2000ApJ...532..136C

We present the first results of a long-term optical spectroscopic monitoring program of a sample of 14 luminous (L_Bol∼10⁴⁵-10⁴⁶ ergs.s^–1) low-redshift (z=0.14-0.59) quasars. Our spectra have been taken annually over a period of 5 yr and for four objects have been supplemented by digital spectra obtained approximately 20 years earlier. For these four objects, we find relatively little change in both the shapes and fluxes of the broad lines from the first observations to the present. However, in one quasar, B2 1721+34, the emission in the core of the Hα profile has decreased while that in the profile wings has increased, suggesting that material has moved from the outer intermediate-line region to the inner very broad line region (VBLR). Our 5 year monitoring campaign reveals variability in the core, but not the wings, of the Hβ profile of several objects in response to variations in the continuum. In a few objects, notably PKS 1451-37, we observe year-to-year variability in the strength of the He II λ4686 line that also appears to be a response to variability in the ionizing continuum. The lack of significant long-term variability in the wings of the Balmer lines supports models in which the VBLR takes the form of a relativistic accretion disk, as opposed to an episodic flow to or from the central engine. In the latter case, current constraints on the gas crossing time of the VBLR indicate that variability would occur on a timescale of months, which we do not observe. The lack of response of the wings of Hβ to continuum variability provides additional evidence that the VBLR is fully ionized in hydrogen.: