Long-term spectroscopic monitoring of low-redshift quasars. 1. 5 year report

We present the first results of a long-term optical spectroscopic monitorin g program of a sample of 14 luminous (L-Bol similar to 10(4.5)-10(4.6) ergs s(-1)) low-redshift (z = 0.14-0.59) quasars. Our spectra have been taken a nnually over a period of 5 yr and for four objects have been supplemented b y digital spectra obtained approximately 20 years earlier. For these four o bjects, we find relatively little change in both the shapes and fluxes of t he broad lines from the first observations to the present. However, in one quasar, B2 1721 + 34, the emission in the core of the H alpha profile has d ecreased while that in the profile wings has increased, suggesting that mat erial has moved from the outer intermediate-line region to the inner very b road line region (VBLR). Our 5 year monitoring campaign reveals variability in the core, but not the wings, of the H beta profile of several objects i n response to variations in the continuum. In a few objects, notably PKS 14 51-37, we observe year-to-year variability in the strength of the He II lam bda 4686 line that also appears to be a response to variability in the ioni zing 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 r elativistic accretion disk, as opposed to an episodic flow to or from the c entral engine. In the latter case, current constraints on the gas crossing time of the VBLR indicate that variability would occur on a timescale of mo nths, which we do not observe. The lack of response of the wings of H beta to continuum variability provides additional evidence that the VBLR is full y ionized in hydrogen.