Polarized light from Sagittarius A* in the near-infrared K_s-band^⋆,⋆⋆

¹ I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
e-mail: shahzaman@ph1.uni-koeln.de
² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
³ Department of Physics and Astronomy, University of California, Los Angeles, CA 90095, USA
⁴ Astronomical Institute, Academy of Sciences, Boční II 1401, 14131 Prague, Czech Republic
⁵ Institut de Recherche en Astrophysique et Planétologie (IRAP) Université de Toulouse, CNRS Observatoire Midi-Pyrénées (OMP), 14 avenue Édouard Belin, 31400 Toulouse, France

Received: 30 October 2014
Accepted: 7 January 2015

Abstract

We present a statistical analysis of polarized near-infrared light from Sgr A*, the radio source associated with the supermassive black hole at the center of the Milky Way. The observations were carried out using the adaptive optics instrument NACO at the VLT UT4 in the infrared K_s-band from 2004 to 2012. Several polarized flux excursions were observed during these years. Linear polarization at 2.2 μm, its statistics, and time variation, can be used constrain the physical conditions of the accretion process onto this supermassive black hole. With an exponent of about 4 for the number density histogram of fluxes above 5 mJy, the distribution of polarized flux density is closely linked to the single state power-law distribution of the total K_s-band flux densities reported earlier. We find typical polarization degrees on the order of 20% ± 10% and a preferred polarization angle of 13° ± 15°. Simulations show the uncertainties under a total flux density of ~2 mJy are probably dominated by observational effects. At higher flux densities there are intrinsic variations of polarization degree and angle within well constrained ranges. Since the emission is most likely due to optically thin synchrotron radiation, the preferred polarization angle we find is very likely coupled to the intrinsic orientation of the Sgr A* system, i.e. a disk or jet/wind scenario associated with the supermassive black hole. If they are indeed linked to structural features of the source the data imply a rather stable geometry and accretion process for the Sgr A* system.