======================================================================== G C N E W S * Newsflash * - The Newsletter for Galactic Center Research - gcnews@aoc.nrao.edu http://www.aoc.nrao.edu/~gcnews ======================================================================== Vol. 28, No. 11 Sep 25, 2008 Recently submitted papers: -------------------------- 1) Event-horizon-scale structure in the supermassive black hole candidate at the Galactic Centre (Doeleman et al., Nature) 2) GRAVITY: getting to the event horizon of Sgr A* (Eisenhauer et al., proceedings) 3) Measuring Distance and Properties of the Milky Way's Central Supermassive Black Hole with Stellar Orbits (Ghez et al., ApJ) 4) Modulated X-ray Emissivity near the Stress Edge in Sgr A* (Falanga et al., Apj) 5) Magnetic Field Configuration at the Galactic Center Investigated by Wide Field Near-Infrared Polarimetry (Nishiyama et al., ApJ) ------------------------------------------------------------------------ Email : dole@haystack.mit.edu Title : Event-horizon-scale structure in the supermassive black hole candidate at the Galactic Centre Author(s): Doeleman, Sheperd S.; Weintroub, Jonathan; Rogers, Alan E. E.; Plambeck, Richard; Freund, Robert; Tilanus, Remo P. J.; Friberg, Per; Ziurys, Lucy M.; Moran, James M.; Corey, Brian; Young, Ken H.; Smythe, Daniel L.; Titus, Michael; Marrone, Daniel P.; Cappallo, Roger J.; Bock, Douglas C.-J.; Bower, Geoffrey C.; Chamberlin, Richard; Davis, Gary R.; Krichbaum, Thomas P.; Lamb, James; Maness, Holly; Niell, Arthur E.; Roy, Alan; Strittmatter, Peter; Werthimer, Daniel; Whitney, Alan R.; Woody, David Paper : Nature, v455, p78, 2008 Abstract: The cores of most galaxies are thought to harbour supermassive black holes, which power galactic nuclei by converting the gravitational energy of accreting matter into radiation. Sagittarius A* (SgrA*), the compact source of radio, infrared and X-ray emission at the centre of the Milky Way, is the closest example of this phenomenon, with an estimated black hole mass that is 4,000,000 times that of the Sun. A long-standing astronomical goal is to resolve structures in the innermost accretion flow surrounding SgrA*, where strong gravitational fields will distort the appearance of radiation emitted near the black hole. Radio observations at wavelengths of 3.5mm and 7mm have detected intrinsic structure in SgrA*, but the spatial resolution of observations at these wavelengths is limited by interstellar scattering. Here we report observations at a wavelength of 1.3mm that set a size of microarcseconds on the intrinsic diameter of SgrA*. This is less than the expected apparent size of the event horizon of the presumed black hole, suggesting that the bulk of SgrA* emission may not be centred on the black hole, but arises in the surrounding accretion flow. ------------------------------------------------------------------------ Email : eisenhau@mpe.mpg.de Title : GRAVITY: getting to the event horizon of Sgr A* Author(s): F. Eisenhauer, G. Perrin, W. Brandner, C. Straubmeier, A. Richichi, S. Gillessen, J.P. Berger, S. Hippler, A. Eckart, M. Schoeller, S. Rabien, F. Cassaing, R. Lenzen, M. Thiel, Y. Clenet, J.R. Ramos, S. Kellner, P. Fedou, H. Baumeister, R. Hofmann, E. Gendron, A. Boehm, H. Bartko, X. Haubois, R. Klein, K. Dodds-Eden, K. Houairi, F. Hormuth, A. Graeter, L. Jocou, V. Naranjo, R. Genzel, P. Kervella, T. Henning, N. Hamaus, S. Lacour, U. Neumann, M. Haug, F. Malbet, W. Laun, J. Kolmeder, T. Paumard, R.-R. Rohloff, O. Pfuhl, K. Perraut, J. Ziegleder, D. Rouan, G. Rousset Paper : proceedings EPrint : 0808.0063 Abstract: We present the second-generation VLTI instrument GRAVITY, which currently is in the preliminary design phase. GRAVITY is specifically designed to observe highly relativistic motions of matter close to the event horizon of Sgr A*, the massive black hole at center of the Milky Way. We have identified the key design features needed to achieve this goal and present the resulting instrument concept. It includes an integrated optics, 4-telescope, dual feed beam combiner operated in a cryogenic vessel; near infrared wavefront sensing adaptive optics; fringe tracking on secondary sources within the field of view of the VLTI and a novel metrology concept. Simulations show that the planned design matches the scientific needs; in particular that 10 microarcsecond astrometry is feasible for a source with a magnitude of K=15 like Sgr A*, given the availability of suitable phase reference sources. ------------------------------------------------------------------------ Email : ghez@astro.ucla.edu Title : Measuring Distance and Properties of the Milky Way's Central Supermassive Black Hole with Stellar Orbits Author(s): A. M. Ghez(1,2), S. Salim(1,4), N. N. Weinberg(3,5), J. R. Lu(1), T. Do(1), J. K. Dunn(1), K. Matthews(3), M. Morris(1), S. Yelda(1), E. E. Becklin(1), T. Kremenek(1), M. Milosavljevic(6), J. Naiman(1,7) Institute: (1) UCLA Department of Physics and Astronomy, Los Angeles, CA 90095-1547 (2) UCLA Institute of Geophysics and Planetary Physics, Los Angeles, CA 90095-1565 (3) California Institute of Technology, Division of Mathematics, Physics and Astronomy, Pasadena, CA 91125 (4) NOAO, 950 N Cherry Ave, Tucson, AZ 85719 (5) University of California Berkeley, Department of Astronomy, Berkeley, CA 94720-3411 (6) University of Texas, Department of Astronomy, Austin, TX 78712 (7) UCSC, Department of Astronomy \& Astrophysics, Santa Cruz, CA 95064 Paper : ApJ, in press EPrint : 0808.2870 Abstract: We report new precision measurements of the properties of our Galaxy's supermassive black hole. Based on astrometric (1995-2007) and radial velocity (2000-2007) measurements from the W. M. Keck 10-meter telescopes, a fully unconstrained Keplerian orbit for the short period star S0-2 provides values for the distance (R_0) of 8.0 +/- 0.6 kpc, the enclosed mass (M_bh) of 4.1 +/- 0.6 * 10^6 M_o, and the black hole's radial velocity, which is consistent with zero with 30 km/s uncertainty. If the black hole is assumed to be at rest with respect to the Galaxy (e.g., has no massive companion to induce motion), we can further constrain the fit and obtain R_0 = 8.4 +/- 0.4 kpc and M_bh = 4.5 +/- 0.4 * 10^6 M_o. More complex models constrain the extended dark mass distribution to be less than 3-4 * 10^5 M_o within 0.01 pc, 100x higher than predictions from stellar and stellar remnant models. For all models, we identify transient astrometric shifts from source confusion (up to 5x the astrometric error) and the assumptions regarding the black hole's radial motion as previously unrecognized limitations on orbital accuracy and the usefulness of fainter stars. Future astrometric and RV observations will remedy these effects. Our estimates of R_0 and the Galaxy's local rotation speed, which it is derived from combining R_0 with the apparent proper motion of Sgr A*, ( theta _0 = 229 +/- 18 km s^-1), are compatible with measurements made using other methods. The increased black hole mass found in this study, compared to that determined using projected mass estimators, implies a longer period for the innermost stable orbit, longer resonant relaxation timescales for stars in the vicinity of the black hole and a better agreement with the M_bh- sigma relation. ------------------------------------------------------------------------ Email : mfalanga@cea.fr Title : Modulated X-ray Emissivity near the Stress Edge in Sgr A* Author(s): M. Falanga (1), F. Melia (2), M. Prescher (2), G. Belanger (3), A. Goldwurm (1) Institute: (1) CEA Saclay, DSM/IRFU/Service d'Astrophysique, 91191 Gif-sur-Yvette, France (2) Physics Department and Steward Observatory, The University of Arizona, Tucson, AZ 85721 (3) ESA/ESAC, Apartado 50727, 28080 Madrid, Spain Paper : Apj 679 Abstract: Sgr A* is thought to be the radiative manifestation of a 3.6x10E6 Msun supermassive black hole at the Galactic center. Its mm/sub-mm spectrum and its flare emission at IR and X-ray wavelengths may be produced within the inner ten Schwarzschild radii of a hot, magnetized Keplerian flow. The lightcurve produced in this region may exhibit quasi-periodic variability. We present ray-tracing simulations to determine the general-relativistically modulated X-ray luminosity expected from plasma coupled magnetically to the rest of the disk as it spirals inwards below the innermost stable circular orbit towards the ``stress edge" in the case of a Schwarzschild metric. The resulting lightcurve exhibits a modulation similar to that observed during a recent X-ray flare from Sgr A*. ------------------------------------------------------------------------ Email : shogo@kusastro.kyoto-u.ac.jp Title : Magnetic Field Configuration at the Galactic Center Investigated by Wide Field Near-Infrared Polarimetry Author(s): Shogo Nishiyama(1,2), Motohide Tamura(2), Hirofumi Hatano(3), Saori Kanai(3), Mikio Kurita(3), Shuji Sato(3), Noriyuki Matsunaga(1), Tetsuya Nagata(1), Takahiro Nagayama(1), Ryo Kandori(2), Yasushi Nakajima(2), Nobuhiko Kusakabe(4), Yaeko Sato(4), James H. Hough(5), Koji Sugitani(6), and Haruyuki Okuda(7) Institute: (1) Department of Astronomy, Kyoto University, Kyoto 606-8502, Japan (2)National Astronomical Observatory of Japan, Mitaka, Tokyo 181-8588, Japan (3) Department of Astrophysics, Nagoya University, Nagoya 464-8602, Japan (4) Department of Astronomical Sciences, Graduate University for Advanced Studies (Sokendai), Mitaka, Tokyo 181-8588, Japan (5) Centre for Astrophysics Research, University of Hertfordshire, Hatfield, Herts AL10 9AB, UK (6) Graduate School of Natural Sciences, Nagoya City University, Nagoya 467-8501, Japan (7) Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara, Kanagawa 229-8510, Japan Paper : ApJ, accepted EPrint : 0809.3089 Web : http://www.kusastro.kyoto-u.ac.jp/~shogo/papers/08NishiyamaMa gFGC.pdf Abstract: We present a polarimetric map of a 20' * 20' area toward the Galactic center. The polarization of point sources has been measured in the J, H, and K_S bands using the near-infrared polarimetric camera SIRPOL on the 1.4 m telescope IRSF. One percent or better accuracy of polarization degree is achieved for sources with J<14.5, H<13.5, and K_S<12.0. Comparing the Stokes parameters between high extinction stars and relatively low extinction ones, we have obtained a polarization originating from magnetically aligned dust grains at the central region of our Galaxy of at most 1-2 kpc. The distribution of the position angles shows a peak at 20 DEGr, nearly parallel to the Galactic plane, suggesting a toroidal magnetic configuration. The derived direction of the magnetic field is in good agreement with that obtained from far-infrared/submillimeter observations, which detect polarized thermal emission from dust in the molecular clouds at the Galactic center. Our results show that by subtracting foreground components, near-infrared polarimetry allows investigation of the magnetic field structure at 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, Masaaki Sakano, Feng Yuan - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - For Abstract submission please follow the instructions which are at http://www.aoc.nrao.edu/~gcnews/home/submission.shtml ========================================================================