The Low-mass, Highly Accreting Black Hole Associated with the Active
  Galactic Nucleus 2XMM J123103.2+110648
  Optical spectra and images taken with the Baade 6.5 meter Magellan telescope
confirm that 2XMM J123103.2+110648, a highly variable X-ray source with an
unusually soft spectrum, is indeed associated with a type 2 (narrow-line)
active nucleus at a redshift of z = 0.11871. The absence of broad Halpha or
Hbeta emission in an otherwise X-ray unabsorbed source suggests that it
intrinsically lacks a broad-line region. If, as in other active galaxies, the
ionized gas and stars in J1231+1106 are in approximate virial equilibrium, and
the black hole mass versus stellar velocity dispersion relation holds, the
exceptionally small velocity dispersion of 33.5 km/s for [O III] 5007 implies
that the black hole mass is approximately 10^5 solar masses, among the lowest
ever detected. Such a low black hole mass is consistent with the general
characteristics of the host, a small, low-luminosity, low-mass disk galaxy. We
estimate the Eddington ratio of the black hole to be > 0.5, in good agreement
with expectations based on the X-ray properties of the source.
