X-ray Diagnostics of Giant Molecular Clouds in the Galactic Center Region and Past Activity of Sgr A*

TL;DR

This paper introduces a Monte Carlo simulation framework to model X-ray reflection from molecular clouds in the Galactic center, enabling detailed interpretation of observational data and insights into past activity of Sgr A*.

Contribution

The authors develop a novel Monte Carlo simulation method for accurate modeling of X-ray reflection in complex molecular cloud geometries, advancing analysis of Galactic center emissions.

Findings

Simulated X-ray morphologies differ above and below 20 keV, revealing cloud structure.

Spectral analysis constrains cloud mass, composition, and source luminosity.

Predictions are testable with upcoming X-ray missions like NuStar and ASTRO-H.

Abstract

Strong iron fluorescence at 6.4 keV and hard-X-ray emissions from giant molecular clouds in the Galactic center region have been interpreted as reflections of a past outburst of the Sgr A* supermassive black hole. Careful treatment of multiple interactions of photons in a complicated geometry is essential to modeling the reprocessed emissions from the dense clouds. We develop a new calculation framework of X-ray reflection from molecular clouds based on Monte Carlo simulations for accurate interpretation of high-quality observational data. By utilizing this simulation framework, we present the first calculations of morphologies and spectra of the reflected X-ray emission for several realistic models of Sgr B2, which is the most massive molecular cloud in our Galaxy. The morphology of scattered hard X-rays above 20 keV is significantly different from that of iron fluorescence due to their large penetrating power into dense regions of the cloud, probing the structure of the cloud. High-resolution spectra provide quantitative evaluation of the iron line including its Compton shoulder to constrain the mass and the chemical composition of the cloud as well as the luminosity of the illuminating source. These predictions can be checked in the near future with future X-ray missions such as NuStar (hard X-rays) and ASTRO-H (both iron lines and hard X-rays).