======================================================================== G C N E W S * Newsflash * - The Newsletter for Galactic Center Research - gcnews@aoc.nrao.edu http://www.aoc.nrao.edu/~gcnews ======================================================================== Vol. 28, No. 5 Apr 10, 2008 Recently submitted papers: -------------------------- 1) Limits on the Position Wander of Sgr A* (Reid et al., ApJ) 2) Very Large Array Observations of Galactic Center OH 1720 MHz Masers in Sagittarius A East and in the Circumnuclear Disk (Sjouwerman & Pihlstroem, ApJ) ------------------------------------------------------------------------ Email : reid@cfa.harvard.edu Title : Limits on the Position Wander of Sgr A* Author(s): Mark J. Reid^1, Avery E. Broderick^1,2, Abraham Loeb^1, Mareki Honma^3, Andreas Brunthaler^4 Institute: (1) Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA, (2) Canadian Institute for Theoretical Astrophysics, 60 St. George Street, Toronto, ON Canada, (3) Mizusawa VERA Observatory, NAOJ, Mitaka, Tokyo 181-8588, Japan, (4) Max-Planck-Institute fuer Radioastronomie, 69 Auf dem Huegel, Bonn-53121, Germany Paper : ApJ, in press Abstract: We present measurements with the VLBA of the variability in the centroid position of Sgr A* relative to a background quasar at 7 mm wavelength. We find an average centroid wander of 71+/- 45 mu as for time scales between 50 and 100 min and 113+/-50 mu as for timescales between 100 and 200 min, with no secular trend. These are sufficient to begin constraining the viability of the accretion hot-spot model for the radio variability of Sgr A*. It is possible to rule out hot spots with orbital radii above 15 G M_Sgr A*/c^2 that contribute more than 30% of the total 7 mm flux. However, closer or less luminous hot spots remain unconstrained. Since the fractional variability of Sgr A* during our observations was 20% on time scales of hours, the hot-spot model for Sgr A*'s radio variability remains consistent with these limits. Improved monitoring of Sgr A*'s centroid position has the potential to place significant constraints upon the existence and morphology of inhomogeneities in a supermassive black hole accretion flow. ------------------------------------------------------------------------ Email : lsjouwer@nrao.edu Title : Very Large Array Observations of Galactic Center OH 1720 MHz Masers in Sagittarius A East and in the Circumnuclear Disk Author(s): Lorant O. Sjouwerman(1) Ylva M. Pihlstroem(2) Institute: (1) National Radio Astronomy Observatory, 1003 Lopezville Rd., Socorro, NM 87801, (2) Department of Physics and Astronomy, University of New Mexico, 800 Yale Boulevard NE, Albuquerque, NM 87131, Paper : ApJ, accepted EPrint : 0804.0445 Abstract: We present Very Large Array (VLA) radio interferometry observations of the 1720 MHz OH masers in the Galactic Center (GC). Most 1720 MHz OH masers arise in regions where the supernova remnant Sgr A East is interacting with the interstellar medium. The majority of the newly found 1720 MHz OH masers are located to the northeast, independently indicating and confirming an area of shock interaction with the +50 km/s molecular cloud (M-0.02-0.07) on the far side of Sgr A East. The previously known bright masers in the southeast are suggested to be the result of the interaction between two supernova remnants, instead of between Sgr A East and the surrounding molecular clouds as generally found elsewhere in the Galaxy. Together with masers north of the circumnuclear disk (CND) they outline an interaction on the near side of Sgr A East. In contrast to the interaction between the +50 km/s cloud and Sgr A East, OH absorption data do not support a direct interaction between the CND material and Sgr A East. We also present three new high-negative velocity masers, supporting a previous single detection. The location and velocities of the high-negative and high-positive velocity masers are consistent with being near the tangent points of, and physically located in the CND. We argue that the high velocity masers in the CND are pumped by dissipation between density clumps in the CND instead of a shock generated by the supernova remnant. That is, the CND masers are not coupled to the supernova remnant and are sustained independently. ------------------------------------------------------------------------ (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 ========================================================================