orbit (e.g., Rossi X-ray Timing Explorer; INTEGRAL). Our first monitoring observations (using the HRC-I aboard Chandra) were obtained on June 5, 2005. Most of the sources detected could be identified with foreground sources, such as X-ray active stars. In addition we detected two persistent X-ray binaries (1E 1743.1-2843; 1A 1742-294), two faint X-ray transients (GRS 1741.9-2843; XMM J174457-2850.3), as well as a possible new transient source at a luminosity of a few times 10^34 erg s^-1. We report on the X-ray results on these systems and on the non detection of the transients in follow-up radio data using the Very Large Array. We discuss how our monitoring campaign can help to improve our understanding of the different types of X-ray transients (i.e., the very faint ones). ------------------------------------------------------------------------ Email : tal.alexander@weizmann.ac.il Title : Probing Post-Newtonian Gravity near the Galactic Black Hole with Stellar Doppler Measurements Author(s): Shay Zucker(1), Tal Alexander(1,2), Stefan Gillessen(3), Frank Eisenhauer(3) and Reinhard Genzel(3) Institute: (1) Faculty of Physics, Weizmann Institute of Science, PO Box 26, Rehovot 76100, Israel (2) The William Z. \& Eda Bess Novick career development chair (3) Max-Planck-Institut fuer extraterrestriche Physik, Postfach 1312, Garching D-85741, Germany Paper : ApJL, submitted EPrint : astro-ph/0509105 Abstract: Stars closely approaching the massive black hole in the center of the Galaxy provide a unique opportunity to probe post-Newtonian physics in a yet unexplored regime of celestial mechanics. Recent advances in infrared stellar spectroscopy allow the precise measurement of stellar Doppler shift curves and thereby the detection of beta ^2 post-Newtonian effects (gravitational redshift in the black hole's potential and the transverse Doppler shift). We formulate a detection procedure in terms of a simplified post-Newtonian parametrization. We then use simulations to show that these effects can be decisively detected with existing instruments after about a decade of observations. We find that neglecting these effects can lead to statistically significant systematic errors in the derived black hole mass and distance. ------------------------------------------------------------------------ (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 ========================================================================