The Variability of Polarized Radiation from Sgr A*

TL;DR

This study investigates the polarization variability of Sgr A* at radio frequencies, revealing polarization levels, rotation measure, and potential causes related to flare events and expanding synchrotron blobs.

Contribution

First detailed polarization measurements of Sgr A* at multiple radio frequencies, analyzing variability and rotation measure changes during flares.

Findings

Detected polarized emission at 22 and 43 GHz with specific polarization levels.

Measured rotation measure of -2.5 x10^3 rad m^-2 between 22 and 43 GHz.

Observed polarization variability correlated with total intensity fluctuations.

Abstract

Sgr A* is variable at radio and submillimeter wavelengths on hourly time scales showing time delays between the peaks of flare emission as well as linearly polarized emission at millimeter and sub-mm wavelengths. To determine the polarization characteristics of this variable source at radio frequencies, we present VLA observations of Sgr A* and report the detection of polarized emission at a level of 0.77\pm0.01% and 0.2\pm0.01% at 43 and 22 GHz, respectively. The change in the time averaged polarization angle between 22 and 43 GHz corresponds to a RM of -2.5\pm0.6 x10^3 rad m{-2} with no phase wrapping (or \sim 5x10^4 rad m^2 with 2\pi phase wrap). We also note a rise and fall time scale of 1.5 -- 2 hours in the total polarized intensity. The light curves of the degree of linearly polarized emission suggests a a correlation with the variability of the total intensity at 43 GHz. The available polarization data at radio and sub-mm wavelengths suggest that the rotation measure decreases with decreasing frequency. This frequency dependence, and observed changes in polarization angle during flare events, may be caused by the reduction in rotation measure associated with the expansion of synchrotron-emitting blobs.