Non-Equilibrium Chemistry and Destruction of CO by X-ray Flares
  Sources of X-rays such as active galactic nuclei and X-ray binaries are often
variable by orders of magnitude in luminosity over timescales of years. During
and after these flares the surrounding gas is out of chemical and thermal
equilibrium. We introduce a new implementation of X-ray radiative transfer
coupled to a time-dependent chemical network for use in 3D
magnetohydrodynamical simulations. A static fractal molecular cloud is
irradiated with X-rays of different intensity, and the chemical and thermal
evolution of the cloud are studied. For a simulated $10^5$ M$_\odot$ fractal
cloud an X-ray flux $<0.01$ erg cm$^{-2}$ s$^{-1}$ allows the cloud to remain
molecular, whereas most of the CO and H$_2$ are destroyed for a flux of $>1$
erg cm$^{-2}$ s$^{-1}$. The effects of an X-ray flare, which suddenly increases
the X-ray flux by $10^5 \times$ are then studied. A cloud exposed to a bright
flare has 99% of its CO destroyed in 10-20 years, whereas it takes $>10^3$
years for 99% of the H$_2$ to be destroyed. CO is primarily destroyed by
locally generated far-UV emission from collisions between non-thermal electrons
and H$_2$; He$^+$ only becomes an important destruction agent when the CO
abundance is already very small. After the flare is over, CO re-forms and
approaches its equilibrium abundance after $10^3-10^5$ years. This implies that
molecular clouds close to Sgr A$^*$ in the Galactic Centre may still be out of
chemical equilibrium, and we predict the existence of clouds near flaring X-ray
sources in which CO has been mostly destroyed but H is fully molecular.
