Probing the Galactic center with X-ray polarimetry

TL;DR

This paper discusses how future X-ray polarimetry can reveal the environment and past activity of the supermassive black hole at the Galactic center by analyzing polarized X-ray emissions from surrounding gas clouds.

Contribution

It summarizes polarimetric simulation results demonstrating how imaging X-ray polarimeters can enhance understanding of the Galactic center's high-energy phenomena.

Findings

Polarized X-ray emission can trace the geometry of the GC environment.

X-ray polarimetry can reveal the SMBH's past activity.

Simulations show potential improvements with future polarimeters.

Abstract

The Galactic center (GC) holds the closest-to-Earth supermassive black hole (SMBH), which makes it the best laboratory to study the close environment of extremely massive compact objects. Polarimetry is sensitive to geometry of the source, which makes it a particularly suitable technique to probe the medium surrounding the GC SMBH. The detection of hard X-ray spectra and prominent iron K$\alpha$ fluorescence features coincident with localized gas clouds (e.g. Sgr~B2, Sgr~C) is known for nearly twenty years now and is commonly associated with a past outburst of the SMBH whose radiation is reprocessing onto the so-called "reflection nebulae". Since scattering leads to polarization, the re-emitted signal from the giant molecular clouds in the first 100~pc of the GC is expected to be polarized. X-ray polarization measurement is thus particularly adapted to probe the origin of the diffuse X-ray emission from the GC reflection nebulae and reveal the past activity of the central SMBH. In this research note, we summarize the results from past and current polarimetric simulations in order to show how a future X-ray polarimeter equipped with imaging detectors could improve our understanding of high-energy astrophysics.