======================================================================== G C N E W S * Newsflash * - The Newsletter for Galactic Center Research - gcnews@aoc.nrao.edu http://www.aoc.nrao.edu/~gcnews ======================================================================== Vol. 23, No. 2 Dec 21, 2005 Recently submitted papers: -------------------------- 1) High Spectral Resolution Observations of the Massive Stars in the Galactic Center (Tanner et al., in press) 2) The Flare Activity of SgrA*; New Coordinated mm to X-Ray Observations (Eckart et al., A&A) 3) Observation of Gamma Rays from the Galactic Center with the MAGIC Telescope (Albert et al., ApJ) 4) Understanding the Radio Variability of Sgr A* (Macquart et al., ApJ) 5) The Double Helix Nebula: a magnetic torsional wave propagating out of the Galactic centre (Morris et al., accepted) 6) The ecology of star clusters and intermediate mass black holes in the Galactic bulge (Portegies Zwart et al., ApJ) 7) GCIRS34W: An irregular variable in the Galactic Centre (Trippe et al., A&A) 8) Millisecond pulsar population in the Galactic center and high energy contributions (Wang, ChJAA) ------------------------------------------------------------------------ Email : Angelle.M.Tanner-120278@jpl.nasa.gov Title : High Spectral Resolution Observations of the Massive Stars in the Galactic Center Author(s): Angelle Tanner Donald F. Figer, Francisco Najarro, Rolf P. Kudritzki, Diane Gilmore, Mark Morris, E. E. Becklin, Ian S. McLean, Andrea M. Gilbert, James R. Graham, James E. Larkin, N. A. Levenson, Harry I. Teplitz Institute: (1) Jet Propulsion Lab, 4800 Oak Grove Dr., Pasadena, CA 91106 (2) Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 (3) Instituto de Estructura de la Materia, CSIC, Serrano 121, 28006, Madrid, Spain (4) Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822 (5) Department of Astronomy and Astrophysics, UCLA, 405 Hilgard Ave., Los Angeles, CA 90095 (6) Max-Planck-Institut für extraterrestrische Physik, Giessenbachstr. 1, 85748 Garching, Germany (7) Department of Astronomy, University of California, Berkeley, 601 Campbell Hall, Berkeley, CA, 94720-3411 (8) Department of Physics and Astronomy, University of Kentucky, 600 Rose Street, Lexington, Kentucky, 40506-0055 (9) Spitzer Science Center, MS 220-6, California Institute of Technology, Pasadena, CA, 91125 Paper : April 2006, in press EPrint : astro-ph/0510028 Abstract: We present high-resolution near-infrared spectra, obtained with the NIRSPEC spectrograph on the W. M. Keck II Telescope, of a collection of hot, massive stars within the central 25 arcseconds of the Galactic center. We have identified a total of twenty-one emission-line stars, seven of which are new radial velocity detections with five of those being classified as He I emission-line stars for the first time. These stars fall into two categories based on their spectral properties: 1) those with narrow lambda lambda 2.112, 2.113 micron \ionHe1 doublet absorption lines, and 2) those with broad 2.058 micron \ionHe1 emission lines. These data have the highest spectral resolution ever obtained for these sources and, as a result, both components of the absorption doublet are separately resolved for the first time. We use these spectral features to measure radial velocities. The majority of the measured radial velocities have relative errors of 20 km/sec , smaller than those previously obtained with proper-motion or radial velocity measurements for similar stellar samples in the Galactic center. The radial velocities estimated from the \ionHe1 absorption doublet are more robust than those previously estimated from the 2.058 micron emission line, since they do not suffer from confusion due to emission from the surrounding ISM. Using this velocity information, we agree with previous stellar velocity studies that the stars are orbiting in a somewhat coherent manner but are not as defined into a disk or disks as previously thought. Finally, multi-epoch radial velocity measurements for IRS 16NE show a change in its velocity presumably due to an unseen stellar companion. ------------------------------------------------------------------------ Email : eckart@ph1.uni-koeln.de Title : The Flare Activity of SgrA*; New Coordinated mm to X-Ray Observations Author(s): A. Eckart, F. K. Baganoff, R. Schoedel, M. Morris, R. Genzel, G.C. Bower, D. Marrone, J.M. Moran, T. Viehmann, M.W. Bautz, W.N. Brandt, G.P. Garmire, T. Ott, S. Trippe, G.R. Ricker, C. Straubmeier, D.A. Roberts, F. Yusef-Zadeh, J.H. Zhao, R. Rao Paper : A&A Dec.2005 EPrint : astro-ph/0512440 Abstract: We report new simultaneous near-infrared/sub-millimeter/X-ray observations of the SgrA* counterpart associated with the massive 3-4x10**6 solar mass black hole at the Galactic Center. The main aim is to investigate the physical processes responsible for the variable emission from SgrA*. The observations have been carried out using the NACO adaptive optics (AO) instrument at the European Southern Observatory's Very Large Telescope and the ACIS-I instrument aboard the Chandra X-ray Observatory as well as the Submillimeter Array SMA on Mauna Kea, Hawaii, and the Very Large Array in New Mexico. We detected one moderately bright flare event in the X-ray domain and 5 events at infrared wavelengths. ------------------------------------------------------------------------ Email : hbartko@mppmu.mpg.de Title : Observation of Gamma Rays from the Galactic Center with the MAGIC Telescope Author(s):  J. Albert and another 130 authors Paper : ApJ submitted Abstract: Recently, the Galactic Center has been reported to be a source of very high energy (VHE) gamma -rays by the VERITAS, CANGAROO and HESS experiments. The energy spectra as measured by these experiments show substantial differences. In this Letter we present MAGIC observations of the Galactic Center, resulting in the detection of a differential gamma -ray flux consistent with a steady, hard-slope power law, described as dN_ gamma /(dA dt dE) = (2.9 +/- 0.6)* 10^-12 (E/TeV)^-2.2 +/- 0.2 cm^-2s^-1 TeV^-1. The gamma -ray source is centered at (Ra, Dec)=(17^h45^m20^s, -29^o2'). This result confirms the previous measurements by the HESS experiment and indicates a steady source of TeV gamma -rays. We briefly describe the observational technique used, the procedure implemented for the data analysis, and discuss the results in the perspective of different models proposed for the acceleration of the VHE gamma -rays. ------------------------------------------------------------------------ Email : jmacquar@aoc.nrao.edu Title : Understanding the Radio Variability of Sgr A* Author(s): Jean-Pierre Macquart, Geoffrey C. Bower 1 Institute: (1) National Radio Astronomy Observatory, P.O. Box 0, Socorro NM 87801, U.S.A. (2) Astronomy Department and Radio Astronomy Laboratory, University of California, Berkeley, Berkeley, CA 94720, U.S.A. Paper : ApJ, Dec 2005, in press EPrint : astro-ph/0512288 Web : http://www.aoc.nrao.edu/~jmacquar/SgrA.pdf Abstract: We determine the characteristics of the 7 mm to 20 cm wavelength radio variability in Sgr A* on time scales from days to three decades. The amplitude of the intensity modulation is between 30 and 39% at all wavelengths. Analysis of uniformly sampled data with proper accounting of the sampling errors associated with the lightcurves shows that Sgr A* exhibits no 57- or 106-day quasi-periodic oscillations, contrary to previous claims. The cause of the variability is investigated by examining a number of plausible scintillation models, enabling those variations which could be attributed to interstellar scintillation to be isolated from those that must be intrinsic to the source. Thin-screen scattering models do not account for the variability amplitude on most time scales. However, models in which the scattering region is extended out to a radius of 50-500 pc from the Galactic Center account well for the broad characteristics of the variability on >4-day time scales. The 10% variability on <4-day time scales at 0.7-3 cm appears to be intrinsic to the source. The degree of scintillation variability expected at millimeter wavelengths depends sensitively on the intrinsic source size; the variations, if due to scintillation, would require an intrinsic source size smaller than that expected. ------------------------------------------------------------------------ Email : morris@astro.ucla.edu Title : The Double Helix Nebula: a magnetic torsional wave propagating out of the Galactic centre Author(s): Mark Morris (UCLA), Keven Uchida (Cornell U.), and Tuan Do (UCLA) Paper : accepted EPrint : astro-ph/0512452 Abstract: Radioastronomical studies have indicated that the magnetic field in the central few hundred parsecs of our Milky Way Galaxy has a dipolar geometry and a strength substantially larger than elsewhere in the Galaxy, with estimates ranging up to a milligauss. A strong, large- scale magnetic field can affect the Galactic orbits of molecular clouds by exerting a drag on them, it can inhibit star formation, and it can guide a wind of cosmic rays away from the central region, so a characterization of the magnetic field at the Galactic center is important for understanding much of the activity there. Here, we report Spitzer Space Telescope observations of an unprecedented infrared nebula having the morphology of an intertwined double helix. This feature is located about 100 pc from the Galaxy's dynamical centre toward positive Galactic latitude, and its axis is oriented perpendicular to the Galactic plane. The observed segment is about 25 pc in length, and contains about 1.25 full turns of each of the two continuous, helically wound strands. We interpret this feature as a torsional AlfvĂ©n wave propagating vertically away from the Galactic disk, driven by rotation of the magnetized circumnuclear gas disk. As such, it offers a new morphological probe of the Galactic center magnetic field. The direct connection between the circumnuclear disk and the double helix is ambiguous, but the MSX images show a possible meandering channel that warrants further investigation. ------------------------------------------------------------------------ Email : spz@science.uva.nl Title : The ecology of star clusters and intermediate mass black holes in the Galactic bulge Author(s): Simon F. Portegies Zwart, Holger Baumgardt, Stephen L. W. McMillan, Junichiro Makino, Piet Hut, Toshi Ebisuzaki Paper : ApJ, in press EPrint : astro-ph/0511397 Abstract: We simulate the inner 100pc of the Milky-Way Galaxy to study the formation and evolution of the population of star clusters and intermediate mass black holes. For this study we perform extensive direct N-body simulations of the star clusters which reside in the bulge, and of the inner few tenth of parsecs of the super massive black hole in the Galactic center. In our N-body simulations the dynamical friction of the star cluster in the tidal field of the bulge are taken into account via (semi)analytic soluations. The N-body calculations are used to calibrate a (semi)analytic model of the formation and evolution of the bulge. We find that about 10% of the clusters born within 100pc of the Galactic center undergo core collapse during their inward migration and form intermediate-mass black holes (IMBHs) via runaway stellar merging. After the clusters dissolve, these IMBHs continue their inward drift, carrying a few of the most massive stars with them. We predict that region within about 10 parsec of the SMBH is populated by about 50IMBHs of some 1000Msun. Several of these are expected to be accompanied still by some of the most massive stars from the star cluster. We also find that within a few milliparsec of the SMBH there is a steady population of several IMBHs. This population drives the merger rate between IMBHs and the SMBH at a rate of about one per 10Myr, sufficient to build the accumulate majority of mass of the SMBH. Mergers of IMBHs with SMBHs throughout the universe are detectable by LISA, at a rate of about two per week. ------------------------------------------------------------------------ Email : trippe@mpe.mpg.de Title : GCIRS34W: An irregular variable in the Galactic Centre Author(s): S. Trippe (1) and F. Martins (1) and T. Ott (1) and T. Paumard (1) and R. Abuter (1) and F. Eisenhauer (1) and S. Gillessen (1) and R. Genzel (1,2) and A. Eckart (3) and R. Schoedel (3) Institute: (1) Max-Planck-Institut fuer extraterrestrische Physik, Giessenbachstrasse, Postfach 1312, D-85741 Garching, Germany and Department of Physics, University of California, Berkeley, USA and 1. Physikalisches Institut, Universitaet zu Koeln, Zuelpicher Strasse 77, D-50937 Koeln, Germany Paper : A&A, accepted EPrint : astro-ph/0510478 Abstract: We report the results of time-resolved photometric and spectroscopic near-infrared observations of the Ofpe/WN9 star and LBV candidate GCIRS34W in the Galactic Centre star cluster. Diffraction limited resolution photometric observations obtained in H- and K-bands show a strong, non-periodic variability on time scales from months to years in both bands, accompanied by variations of the stellar colour. Three K-band spectra obtained in 1996, 2003, and 2004 with integral field spectrometers are identical within their accuracies and exclude significant spectroscopic variability. The most probable explanation of the stellar photometric variability is obscuration by circumstellar material ejected by the star. The approximated position of GCIRS34W in an HR diagram is located between O supergiants and LBVs, suggesting that this star is a transitional object between these two phases of stellar evolution. ------------------------------------------------------------------------ Email : wwang@mpe.mpg.de Title : Millisecond pulsar population in the Galactic center and high energy contributions Author(s): Wei Wang Institute: (1) National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012 Max-Planck-Institut fuer extraterrestrische Physik, Postfach 1312, 85741 Garching, Germany Paper : ChJAA 2006 in press EPrint : astro-ph/0510461 Abstract: We propose that there possibly exists a population of millisecond pulsars in the Galactic center region. Millisecond pulsars (MSPs) could emit GeV gamma-rays through synchrotron-curvature radiation as predicted by outer gap models. In the same time, the compact wind nebulae around millisecond pulsars can emit X-rays though synchrotron radiation and TeV photons through inverse Compton processes. Besides, millisecond pulsar winds provide good candidates for the electron-positrons sources in the Galactic center. Therefore, we suggest that the millisecond pulsar population could contribute to the weak unidentified Chandra X-ray sources, the diffuse gamma-rays detected by EGRET, electron-positron annihilation lines and TeV photons detected by HESS toward 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, Robin Herrnstein - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - For Abstract submission please follow the instructions which are at http://www.aoc.nrao.edu/~gcnews/home/submission.shtml ========================================================================