Polarimetric Imaging of Sgr A* in its Flaring State

TL;DR

This paper uses relativistically correct polarimetric imaging to analyze Sgr A*'s flaring activity, aiming to understand the emission mechanisms and geometry of the infrared and X-ray flares.

Contribution

It provides detailed predictions of polarization properties of Sgr A*'s flares, aiding future X-ray polarimetry observations to distinguish emission models.

Findings

Polarization fraction and angle are similar for synchrotron-cooled X-ray and NIR emissions.

X-ray polarization differs when arising from scattering medium versus synchrotron processes.

X-ray polarimetry can significantly advance understanding of black hole near-horizon physics.

Abstract

The Galaxy's supermassive black hole, Sgr A*, produces an outburst of infrared radiation about once every 6 hours, sometimes accompanied by an even more energetic flurry of X-rays. The NIR photons are produced by nonthermal synchrotron processes, but we still don't completely understand where or why these flares originate, nor exactly how the X-rays are emitted. The power-law electrons radiating the infrared light may be partially cooled, so the distribution may be a broken power law with a ("cooling break") transition frequency. In addition, the emission region appears to be rather compact, possibly restricted to the inner edge of the accretion disk. In that case, the X-ray outburst may itself be due to synchrotron processes by the most energetic particles in this population. In this paper, we examine several key features of this proposal, producing relativistically correct polarimetric images of Sgr A*'s NIR and X-ray flare emission, in order to determine (1) whether the measured NIR polarization fraction is consistent with this geometry, and (2) whether the predicted X-ray to NIR peak fluxes are confirmed by the currently available multi-wavelength observations. We also calculate the X-ray polarization fraction and position angle (relative to that of the NIR photons) in anticipation of such measurements in the coming years. We show that whereas the polarization fraction and position angle of the X-rays are similar to those of the NIR component for synchrotron-cooled emission, these quantities are measurably different when the X-rays emerge from a scattering medium. It is clear, therefore, that the development of X-ray polarimetry will represent a major new tool for studying the spacetime near supermassive black holes.