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Optical spectra and images taken with the Baade 6.5 meter Magellan telescopeconfirm that 2XMM J123103.2+110648, a highly variable X-ray source with anunusually 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 orHbeta emission in an otherwise X-ray unabsorbed source suggests that itintrinsically lacks a broad-line region. If, as in other active galaxies, theionized gas and stars in J1231+1106 are in approximate virial equilibrium, andthe black hole mass versus stellar velocity dispersion relation holds, theexceptionally small velocity dispersion of 33.5 km/s for [O III] 5007 impliesthat the black hole mass is approximately 10^5 solar masses, among the lowestever detected. Such a low black hole mass is consistent with the generalcharacteristics of the host, a small, low-luminosity, low-mass disk galaxy. Weestimate the Eddington ratio of the black hole to be > 0.5, in good agreementwith expectations based on the X-ray properties of the source.

The Low-mass, Highly Accreting Black Hole Associated with the Active Galactic Nucleus 2XMM J123103.2+110648