Ildar Khabibullin
Reflection of X-ray emission on molecular clouds in the Galactic Center has established itself as a powerful tool for joint exploration of the past Sgr A*'s activity and properties of the dense gas distribution under extreme conditions of the Central Molecular Zone. Thanks to extensive observational efforts and improvements in modelling, parameters of the recent flare(s) and characteristics of the brightest molecular complexes have been both reliably inferred. Upcoming measurements of the polarisation in reflected X-ray continuum will help to separate the signal of interest from the contaminating background and to check the underlying assumptions of the previously used techniques in an independent manner. Probing the intrinsic polarisation properties of the primary emission might become feasible, helping us to reveal its physical origin and connection to the regularly observed Sgr A*’s X-ray and infrared flares of smaller amplitude. On the other hand, high resolution imaging and spectral mapping of the reflected X-ray emission are capable of shedding the light on characteristics of the supersonic gas motions which shape the inner structure of the molecular clouds and are intricately related to the whole cycle of massive star formation. We describe the current status and highlight the future prospects of observing the unique “Galactic Center X-ray Time-Lapse” in its full power, i.e. in 3D, HD, and polarised.
The implications of this paradigm allow us to use statistical properties of the reflected emission as a proxy for statistical properties of the molecular gas distribution. We demonstrate the results of using this for reconstruction of the 3D density field of the molecular gas on the scale of tens of pc and to explore inner properties of individual clouds down to sub-pc scales. Spectral mapping in the fluorescent line of iron opens a possibility to explore line-of-sight velocity field field of the molecular gas, that can bear traces of enhanced shear or tidal distortions
(as exemplified by the velocity field in the Brick cloud). We end up by illustrating prospects of studying polarisation of the reflected emission, which not only provides an independent test for the inferred 3D location of the clouds, but also might reveal intrinsic polarisation of the primary source. Future advances in data and modelling will allow us to start shedding the light on physical processes behind Sgr A* flares and CMZ molecular gas characteristics.