Near-infrared polarimetry setting constraints on the orbiting spot model for Sgr A* flares

¹ I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany e-mail: leo@ph1.uni-koeln.de
² Institut für Theoretische Physik und Astrophysik, Universität 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, Boční II, 14131 Prague, Czech Republic

Received: 28 June 2006
Accepted: 21 August 2006

Abstract

Context.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 min in agreement with previous observations.

Aims.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.

Methods.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.

Results. 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 on a  level.