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Issue A&A
Volume 458, Number 2, November I 2006
Page(s) L25 - L28
Section Letters
DOI http://dx.doi.org/10.1051/0004-6361:20066251



A&A 458, L25-L28 (2006)
DOI: 10.1051/0004-6361:20066251

Letter

K-band polarimetry of an Sgr A* flare with a clear sub-flare structure

L. Meyer1, R. Schödel1, A. Eckart1, V. Karas2, M. Dovciak2 and W. J. Duschl3, 4

1  I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
2  Astronomical Institute, Academy of Sciences, Bocní II, 14131 Prague, Czech Republic
3  Institut für Theoretische Physik und Astrophysik, Universität zu Kiel, 24098 Kiel, Germany
4  Steward Observatory, The University of Arizona, 933 N. Cherry Ave., Tucson, AZ 85721, USA

(Received 15 August 2006 / Accepted 6 September 2006 )

Abstract
Context.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.
Aims.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 ~$18\pm 3$ min reported earlier.
Methods.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.
Results.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 $15.5\pm2$ 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 $i > 20\degr$ on a 3$\sigma$ level, and the dimensionless spin parameter of the black hole is derived to be $a_\star > 0.5$.


Key words: black hole physics -- accretion, accretion disks -- Galaxy: center



© ESO 2006

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