Destruction and multiple ionization of PAHs by X-rays in circumnuclear
  regions of AGNs
  The infrared signatures of polycyclic aromatic hydrocarbons (PAHs) are
observed in a variety of astrophysical objects, including the circumnuclear
medium of active galactic nuclei (AGNs). These are sources of highly energetic
photons (0.2 to 10 keV), exposing the PAHs to a harsh environment. In this
work, we examined experimentally the photoionization and photostability of
naphthalene (C$_{10}$H$_{8}$), anthracene (C$_{14}$H$_{10}$),
2-methyl-anthracene (C$_{14}$H$_{9}$CH$_{3}$) and pyrene (C$_{16}$H$_{10}$)
upon interaction with photons of 275, 310 and 2500 eV. The measurements were
performed at the Brazilian Synchrotron Light Laboratory using time-of-flight
mass-spectrometry (TOF-MS). We determined the absolute photoionization and
photodissociation cross sections as a function of the incident photon energy;
the production rates of singly, doubly and triply charged ions; and the
molecular half-lives in regions surrounding AGNs. Even considering moderate
X-ray optical depth values ($\tau = 4.45$) due to attenuation by the dusty
torus, the half-lives are not long enough to account for PAH detection. Our
results suggest that a more sophisticated interplay between PAHs and dust
grains should be present in order to circumvent molecular destruction. We could
not see any significant difference in the half-life values by increasing the
size of the PAH carbon backbone, N$_C$, from 10 to 16. However, we show that
the multiple photoionization rates are significantly greater than the single
ones, irrespective of the AGN source. We suggest that an enrichment of multiply
charged ions caused by X-rays can occur in AGNs.
