X-raying molecular clouds with a short flare: probing statistics of gas
  density and velocity fields
  We take advantage of a set of molecular cloud simulations to demonstrate a
possibility to uncover statistical properties of the gas density and velocity
fields using reflected emission of a short (with duration much less than the
cloud's light-crossing time) X-ray flare. Such situation is relevant for the
Central Molecular Zone of our Galaxy where several clouds get illuminated by a
$\sim110$ yr-old flare from the supermassive black hole Sgr A*. Due to
shortness of the flare ($\Delta t\lesssim1.6$ yrs), only a thin slice ($\Delta
z\lesssim0.5$ pc) of the molecular gas contributes to the X-ray reflection
signal at any given moment, and its surface brightness effectively probes the
local gas density. This allows reconstructing the density probability
distribution function over a broad range of scales with virtually no influence
of attenuation, chemo-dynamical biases and projection effects. Such measurement
is key to understanding the structure and star-formation potential of the
clouds evolving under extreme conditions in the CMZ. For cloud parameters
similar to the currently brightest in X-ray reflection molecular complex Sgr A,
the sensitivity level of the best available data is sufficient only for
marginal distinction between solenoidal and compressive forcing of turbulence.
Future-generation X-ray observatories with large effective area and high
spectral resolution will dramatically improve on that by minimising systematic
uncertainties due to contaminating signals. Furthermore, measurement of the
iron fluorescent line centroid with sub-eV accuracy in combination with the
data on molecular line emission will allow direct investigation of the gas
velocity field.
