Probing the clumping structure of Giant Molecular Clouds through the
  spectrum, polarisation and morphology of X-ray Reflection Nebulae
  We suggest a method for probing global properties of clump populations in
Giant Molecular Clouds (GMCs) in the case where these act as X-ray reflection
nebulae (XRNe), based on the study of the clumping's overall effect on the
reflected X-ray signal, in particular on the Fe K-alpha line's shoulder. We
consider the particular case of Sgr B2, one of the brightest and most massive
XRN in our Galaxy. We parametrise the gas distribution inside the cloud using a
simple clumping model, with the slope of the clump mass function (alpha), the
minimum clump mass (m_{min}), the fraction of the cloud's mass contained in
clumps (f_{DGMF}), and the mass-size relation of individual clumps as free
parameters, and investigate how these affect the reflected X-ray spectrum. In
the case of very dense clumps, similar to those presently observed in Sgr B2,
these occupy a small volume of the cloud and present a small projected area to
the incoming X-ray radiation. We find that these contribute negligibly to the
scattered X-rays. Clump populations with volume filling factors of > 10^{-3},
do leave observational signatures, that are sensitive to the clump model
parameters, in the reflected spectrum and polarisation. Future high-resolution
X-ray observations could therefore complement the traditional optical and radio
observations of these GMCs, and prove to be a powerful probe in the study of
their internal structure. Finally, clumps in GMCs should be visible both as
bright spots and regions of heavy absorption in high resolution X-ray
observations. We therefore further study the time-evolution of the X-ray
morphology, under illumination by a transient source, as a probe of the 3d
distribution and column density of individual clumps by future X-ray
observatories.
