======================================================================== G C N E W S * Newsflash * - The Newsletter for Galactic Center Research - gcnews@aoc.nrao.edu http://www.aoc.nrao.edu/~gcnews ======================================================================== Vol. 21, No. 2 Apr 12, 2005 Dear subscriber, Below we have summarized the new abstracts received since March 25th. Another new item is the audio CD from the Galactic Center workshop in Hawaii (2002), which we have put on the GCNEWS conference web page (http://www.aoc.nrao.edu/~gcnews/home/conferences.shtml) under GC02. Please continue to send in your abstracts to GCNEWS, for example from the upcoming KITP/UCSB meeting this week. Thanks in advance! Recently submitted papers: -------------------------- 1) INTEGRAL/IBIS search for e^-e^+ annihilation radiation from the Galactic Center Region (De Cesare et al., Advances in Space Research) 2) Contribution to diffuse gamma-rays in the Galactic center region from unresolved millisecond pulsars (Wang et al., MNRAS) 3) Stellar Orbits Around the Galactic Center Black Hole (Ghez et al., ApJ 2005) 4) Intra-day Variability of Sagittarius A* at 3 Millimeters (Mauerhan et al., ApJL) ------------------------------------------------------------------------ Email : Giovanni.Decesare@rm.iasf.cnr.it Title : INTEGRAL/IBIS search for e^-e^+ annihilation radiation from the Galactic Center Region Author(s): G. De Cesare(1), A. Bazzano(1), F. Capitanio(1), M. Del Santo(1), V. Lonjou(2), L. Natalucci(1), P. Ubertini(1), P. Von Ballmoos(2) Institute: (1) Istituto di Astrofisica Spaziale e Fisica cosmica (IASF) - CNR, via Fosso del Cavaliere 100, 00133 Roma, Italy (2) Centre d'Etude Spatiale des Raynnements, CNRS/UPS, B.P. 4346, 31028 Toulouse Cedex 4, France Paper : Advances in Space Research , 2005, in press EPrint : astro-ph/0501123 Web : http://arxiv.org/abs/astro-ph/0501123 Abstract: Electron-positron annihilation radiation from the Galactic Center region has been detected since the seventies, but its astrophysical origin is still a topic of a scientific debate. We have analyzed data of the gamma-ray imager IBIS/ISGRI onboard of ESA's INTEGRAL platform in the e^-e^+ line. During the first year of the missions Galactic Center Deep Exposure no evidence for point sources at 511 keV has been found in the ISGRI data; the 2 sigma upper limit for resolved single point sources is estimated to be 1.6* 10^-4 ph cm^-2 s^-1. ------------------------------------------------------------------------ Email : anisia@bohr.physics.hku.hk Title : Contribution to diffuse gamma-rays in the Galactic center region from unresolved millisecond pulsars Author(s): Wang, W., Jiang, Z.J. and Cheng, K.S. Institute: Department of Physics, The University of Hong Kong, Pokfulam Road, Hong Kong Paper : MNRAS, Mar 2005, 358 (1), 263-269 EPrint : astro-ph/0501245 Abstract: The diffuse gamma-rays in the Galactic center region have been studied. We propose that there exists a population of millisecond pulsars (MSPs) in the Galactic Center, which will emit GeV gamma-rays through the synchrotron-curvature radiation as predicted by outer gap models. These GeV gamma-rays from unresolved millisecond pulsars probably contribute to the diffuse gamma-ray spectrum detected by EGRET which displays a break at a few GeV. We have used the Monte Carlo method to obtain the simulated samples of millisecond pulsars in the Galactic center region covered by EGRET ( 1.5^o) according to the different period and magnetic field distributions from the observed millisecond pulsars in the Galactic field and globular clusters, and superposed their synchrotron-curvature spectra to derive the total GeV flux. Our simulated results suggest that there probably exist about 6000 unresolved millisecond pulsars in the region of the angular resolution for EGRET, whose emissions could contribute significantly to the observed diffuse gamma-rays in the Galactic center. ------------------------------------------------------------------------ Email : ghez@astro.UCLA.EDU Title : Stellar Orbits Around the Galactic Center Black Hole Author(s): A. M. Ghez(1), S. Salim, S. D. Hornstein, A. Tanner, J. R. Lu, M. Morris, E. E. Becklin, G. Duchene Institute: (1) UCLA Division of Astronomy and Astrophysics, Los Angeles, CA 90095-1574 (2) Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90095-1565 Paper : 2005 ApJ, 620, 744 Abstract: We present new diffraction-limited images of the Galactic Center, obtained with the W. M. Keck I 10-meter telescope. Within 0.'' 4 of the Galaxy's central dark mass, 17 proper motion stars, with K magnitudes ranging from 14.0 to 16.8, are identified and 10 of these are new detections (6 were also independently discovered by Schoedel et al. 2003). In this sample, three newly identified (S0-16, S0-19, and S0-20) and four previously known (S0-1, S0-2, S0-4, and S0-5) sources have measured proper motions that reveal orbital solutions. Orbits are derived simultaneously so that they jointly constrain the central dark object's properties: its mass, its position, and, for the first time using orbits, its motion on the plane of the sky. This analysis pinpoints the Galaxy's central dark mass to within 1.3 mas (10 AU) and limits its proper motion to 1.5 +/- 0.5 mas y^-1 (or equivalently 60 +/- 20 km s^-1) with respect to the central stellar cluster. This localization of the central dark mass is consistent with our derivation of the position of the radio source Sgr A* in the infrared reference frame (+/- 10 mas), but with an uncertainty that is a factor 8 times smaller, which greatly facilitates searches for near-infrared counterparts to the central black hole. Consequently, one previous claim for such a counterpart can now be ascribed to a close stellar passage in 1996. Furthermore, we can place a conservative upper limit of 15.5 mag on any steady-state counter-part emission. The estimated central dark mass from orbital motions is 3.7 (+/- 0.2 ) * 10^6 (\fracR_o8kpc)^3 M_o; this is a more direct measure of mass than those obtained from velocity dispersion measurements, which are as much as a factor of two smaller. The Galactic Center's distance, which adds an additional 19% uncertainty in the estimated mass, is now the limiting source of uncertainty in the absolute mass. For stars in this sample, the closest approach is achieved by S0-16, which came within a mere 45 AU (= 0.0002 pc = 600 R_s) at a velocity of 12,000 km s^-1. This increases the inferred dark mass density by four orders of magnitude compared to earlier analyses based on velocity and acceleration vectors, making the Milky Way the strongest existing case for a supermassive black hole at the center of a normal type galaxy. Well determined orbital parameters for these seven Sgr A* cluster stars provide new constraints on how these apparently massive, young (<10 Myr) stars formed in a region that seems to be hostile to star formation. Unlike the more distant He-I emission line stars - another population of young stars in the Galactic Center - that appear to have co-planar orbits, the Sgr A* cluster stars have orbital properties (eccentricities, angular momentum vectors, and apoapse directions) that are consistent with an isotropic distribution. Therefore many of the mechanisms proposed for the formation of the He-I stars, such as formation from a pre-existing disk, are unlikely solutions for the Sgr A* cluster stars. Unfortunately, alternative theories for producing young stars, or old stars that look young, in close proximity to a central supermassive black hole, are all also somewhat problematic. Understanding the apparent youth of stars in the Sgr A* cluster, as well as the more distant He I emission line stars, has now become one of the major outstanding issues in the study of the Galactic Center. ------------------------------------------------------------------------ Email : mauerhan5000@yahoo.com Title : Intra-day Variability of Sagittarius A* at 3 Millimeters Author(s): Jon C. Mauerhan(1), Mark Morris(1), Fabian Walter(2), and Frederick K. Baganoff(3) Institute: (1) Department of Physics \& Astronomy, University of California, Los Angeles, Los Angeles, CA 90095 (2) Max Planck Institut fuer Astronomie, Koenigstuhl 17, 69117 Heidelberg, Germany (3) Center for Space Research, Massachusetts Institute of Technology, Cambridge MA 02139 Paper : ApJL, Mar 2005, in press EPrint : astro-ph/0503124 Abstract: We report observations and analysis of flux monitoring of Sagittarius A* at 3-mm wavelength using the OVRO millimeter interferometer over a period of eight days (2002 May 23-30). Frequent phase and flux referencing (every 5 minutes) with the nearby calibrator source J1744-312 was employed to control for instrumental and atmospheric effects. Time variations are sought by computing and subtracting, from each visibility in the database, an average visibility obtained from all the data acquired in our monitoring program having similar uv-spacings. This removes the confusing effects of baseline-dependent, correlated flux interference caused by the static, thermal emission from the extended source Sgr A West. Few-day variations up to 20% and intra-day variability of 20% and in some cases up to 40% on few-hour time scales emerge from the differenced data on SgrA*. Power spectra of the residuals indicate the presence of hourly variations on all but two of the eight days. Monte Carlo simulation of red-noise light curves indicates that the hourly variations are well described by a red-noise power spectrum with P(f) proportional to f^-1. Of particular interest is a 2.5 hour variation seen prominently on two consecutive days. An average power spectrum from all eight days of data reveals noteworthy power on this time scale. There is some indication that few-hour variations are more pronounced on days when the average daily flux is highest. We briefly discuss the possibility that these few-hour variations are due to the dynamical modulation of accreting gas around the central supermassive black hole, as well as the implications for the structure of the SgrA* photosphere at 3 mm. Finally, these data have enabled us to produce a high sensitivity 3-mm map of the extended thermal emission surrounding SgrA*. ------------------------------------------------------------------------ (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 ========================================================================