# Can Sgr A* flares reveal the molecular gas density PDF?

## TL;DR

X-ray observations of Sgr A* flares illuminate the molecular gas in the Galactic Center, enabling the study of past black hole activity and the molecular cloud density distribution, especially the PDF of gas density.

## Contribution

The paper demonstrates how X-ray flare reflections can be used to probe the molecular gas density PDF and past activity of Sgr A*, offering new insights beyond traditional methods.

## Key findings

- Existing data estimate flare age and luminosity.
- Void and wall structures in molecular clouds are observed.
- The gas density PDF can be measured using X-ray reflections.

## Abstract

Illumination of dense gas in the Central Molecular Zone (CMZ) by powerful X-ray flares from Sgr A* leads to prominent structures in the reflected emission that can be observed long after the end of the flare. By studying this emission we learn about past activity of the supermassive black hole in our Galactic Center and, at the same time, we obtain unique information on the structure of molecular clouds that is essentially impossible to get by other means. Here we discuss how X-ray data can improve our knowledge of both sides of the problem. Existing data already provide: i) an estimate of the flare age, ii) a model-independent lower limit on the luminosity of Sgr A* during the flare and iii) an estimate of the total emitted energy during Sgr A* flare. On the molecular clouds side, the data clearly show a voids-and-walls structure of the clouds and can provide an almost unbiased probe of the mass/density distribution of the molecular gas with the hydrogen column densities lower than few $10^{23}\;{\rm cm^{-2}}$. For instance, the probability distribution function of the gas density $PDF(\rho)$ can be measured this way. Future high energy resolution X-ray missions will provide the information on the gas velocities, allowing, for example a reconstruction of the velocity field structure functions and cross-matching the X-ray and molecular data based on positions and velocities.