======================================================================== G C N E W S * Newsflash * - The Newsletter for Galactic Center Research - gcnews@aoc.nrao.edu http://www.aoc.nrao.edu/~gcnews ======================================================================== Vol. 27, No. 7 Sep 20, 2007 Recently submitted papers: -------------------------- 1) On the orientation of the Sagittarius A* system (Eckart et al., A&A) 2) Hypervelocity Stars III. The Space Density and Ejection History of Main Sequence Stars from the Galactic Center (Brown et al., ApJ) 3) Suzaku Detection of Extended/Diffuse Hard X-Ray Emission from the Galactic Center (Yuasa et al., PASJ) 4) The Variability of Polarized Radiation from Sgr A* (Yusef-Zadeh et al., ApJL) ------------------------------------------------------------------------ Email : leo@ph1.uni-koeln.de Title : On the orientation of the Sagittarius A* system Author(s): L. Meyer(1) and R. Schoedel(1) and A. Eckart(1,2) and W. J. Duschl(3,4) and V. Karas(5) and M. Dov\vciak(5) Institute: (1) I.Physikalisches Institut, Universitaet zu Koeln, Zuelpicher Str. 77, 50937 Koeln, Germany (2) Max-Planck-Institut fuer Radioastronomie, Auf dem Huegel 69, 53121 Bonn, Germany (3) Institut fuer Theoretische Physik und Astrophysik, Universitaet zu Kiel, 24098 Kiel, Germany (4) Steward Observatory, The University of Arizona, 933 N. Cherry Ave. Tucson, AZ 85721, USA (5) Astronomical Institute, Academy of Sciences, Bo\vcni II, CZ-14131 Prague, Czech Republic Paper : A&A, 2007, in press Abstract: The near-infrared emission from the black hole at the Galactic center (Sgr A*) has unique properties. The most striking feature is a suggestive periodic sub-structure that has been observed in a couple of flares so far. Using near-infrared polarimetric observations and modelling the quasi-periodicity in terms of an orbiting blob, we try to constrain the three dimensional orientation of the Sgr A* system. We report on so far unpublished polarimetric data from 2003. They support the observations of a roughly constant mean polarization angle of 60 DEGr +/- 20 DEGr from 2004 - 2006. Prior investigations of the 2006 data are deepened. In particular, the blob model fits are evaluated such that constraints on the position angle of Sgr A* can be derived. Confidence contours in the position - inclination angle plane are derived. On a 3 sigma level the position angle of the equatorial plane normal is in the range 60 DEGr - 108 DEGr (east of north) in combination with a large inclination angle. This agrees well with recent independent work in which radio spectral/morphological properties of Sgr A* and X-ray observations, respectively, have been used. However, the quality of the presently available data and the uncertainties in our model bring some ambiguity to our conclusions. ------------------------------------------------------------------------ Email : wbrown@cfa.harvard.edu Title : Hypervelocity Stars III. The Space Density and Ejection History of Main Sequence Stars from the Galactic Center Author(s): Warren R. Brown, Margaret J. Geller, Scott J. Kenyon, Michael J. Kurtz Benjamin C. Bromley Institute: (1) Smithsonian Astrophysical Observatory, 60 Garden St, Cambridge, MA 02138 (2) Department of Physics, University of Utah, 115 S 1400 E, Salt Lake City, UT 84112 Paper : ApJ, 2007, accepted EPrint : 0709.1471 Abstract: We report the discovery of 3 new unbound hypervelocity stars (HVSs), stars traveling with such extreme velocities that dynamical ejection from a massive black hole (MBH) is their only suggested origin. We also detect a population of possibly bound HVSs. The significant asymmetry we observe in the velocity distribution - we find 26 stars with v_rf>275 km s^-1 and 1 star with v_rf<-275 km s^-1 - shows that the HVSs must be short-lived, probably 3 - 4 M_\sun main sequence stars. Any population of hypervelocity post-main sequence stars should contain stars falling back onto the Galaxy, contrary to the observations. The spatial distribution of HVSs also supports the main sequence interpretation: longer-lived 3 M_\sun HVSs fill our survey volume; shorter-lived 4 M_\sun HVSs are missing at faint magnitudes. We infer that there are 96+/-10 HVSs of mass 3 - 4 M_\sun within R<100 kpc, possibly enough HVSs to constrain ejection mechanisms and potential models. Depending on the mass function of HVSs, we predict that SEGUE may find up to 5 - 15 new HVSs. The travel times of our HVSs favor a continuous ejection process, although a 120 Myr-old burst of HVSs is also allowed. ------------------------------------------------------------------------ Email : yuasa@amalthea.phys.s.u-tokyo.ac.jp Title : Suzaku Detection of Extended/Diffuse Hard X-Ray Emission from the Galactic Center Author(s): Takayuki Yuasa(1), Ken-ichi Tamura(2,1), Kazuhiro Nakazawa(1), Motohide Kokubun(2), Kazuo Makishima(1,3), Aya Bamba(2), Yoshitomo Maeda(2), Tadayuki Takahashi(2,1), Ken Ebisawa(2,1), Atsushi Senda(3), Yoshiaki Hyodo(4), Takeshi Go Tsuru(4), Katsuji Koyama(4), Shigeo Yamauchi(5), Hiromitsu Takahashi(6) Institute: (1) Department of Physics, School of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033 (2) Department of High Energy Astrophysics, Institute of Space and Astronautical Science (ISAS),,Japan Aerospace Exploration Agnency (JAXA), 3-1-1 Yoshinodai, Sagamihara, Kanagawa 229-8510 (3) Cosmic Radiation Laboratory, The Institute of Physical and Chemical Research (RIKEN), 2-1 Hirosawa, Wako, Saitama 351-0198 (4) Department of Physics, Graduate School of Science, Kyoto University, Kita Shirakawa Oiwake-cho, Sakyo-ku, Kyoto 606-8502 (5) Faculty of Humanities and Social Sciences, Iwate University, 3-18-34 Ueda, Morioka, Iwate 020-8550 (6) Department of Physical Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526 Paper : PASJ, in press EPrint : 0709.1580 Web : http://arxiv.org/pdf/0709.1580 Abstract: Five on-plane regions within +/- \timeform0o.8 of the Galactic center were observed with the Hard X-ray Detector (HXD) and the X-ray Imaging Spectrometer (XIS) onboard Suzaku. From all regions, significant hard X-ray emission was detected with HXD-PIN up to 40 keV, in addition to the extended plasma emission which is dominant in the XIS band. The hard X-ray signals are inferred to come primarily from a spatially extended source, rather than from a small number of bright discrete objects. Contributions to the HXD data from catalogued X-ray sources, typically brighter than 1 mCrab, were estimated and removed using information from Suzaku and other satellites. Even after this removal, the hard X-ray signals remained significant, exhibiting a typical 12-40 keV surface brightness of 4*10^-10 \ergcms \mathrmdeg^-2 and power-law-like spectra with a photon index of 1.8. Combined fittings to the XIS and HXD-PIN spectra confirm that a separate hard tail component is superposed onto the hot thermal emission, confirming a previous report based on the XIS data. Over the 5-40 keV band, the hard tail is spectrally approximated by a power law of photon index 2, but better by those with somewhat convex shapes. Possible origins of the extended hard X-ray emission are discussed. ------------------------------------------------------------------------ Email : zadeh@northwestern.edu Title : The Variability of Polarized Radiation from Sgr A* Author(s): F. Yusef-Zadeh, M. Wardle, W. D. Cotton, C. O. Heinke and D. A. Roberts Paper : ApJL, in press Web : http://arxiv.org/abs/0708.3492 Abstract: Sgr A* is variable at radio and submillimeter wavelengths on hourly time scales showing time delays between the peaks of flare emission as well as linearly polarized emission at millimeter and sub-mm wavelengths. To determine the polarization characteristics of this variable source at radio frequencies, we present VLA observations of Sgr A* and report the detection of polarized emission at a level of 0.77+/-0.01% and 0.2+/-0.01% at 43 and 22 GHz, respectively. The change in the time averaged polarization angle between 22 and 43 GHz corresponds to a RM of -2.5+/-0.6 *10^3 rad m^-2 with no phase wrapping (or 5*10^4 rad m^2 with 2\pi phase wrap). We also note a rise and fall time scale of 1.5 - 2 hours in the total polarized intensity. The light curves of the degree of linearly polarized emission suggests a a correlation with the variability of the total intensity at 43 GHz. The available polarization data at radio and sub-mm wavelengths suggest that the rotation measure decreases with decreasing frequency. This frequency dependence, and observed changes in polarization angle during flare events, may be caused by the reduction in rotation measure associated with the expansion of synchrotron-emitting blobs. ------------------------------------------------------------------------ (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 ========================================================================