======================================================================== G C N E W S * Newsflash * - The Newsletter for Galactic Center Research - gcnews@aoc.nrao.edu http://www.aoc.nrao.edu/~gcnews ======================================================================== Vol. 25, No. 6 Oct 12, 2006 Recently submitted papers: -------------------------- 1) Near-infrared polarimetry setting constraints on the orbiting spot model for Sgr A* flares (Meyer et al., A&A) 2) K-band polarimetry of an Sgr A* flare with a clear sub-flare structure (Meyer et al., A&AL) 3) Polarimetry of Near-Infrared Flares from Sgr A* (Eckart et al., A&A) ------------------------------------------------------------------------ Email : leo@ph1.uni-koeln.de Title : Near-infrared polarimetry setting constraints on the orbiting spot model for Sgr A* flares Author(s): L. Meyer(1), A. Eckart(1), R. Schoedel(1), W. J. Duschl(2,3), K. Mu\vzi\'c(1) , M. Dov\vciak(4), V. Karas(4) Institute: (1) I.Physikalisches Institut, Universitaet zu Koeln, Zuelpicher Str. 77, 50937 Koeln, Germany (2) Institut fuer Theoretische Physik und Astrophysik, Universitaet Kiel, 24098 Kiel, Germany (3) Steward Observatory, The University of Arizona, 933 N. Cherry Ave. Tucson, AZ 85721, USA (4) Astronomical Institute, Academy of Sciences, Bo\vcni II, CZ-14131 Prague, Czech Republic Paper : A&A, 2006, in press EPrint : astro-ph/0610104 Abstract: Recent near-infrared polarization measurements of Sgr A* show that its emission is significantly polarized during flares and consists of a non- or weakly polarized main flare with highly polarized sub-flares. The flare activity suggests a quasi-periodicity of 20 minutes in agreement with previous observations. By simultaneous fitting of the lightcurve fluctuations and the time-variable polarization angle, we address the question of whether these changes are consistent with a simple hot spot/ring model, in which the interplay of relativistic effects plays the major role, or whether some more complex dependency of the intrinsic emissivity is required. We discuss the significance of the 20 min peak in the periodogram of a flare from 2003. We consider all general relativistic effects that imprint on the polarization degree and angle and fit the recent polarimetric data, assuming that the synchrotron mechanism is responsible for the intrinsic polarization and considering two different magnetic field configurations. Within the quality of the available data, we think that the model of a single spot in addition to an underlying ring is favoured. In this model the broad near-infrared flares of Sgr A* are due to a sound wave that travels around the MBH once while the sub-flares, superimposed on the broad flare, are due to transiently heated and accelerated electrons which can be modeled as a plasma blob. Within this model it turns out that a strong statement about the spin parameter is difficult to achieve, while the inclination can be constrained to values > 35 DEG on a 3 sigma level. ------------------------------------------------------------------------ Email : leo@ph1.uni-koeln.de Title : K-band polarimetry of an Sgr A* flare with a clear sub-flare structure Author(s): L. Meyer(1), R. Schoedel(1), A. Eckart(1), V. Karas(2), M. Dov\vciak(2), W. J. Duschl(3,4) Institute: (1) I.Physikalisches Institut, Universitaet zu Koeln, Zuelpicher Str. 77, 50937 Koeln, Germany (2) Astronomical Institute, Academy of Sciences, Bo\vcni II, CZ-14131 Prague, Czech Republic (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 Paper : A&A Letters, 2006, in press EPrint : astro-ph/0610147 Abstract: The supermassive black hole at the Galactic center, Sgr A*, shows frequent radiation outbursts, often called 'flares'. In the near-infrared some of these flares were reported as showing intrinsic quasi-periodicities. The flux peaks associated with the quasi-periodic behavior were found to be highly polarized. The aim of this work is to present new evidence to support previous findings of the properties of the polarized radiation from Sgr A* and to again provide strong support for the quasi-periodicity of 18+/- 3 min reported earlier. Observations were carried out at the European Southern Observatory's Very Large Telescope on Paranal, Chile. We used the NAOS/CONICA adaptive optics/near-infrared camera instrument. By fitting the polarimetric lightcurves with a hot-spot model, we addressed the question of whether the data are consistent with this model. To fit the observed data we used a general relativistic ray-tracing code in combination with a simple hot-spot/ring model. We report on new polarization measurements of a K-band flare from the supermassive black hole at the Galactic center. The data provide very strong support for a quasi-periodicity of 15.5+/-2 min. The mean polarization of the flare is consistent with the direction of the electric field vector that was reported in previous observations. The data can be modeled successfully with a combined blob/ring model. The inclination i of the blob orbit must be i > 20 DEG on a 3 sigma level, and the dimensionless spin parameter of the black hole is derived to be a_\star > 0.5. ------------------------------------------------------------------------ Email : leo@ph1.uni-koeln.de Title : Polarimetry of Near-Infrared Flares from Sgr A* Author(s): A. Eckart(1), R. Schoedel(1), L. Meyer(1), S. Trippe(2), T. Ott(2), and R. Genzel(2,3) Institute: (1) I.Physikalisches Institut, Universitaet zu Koeln, Zuelpicher Str.77, 50937 Koeln, Germany, (2) Max Planck Institut fuer extraterrestrische Physik, Giessenbachstra\sse, 85748 Garching, Germany (3) Physics Department, University of California at Berkeley, Le Conte Hall, Berkeley, CA 94720, USA Paper : A&A, 2006, 455,1 EPrint : astro-ph/0610103 Abstract: We report new polarization measurements of the variable near-infrared emission of the SgrA* counterpart associated with the massive 3-4*10^6M_o Black Hole at the Galactic Center. We 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. We find that the variable NIR emission of SgrA* is highly polarized and consists of a contribution of a non- or weakly polarized main flare with highly polarized sub-flares. The flare activity shows a possible quasi-periodicity of 20+/-3 minutes consistent with previous observations. The highly variable and polarized emission supports that the NIR emission is non-thermal. The observations can be interpreted in a jet or temporary disk model. In the disk model the quasi-periodic flux density variations can be explained due to spots on relativistic orbits around the central MBH. Alternative explanations for the high central mass concentration involving boson or fermion balls are increasingly unlikely. ------------------------------------------------------------------------ (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 ========================================================================