Toward measuring the spin of obscured supermassive black holes: a
  critical assessment with disk megamasers
  Mass and spin are two fundamental properties of astrophysical black holes.
While some established, indirect methods are adopted to measure both these
properties of active galactic nuclei (AGN) when viewed relatively face-on, very
few suggested methods exist to measure these properties when AGN are viewed
highly inclined and potentially obscured by large amounts of gas. In this
context, we explore the accuracy and performance of a recently proposed method
to estimate the spin of AGN through fitting their accretion disk spectral
energy distribution, when adapted for highly inclined and obscured systems, and
in particular to a sample of six, local water megamasers. For these sources,
both the accretion rate and inclination angle are known, allowing us to rely
only on the AGN bolometric luminosity to infer their spin. Using the bolometric
luminosity as a proxy for the accretion disk peak luminosity, we derive the
expected bolometric luminosity as a function of spin. Then, we measure the
bolometric luminosity of each source through X-ray spectroscopy, and compare it
with the expected value to constrain the spin of the AGN. The quality of the
constraints depend critically on the accuracy of the measured bolometric
luminosity, which is difficult to estimate in heavily obscured systems. Three
out of six sources do not show consistency between the expected and measured
bolometric luminosities, while other three (four, when considering the [OIII]
line as tracer of the bolometric luminosity) are formally consistent with high
spin values. Our results suggest that this method, although promising (and
possibly considered as a future calibrator for other methods) needs better
observational data and further theoretical modeling to be successfully applied
to obscured AGN and to infer robust results.
