An estiumate of the central black hole mass in NGC 6814.

Ginga observations have added to our knowledge of the dramatic X-ray variability of the Seyfert galaxy NGC 6814. The X-ray continuum flux has been observed to decrease by a factor of 2 in 50 s. The intensity of the iron Kα line at ~6.5 keV and the X-ray continuum have been found to vary virtually simultaneously, with an upper limit to the delay of less than 250 s. Based on an idealized model consisting of a relativistic accretion disc illuminated by an extended central source of hard radiation, we calculate the stationary characteristics of the line profile, as well as the line intensity variations in response to variability of the central source. A comparison with the observed properties of the iron Kα line allows us to constrain the disc geometry and the mass of the central object. Iron Kα lines are likely to originate from highly ionized iron ions; for this case we estimate an inner radius of the line-emitting region of between 6 and 30 gravitational radii (GM/c^2^), a disc inclination i <~ 40^deg^ and a central black hole mass 8 x 10^4^L_43_ <~ M <~ 3.9 x 10^6^ M_sun_, where L_43_ is the accretion luminosity in units of 10^43^ erg s^-1^. Alternative accretion disc scenarios are also discussed. The energy conversion efficiency implied by the fastest X-ray variations, n ~ 0.1 is suggestive of a storage mechanism of gravitationally released energy or of the presence of a rotating black hole.