3D MC II: X ray echoes reveal a clumpy molecular cloud in the CMZ

Abstract

X-ray observations collected over the last decades have revealed a strongly variable X-ray signal within the Milky Way's Galactic center, interpreted as X-ray echoes from its supermassive black hole, Sgr A*. These echoes are traced by the strong Fe K$\alpha$ fluorescent line at 6.4 keV of which its intensity is proportional to the density of the illuminated molecular gas. Over time, the echo scans through molecular clouds (MCs) in our Galactic center, revealing their 3D structure and highlighting their densest parts. While previous studies have utilized spectral line doppler shifts along with kinematic models to constrain the geometry of the CMZ or to study the structure of individual clouds, these methods have limitations, particularly in the turbulent region of the CMZ. We use archival Chandra X-ray data to construct one of the first 3D representations of one prominent MC, the Stone Cloud, located at (l = 0.068{\deg}, b = -0.076{\deg}) at a distance of ~20pc from Sgr A* in projection. Using the Chandra X-ray Observatory, we followed the X-ray echo in this cloud from 2008 to 2017. We combine this data with 1.3 mm dust continuum emission observed with the Submillimeter Array (SMA) and the Herschel Space Observatory to re-construct the 3D structure of the cloud and estimate the column densities for each year's observed slice. The analysis of the X-ray echoes along with velocities from SMA molecular line data indicate that the structure of the Stone cloud can be described as a very diffuse background with multiple dense clumps throughout.