  A reverberation-mapping program on NGC 4395, the least-luminous known Seyfert
1 galaxy, undertaken with the Space Telescope Imaging Spectrograph on the
Hubble Space Telescope, yields a measurement of the mass of the central black
hole of 360,000 solar masses. The observations consist of two visits of 5
orbits each, in 2004 April and July. During each of these visits, the UV
continuum varied by at least 10% (rms) and only C IV 1549 showed corresponding
variations large enough to reliably determine the emission-line lag, which was
measured to be of order one hour for both visits. The size of the C IV-emitting
region is about a factor of three smaller than expected if the slope of the
broad-line region radius-luminosity relationship is identical to that for the
H-beta emission line. NGC 4395 is underluminous even for its small black hole
mass; the Eddington ratio of 0.0012 is lower than that of any other active
galactic nucleus for which a black hole mass measurement has been made by
emission-line reverberation.
