======================================================================== G C N E W S * Newsflash * - The Newsletter for Galactic Center Research - gcnews@mpifr-bonn.mpg.de http://www.mpifr-bonn.mpg.de/gcnews ======================================================================== Vol. 9, No. 15 Apr 29, 1999 Dear subscriber: we have tried to bring GCNEWS back online! We hope that from now one GCNEWS will be almost back to normal again. To you it should look just as before. It could still be that due to the restructuring certain features are not available (e.g. subscription form) or that pages are not properly formatted. If you notice any dead links, ill-formatted pages, or any other trouble please do not hesitate to let us know. Please note that we were forced to change the endings of all pages from .html to .shtml. Hence if you have personal links to individual pages within gcnews, you will need to change this. The address http://www.mpifr-bonn.mpg.de/gcnews is still working. Please start again sending us your abstracts! Best wishes Heino Falcke & Angela Cotera Recently submitted papers: -------------------------- 1) The Linear Polarization of Sagittarius A* I. VLA Spectro-polarimetry at 4.8 and 8.4 GHz (Bower et al., ApJ) 2) THE STREAM-STREAM COLLISION AFTER THE TIDAL DISRUPTION OF A STAR AROUND A MASSIVE BLACK HOLE (Kim et al., ApJ) ------------------------------------------------------------------------ Email : gbower@aoc.nrao.edu Title : The Linear Polarization of Sagittarius A* I. VLA Spectro-polarimetry at 4.8 and 8.4 GHz Author(s): Geoffrey C. Bower(1,2), Donald C. Backer(3), Jun-Hui Zhao(4), Miller Goss(2) \& Heino Falcke(1) Institute: (1) Max Planck Institut fuer Radioastronomie, Auf dem Huegel 69, D 53121 Bonn Germany (2) National Radio Astronomy Observatory, P.O. Box O, 1003 Lopezville, Socorro, NM 87801 (3) Astronomy Department \& Radio Astronomy Laboratory, University of California, Berkeley, CA 94720 (4) Harvard-Smithsonian Center for Astrophysics, Mail Stop 42, 60 Garden Street, Cambridge, MA 02138 Paper : to appear in ApJ 521 #2, Aug 20, 1999 Weblink : http://www.nrao.edu/~gbower/publications.html EPrint : astro-ph/9904091 Abstract: Synchrotron radiation from active galactic nuclei (AGN) is often highly polarized. We present a search for linear polarization with the Very Large Array (VLA) at 4.8 GHz and 8.4 GHz from the nearest AGN, Sagittarius A*. As a part of this study we used spectro-polarimetric data that were sensitive to a rotation measure (RM) as large as 3.5* 10^6\rdm at 4.8 GHz and 1.5* 10^7\rdm at 8.4 GHz. The upper limit to the linear polarization of Sgr A* over a broad range of RM is 0.2% at both frequencies. We also present continuum observations with the VLA at 4.8 GHz which give an upper limit of 0.1% for RMs less than 10^4 \rdm. We conclude that depolarization is unlikely to occur in the Galacter Center scattering medium. However, it is possible for depolarization to occur in the accretion region of Sgr A* if the outer scale of turbulence is small enough. We also consider the implications of a very low intrinsic polarization for Sgr A*. ------------------------------------------------------------------------ Email : sskim@space.kaist.ac.kr Title : THE STREAM-STREAM COLLISION AFTER THE TIDAL DISRUPTION OF, A STAR AROUND A MASSIVE BLACK HOLE Author(s): Sungsoo S. Kim(1,4), Myeong-Gu Park(2), Hyung Mok Lee(3,5) Institute: (1) Department of Physics, Korea Advanced Institute of Science \& Technology, Daejon 305-701, Korea (2) Department of Astronomy and Atmospheric Sciences, Kyungpook National University, Taegu 702-701, Korea (3) Department of Earth Sciences, Pusan National University, Pusan 609-7 35, Korea (4) This work has been initiated when he was at Institute for Basic Sciences, Pusan National University, Korea (5) Current address: Department of Astronomy, Seoul National University, Seoul 151-742, Korea; hmlee@astro.snu.ac.kr Paper : Accepted for publication in ApJ EPrint : astro-ph/9902132=01 Abstract: A star can be tidally disrupted around a massive black hole. It has been known that the debris forms a precessing stream, which may collide with itself. The stream collision is a key process determining the subsequent evolution of the stellar debris: if the orbital energy is efficiently dissipated, the debris will eventually form a circular disk (or torus). In this paper, we have numerically studied such stream collision resulting from the encoun= ter between a 10^6 M_sun black hole and a 1 M_sun normal star with a pericenter radius of 100 R_sun. A simple treatment for radiative cooling has been adopted for both optically thick and thin regions. We have found that approximately 10 to 15% of the initial kinetic energy of the streams is converted into thermal energy during the collision. The spread in angular momentum of the incoming stream is increased by a factor of 2 to 3, and such increase, together with the decrease in kinetic energy, significantly helps the circularization process. Initial luminosity burst due to the collision may reach as high as 10^41 erg sec^-1 in 10^4 sec, after which the luminosity increases again (but slowly this time) to a steady value of a few 10^40 erg sec^-1 in a few times of 10^5 sec. The radiation from the system is expected to be close to Planckian with effective temperature of ~ 10^5 K. ------------------------------------------------------------------------ (Older versions of the Newsflash can be found at the gcnews web-page) ======================================================================== Edited by Angela Cotera Heino Falcke (cotera@ipac.caltech.edu) (hfalcke@mpifr-bonn.mpg.de) - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - For Abstract submission please send the (La)Tex file of your paper to gcnews@mpifr-bonn.mpg.de ========================================================================