======================================================================== 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. 10 Sep 19, 2008 Dear Subscriber, Please find the usual "Recently submitted papers" after an announcement on "GC Special Sessions at the Long Beach AAS - January 2009": We would like to bring your attention to two Special Sessions at the January 2009 AAS meeting in Long Beach, CA focused on Galactic center science. The sessions will both take place on Monday, 5 January 2009. In particular, the organizers from both sessions invite submission of related research posters to each of the sessions (each special session should be listed as a subject category when you are submitting your abstract). Please feel free to contact either organizer with questions. Hope to see you in January! ----------------------------------------------------------------------------- Special Session: Galactic Center Massive Stars and Magnetized ISM 1/5/2009 10:00 - 11:30 AM In the past few years, the central hundred parsecs of the Galaxy have been surveyed anew at a variety of wavelengths, notably with Spitzer, Chandra, HST, UKIDSS, and the VLA, all with considerably enhanced sensitivity and spatial resolution. These efforts have culminated this year with the emergence of a new large-scale HST/NICMOS survey of Paschen-alpha emission and a deep Chandra mosaic. All of this newly available information provides an exciting new vision of the population of massive young stars and star clusters, and on the dramatic effect that those stars have on the interstellar medium in terms of heating, ionization and dynamical shaping. The proposed special session would bring together investigators to discuss the timely issues that are emerging from the new views of the rich Galactic center region. These issues include: (1) the dominant mode of star formation in the central molecular zone, (2) the effect of the winds and radiation of massive stars on subsequent star formation in neighboring clouds, (3) the likely role of the strong Galactic center magnetic field in determining the morphology of HII regions and PDRs, and (4) the extent to which our Galactic nucleus serves as a template for understanding star formation activity in gas-rich galactic nuclei in general. In addition, the attendees would use the discussions to focus the scientific program for the next international symposium on the Galactic center, to be held in China in October 2009. Organizer: Cornelia Lang, University of Iowa (cornelia-lang@uiowa.edu) See: http://aas.org/meetings/aas213/event_special_ss03.php ----------------------------------------------------------------------------- Special Session: Getting to the Event Horizon: SgrA* 1/5/2009 2:00 - 3:30 PM SgrA*, the super massive black hole candidate at the Galactic Center, presents the nearest example of the AGN phenomenon, and the best opportunity to study black hole physics, accretion, and strong GR effects in detail. Within the past several years there has been a great deal of progress on modeling the innermost accretion regions of SgrA* with linear resolutions much less than the Schwarzschild radius. Recent 230GHz VLBI detections of SgrA* have angular resolutions that are beginning to constrain these models and, hence, fundamental black hole parameters. Multi-wavelength monitoring campaigns are refining models of flaring structures and setting expected size and time scales for variability. The goal of this special session is to bring together researchers in theory and observers across the spectrum to discuss the present and future prospects for pinning down the existence of a super massive black hole, and its characteristics. We also plan to address the issue of future technical advances that would significantly advance the field. Organizer: Sheperd Doeleman, MIT-Haystack Observatory (dole@haystack.mit.edu) See: http://aas.org/meetings/aas213/event_special_ss04.php ----------------------------------------------------------------------------- Recently submitted papers: -------------------------- 1) Timing and Spectral Study of AX J1745.6-2901 with Suzaku (Hyodo et al., PASJ) 2) A 600 minute near-infrared lightcurve of Sagittarius A* (Meyer et al., ApJL) 3) Metallicity in the Galactic Center:, The Quintuplet cluster (Najarro et al., ApJ) 4) Dark stars at the Galactic centre - the main sequence (Pat et al., arXiv:0809.1871) 5) High-frequency VLBI observations of Sgr A* during a multi-frequency campaign in May 2007 (Lu et al., ) 6) On the tidal evolution of the orbits of low-mass satellites around black holes (Cadez et al., A&A) ------------------------------------------------------------------------ Email : hyodo@cr.scphys.kyoto-u.ac.jp Title : Timing and Spectral Study of AX J1745.6-2901 with Suzaku Author(s): Yoshiaki Hyodo (1), Yoshihiro Ueda (1), Takayuki Yuasa (2), Yoshitomo Maeda (3), Kazuo Makishima (2), and Katsuji Koyama (1), Institute: (1) Department of Astronomy, Graduate School of Science, Kyoto University, Kita-shirakawa Oiwake-cho, Sakyo, Kyoto 606-8502 (2) Department of Physics, School of Science, the University of Tokyo, 7-3-1, Hongo, Bunkyo, Tokyo 113-0033 (3) Institute of Space and Astronautical Science, 3-1-1, Yoshinodai, Sagamihara, Kanagawa, 229-8510 Paper : PASJ, 2009, accepted Abstract: The eclipsing low-mass X-ray binary \axj was observed with Suzaku in its outburst phase. Combining the Chandra observation made 1.5 month earlier than Suzaku, we determined the orbital period to be 30063.76+/-0.14 s. We found deep flux dips prior to the eclipse phase of orbit. The X-ray spectrum of the persistent phase is described with a combination of a direct and a scattered-in by dust emissions. During the eclipse, the X-ray spectrum becomes only the dust scattering (scattered-in) component. The optical depth of the dust-scattering is 10.5 at 1 keV. The direct component is composed of a blackbody likely from the neutron star surface and a disk-blackbody. No power-law component is found in the hard energy band up to 30 keV. A clear edge at 7.1 keV in the deep dip spectrum indicates that the major portion of Fe in the absorber is neutral or at low ionization state. We discovered four narrow absorption lines near the K-shell transition energies of Fe\emissiontypeXXV, Fe\emissiontypeXXVI, and Ni\emissiontypeXXVII. The absorption line features are well explained by the solar abundance gas in a bulk motion of 10^3 km s^-1. ------------------------------------------------------------------------ Email : leo@astro.UCLA.EDU Title : A 600 minute near-infrared lightcurve of Sagittarius A* Author(s): L. Meyer(1), T. Do(1), A. Ghez(1), M. R. Morris(1), G. Witzel(2), A. Eckart(2), G. Belanger(3), R. Schoedel(4) Institute: (1) Department of Physics and Astronomy, University of California, Los Angeles, CA 90095-1547 (2) Universitaet zu Koeln, Zuelpicher Str. 77, 50937 Koeln (3) ESA/ESAC, PO Box 78, 28691 Villanueva de la Ca\ nada, Spain (4) Instituto de Astrofisica de Andalucia, Camino Bajo de Huetor 50, 18008 Granada, Spain Paper : ApJL, accepted Abstract: We present the longest, by a factor of two, near-infrared lightcurve from Sgr A* - the supermassive black hole in the Galactic center. Achieved by combining Keck and VLT data from one common night, which fortuitously had simultaneous Chandra and SMA data, this lightcurve is used to address two outstanding problems. First, a putative quasi-periodicity of 20 min reported by groups using ESO's VLT is not confirmed by Keck observations. Second, while the infrared and mm-regimes are thought to be related based on reported time lags between lightcurves from the two wavelength domains, the reported time lag of 20 min inferred using the Keck data of this common VLT/Keck night only is at odds with the lag of 100 min reported earlier. With our long lightcurve, we find that (i) the simultaneous 1.3 millimeter observations are in fact consistent with a 100 min time lag, (ii) the different methods of NIR photometry used by the VLT and Keck groups lead to consistent results, (iii) the Lomb-Scargle periodogram of the whole NIR lightcurve is featureless and follows a power-law with slope -1.6, and (iv) scanning the lightcurve with a sliding window to look for a transient QPO phenomenon reveals for a certain part of the lightcurve a 25 min peak in the periodogram. Using Monte Carlo simulations and taking the number of trials into account, we find it to be insignificant. ------------------------------------------------------------------------ Email : najarro@damir.iem.csic.es Title : Metallicity in the Galactic Center:, The Quintuplet cluster Author(s): Francisco Najarro(1), Don F. Figer(2), D. John Hillier(3), T. R. Geballe(4), Rolf P. Kudritzki(5) Institute: (1) Instituto de Estructura de la Materia, CSIC, Serrano 121, 29006 Madrid, Spain (2) Chester F. Carlson Center for Imaging Science, Rochester Institute of Technology, 54 Lomb Memorial Drive, Rochester, NY 14623 (3) Department of Physics and Astronomy, University of Pittsburgh, 3941 O'Hara Street, Pittsburgh, PA 15260 (4) Gemini Observatory, Hilo, 670 N. A'ohoku Pl., HI 96720 (5) Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822 Paper : ApJ accepted EPrint : 0809.3185 Abstract: We present a measurement of metallicity in the Galactic center Quintuplet Cluster made using quantitative spectral analysis of two Luminous Blue Variables (LBVs). The analysis employs line-blanketed NLTE wind/atmosphere models fit to high-resolution near-infrared spectra containing lines of H, \HeI, \SiII, \MgII, and \FeII. We are able to break the H/He ratio vs. mass-loss rate degeneracy found in other LBVs and to obtain robust estimates of the He content of both objects. Our results indicate solar iron abundance and roughly twice solar abundance in the alpha -elements. These results are discussed within the framework of recent measurements of oxygen and carbon composition in the nearby Arches Cluster and iron abundances in red giants and supergiants within the central 30 pc of the Galaxy. The relatively large enrichment of alpha -elements with respect to iron is consistent with a history of more nucleosynthesis in high mass stars than the Galactic disk. ------------------------------------------------------------------------ Email : pat@physto.se Title : Dark stars at the Galactic centre - the main sequence Author(s): Pat Scott^1, Malcolm Fairbairn^2,3 and Joakim Edsjoe^1 Institute: ^1Cosmology, Particle Astrophysics and String Theory, Department of Physics, Stockholm University \& High Energy Astrophysics and, Cosmology Centre (HEAC), AlbaNova University Centre, SE-106 91 Stockholm, Sweden, ^2Theory Division, CERN, CH-1211, Geneva 23, Switzerland, ^3Physics, Kings College London, Strand, London WC2R 2LS, UK Paper : arXiv:0809.1871 Web : http://arxiv.org/abs/0809.1871v1 Abstract: In regions of very high dark matter density such as the Galactic centre, the capture and annihilation of WIMP dark matter by stars has the potential to significantly alter their evolution. We describe the dark stellar evolution code DarkStars, and present a series of detailed grids of WIMP-influenced stellar models for main sequence stars. We describe the changes in stellar structure and main sequence evolution which occur as a function of the rate of energy injection by WIMPs, for masses of 0.3-2.0 M_o and metallicities Z = 0.0003-0.02. We show what rates of energy injection can be obtained using realistic orbital parameters for stars at the Galactic centre, including detailed consideration of the velocity and density profiles of dark matter. Capture and annihilation rates are strongly boosted when stars follow elliptical rather than circular orbits. Dark stars are unlikely to exist on any circular orbits in the Milky Way. If there is a spike of dark matter induced by the supermassive black hole at the Galactic centre, solar-mass stars following orbits with periods as long as 50 years and eccentricities as low as 0.9 could be significantly affected. The observation of low-mass stars on such orbits would either provide a detection of WIMP dark matter, or place stringent limits on the combination of the WIMP mass, spin-dependent nuclear-scattering cross-section, halo density and velocity distribution near the Galactic centre. In some cases, the derived limits on the WIMP mass and spin-dependent nuclear-scattering cross-section would be of comparable sensitivity to current direct-detection experiments. ------------------------------------------------------------------------ Email : rslu@mpifr-bonn.mpg.de Title : High-frequency VLBI observations of Sgr A* during a multi-frequency campaign in May 2007 Author(s): R-S Lu^1,2, T P Krichbaum^1, A Eckart^2, S Koenig^2,1, D Kunneriath^2,1, G Witzel^2, A Witzel^1 and J A Zensus^1 Institute: (1) ^1 Max-Planck-Institut fuer Radioastronomie, Auf dem Huegel 69, D-53121 Bonn, Germany (2) ^2 University of Cologne, I. Physikalisches Institut, Zuelpicher Str. 77, D-50937 Koeln, Germany Web : http://arXiv.org/abs/0809.2463 Abstract: In May 2007 the compact radio source Sgr A* was observed in a global multi-frequency monitoring campaign, from radio to X-ray bands. Here we present and discuss first and preliminary results from polarization sensitive VLBA observations, which took place during May 14-25, 2007. Here, Sgr A* was observed in dual polarization on 10 consecutive days at 22, 43, and 86 GHz. We describe the VLBI experiments, our data analysis, monitoring program and show preliminary images obtained at the various frequencies. We discuss the data with special regard also to the short term variability. ------------------------------------------------------------------------ Email : uros.kostic@fmf.uni-lj.si Title : On the tidal evolution of the orbits of low-mass satellites around black holes Author(s): Andrej Cadez(1) and Massimo Calvani(2) and Uros Kosti\'c(1) Institute: (1) Faculty of Mathematics and Physics, University of Ljubljana, Jadranska 19, 1000 Ljubljana, Slovenia (2) INAF - Astronomical Observatory of Padova, Vicolo Osservatorio 5, 35122 Padova, Italy Paper : A&A 487 (2008) 527-532 Web : http://astro.ago.uni-lj.si/bh/uploads/Main/aa_tidal.pdf Abstract: Low-mass satellites, like asteroids and comets, are expected to be present around the black hole at the Galactic center. We consider small bodies orbiting a black hole, and we study the evolution of their orbits due to tidal interaction with the black hole. In this paper we investigate the consequences of the existence of plunging orbits when a black hole is present. We are interested in finding the conditions that exist when capture occurs. Earlier analysis of the evolution of classical Keplerian orbits was extended to relativistic orbits around a Schwarzschild black hole. The main difference between the Keplerian and black hole cases is in the existence of plunging orbits. Orbital evolution, leading from bound to plunging orbits, goes through a ``final" unstable circular orbit. On this orbit, tidal energy is released on a characteristic black hole timescale. This process may be relevant for explaining how small, compact clumps of material can be brought onto plunging orbits, where they may produce individual short duration accretion events. The available energy and the characteristic timescale are consistent with energy released and the timescale typical of Galactic flares. ------------------------------------------------------------------------ (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 ========================================================================