Multiwavelength monitoring of the dwarf Seyfert 1 galaxy NGC 4395. I. A reverberation-based measurement of the black hole mass

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 M-BH = (3.6 +/- 1.1) x 10(5) M-circle dot. The observations consist of two visits of five 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 lambda 1549 showed corresponding variations large enough to reliably determine the emission-line lag, which was measured to be of order 1 hr for both visits. The size of the C IV-emitting region is about a factor of 3 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 under-luminous even for its small black hole mass; the Eddington ratio of similar to 1.2 x 10(-3) is lower than that of any other active galactic nucleus for which a black hole mass measurement has been made by emission-line reverberation.