On the Origin of the 6.4 keV Line in the Galactic Center Region
  We analyse the 6.4 keV iron line component produced in the Galactic Center
(GC) region by cosmic rays in dense molecular clouds (MCs) and in the diffuse
molecular gas. We showed that this component, in principle, can be seen in
several years in the direction of the cloud Srg B2. If this emission is
produced by low energy CRs which ionize the interstellar molecular gas the
intensity of the line is quite small, < 1%. However, we cannot exclude that
local sources of CRs or X-ray photons nearby the cloud may provide much higher
intensity of the line from there. Production of the line emission from
molecular clouds depends strongly on processes of CR penetration into them. We
show that turbulent motions of neutral gas may generate strong magnetic
fluctuations in the clouds which prevent free penetration of CRs into the
clouds from outside. We provide a special analysis of the line production by
high energy electrons. We concluded that these electrons hardly provide the
diffuse 6.4 keV line emission from the GC because their density is depleted by
ionization losses. We do not exclude that local sources of electrons may
provide an excesses of the 6.4 keV line emission in some molecular clouds and
even reproduce a relatively short time variations of the iron line emission.
However, we doubt whether a single electron source provides the simultaneous
short time variability of the iron line emission from clouds which are distant
from each other on hundred pc as observed for the GC clouds. An alternative
speculation is that local electron sources could also provide the necessary
effect of the line variations in different clouds that are seen simultaneously
by chance that seems, however, very unlikely.
