SIMBAD references

We use a maximum-entropy method to show that the transfer function (TF) of the broad-line region (BLR) in the Seyfert-1 galaxy NGC 3516 is time-varying. The TF decreased by nearly a factor of two over a time scale less than half a year during a monitoring campaign in 1990. We conclude that the observed time dependency is likely to be either due to variations of the spectral index of the ionizing continuum spectrum; due to a change in the number or covering fraction of broad-line clouds; due to nonlinear line response; or due to nonstationary anisotropy of the continuum source on time scales of several weeks to months. An extended continuum source could also explain the observed time-dependency. The symmetry of the time lag for variations in the red and blue wings of Hα indicates that the BLR kinematics is not dominated by organized radial inflow or outflow. If we assume circular orbits, the observed time lag of 14 days at velocity v=4000km/s suggests a mass of the central object of ∼2x10⁷M_☉. The Hα TF peaks away from zero delay, indicating that the Hα BLR is either non-spherical or inwardly emitting.