======================================================================== 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. 12 Dec 21, 1998 Recently submitted papers: -------------------------- 1) Relativistic Effects in Proper Motions of Stars Surrounding the Galactic Center (Jaroszynski,Acta Astronomica) 2) Sgr A* ``Visual Binaries'': A Direct Measurement of the Galactocentric Distance (Salim & Gould, ApJ) 3) OBSERVATIONS OF THE GALACTIC CENTER REGION WITH BEPPO-SAX (Sidoli et al., 3rd Integral Proc.) ------------------------------------------------------------------------ Email : mj@mimas.astrouw.edu.pl Title : Relativistic Effects in Proper Motions of Stars Surrounding the Galactic Center Author(s): M. Jaroszynski Institute: Warsaw University Observatory, Al. Ujazdowskie 4, 00-478 Warszawa, Poland Paper : Acta Astronomica, Vol 48, No 4, in press EPrint : astro-ph/9812314 Abstract: We simulate the astrometric observations of stars moving close to the black hole in the Galactic Center. We show, that for orbits <= 10^3 AU and position measurements with the accuracy of the Keck Interferometer, the periastron motion of elliptical orbits will be measurable. The models of star trajectories neglecting the periastron motion will be easy to reject with the high confidence level. The measurement of orbital elements and the periastron motion can be effectively used as an independent estimate the distance to the Galactic Center. The effects of orbit precession may be visible in some cases. The effects of gravitational radiation are completely negligible as well as the influence of the black hole rotation on the propagation of light. ------------------------------------------------------------------------ Email : samir@astronomy.ohio-state.edu Title : Sgr A* ``Visual Binaries'': A Direct Measurement of the Galactocentric Distance Author(s): Samir Salim, Andrew Gould Institute: Ohio State University, Department of Astronomy, 174 West 18th Ave., Columbus, OH 43210 Paper : Submitted to ApJ Abstract: We present a new geometrical method for measuring the distance to the Galactic center (r_0) by solving for the Keplerian orbit of individual stars bound to the black hole associated with the Sgr A* from radial velocity and proper motion measurements. We identify three stars to which the method may be applied, and show that 1-5% accuracy of r_0 can be expected after 15 years of observing, and 0.5-2% after 30 years of observing, depending on what the orbital parameters of these three stars turn out to be. Combining the measurements of the three stars with favorable orbital parameters leads to even more precise values. In the example that we present, such combined solution yields 4% accuracy already by the year 2002. All these estimates assume that annual position measurements will continue to be made with the sigma _\rho = 2 mas precision recently reported by Ghez et al. The precision of the distance measurement is relatively insensitive to the radial velocity errors, provided that the latter are less than 50 km/sec . Besides potentially giving an estimate of r_0 that is better than any currently in use, the greatest advantage of this method is that it is free from systematic errors. ------------------------------------------------------------------------ Email : sidoli@ifctr.mi.cnr.it Title : OBSERVATIONS OF THE GALACTIC CENTER REGION WITH BEPPO-SAX Author(s): L. Sidoli^1,2, S. Mereghetti^2, L. Chiappetti^2, J. Heise^3, G.L. Israel^4, E. Kuulkers^3, M. Orlandini^5, P. Predehl^6, A. Tiengo^1,2 & A. Treves^7 Institute: 1) Dipartimento di Fisica, Universit\`a di Milano, Milano, Italy 2) Istituto di Fisica Cosmica ``G. Occhialini" , Milano, Italy 3) SRON-Utrecht, Utrecht, The Netherlands 4) Osservatorio Astronomico di Roma, Monteporzio Catone, Roma, Italy 5) TeSRE, Bologna, Italy 6) MPE, Garching, Germany 7) Dipartimento di Fisica, Universit\`a di Milano, sede di Como, Italy Paper : to appear in Proc. 3rd INTEGRAL Workshop, Taormina 98 Weblink : http://xxx.lanl.gov/abs/astro-ph/9812349 Abstract: We are performing a survey of the Galactic Center region (Sidoli et al., 1998a) with the BeppoSAX satellite. Several known point sources are visible (including one at the position of Sgr A*), as well as newly discovered sources and diffuse emission. Here we report the preliminary results of the on-going analysis of both the point sources and the diffuse X-ray emission. ------------------------------------------------------------------------ (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 ========================================================================