Water megamaser emission in hard X-ray selected AGN
  Water megamaser emission at 22 GHz has proven to be a powerful tool for
astrophysical studies of AGN allowing an accurate determination of the central
black hole mass and of the accretion disc geometry and dynamics. However, after
searches among thousands of galaxies, only ~ 200 of them have shown such
spectroscopic features, most of them of uncertain classification. In addition,
the physical and geometrical conditions under which maser activates are still
unknown. In this work we aim at characterizing the occurrence of water maser
emission in an unbiased sample of AGN, investigating the relation with the
X-ray properties and the possible favorable geometry needed to detect water
maser. We have searched for 22 GHz maser emission in a hard X-ray selected
sample of AGN, taken from the INTEGRAL/IBIS survey above 20 keV. Of the 380
sources in the sample, only half have water maser data. We have also considered
a sub-sample of 87 sources, volume limited, for which we obtained new Green
Bank Telescope and Effelsberg observations (for 35 sources), detecting one new
maser and increasing its radio coverage to 75%. The detection rate of water
maser emission in the total sample is 15+/-3%, this fraction raises up to
19+/-5% for the complete sub-sample, especially if considering type 2 and
Compton thick AGN. These results demonstrate that the hard X-ray selection may
significantly enhance the maser detection efficiency over comparably large
optical/infrared surveys. A possible decline of the detection fraction with
increasing luminosity might suggest that an extreme luminous nuclear
environment does not favour maser emission. The large fraction of CT AGN with
water maser emission could be explained in terms of geometrical effects, being
the maser medium the very edge-on portion of the obscuring medium.
