THE TIME VARIABILITY OF THE ULTRAVIOLET CONTINUUM AND LYMAN-ALPHA IN 3C-273

This article presents the light curve of the ultraviolet (UV) continuu m flux and of the Lyalpha emission line of 3C 273 between 1978 and 199 2.5. It also presents evidence for a quantitative difference between n earby Seyfert 1 and the quasar 3C 273 regarding the response of the em ission lines to UV continuum flux variations. In contrast to what is o bserved in nearby Seyfert galaxies, the Lyalpha line in 3C 273 does no t significantly respond to flux variations of the continuum by a facto r 2. The largest excursion (minimum or maximum) of the Lyalpha intensi ty is no more than 8% from the local mean, while the 1 sigma error on the Lyalpha intensity is 5%. The near-constancy implies a different di stribution (or different physical conditions) of the Lyalpha emitting gas in 3C 273 on the one hand and in nearby Seyfert 1 galaxies on the other. It points to the near-absence in 3C 273 of Lyalpha emitting gas located at less than c DELTAt from the continuum source, DELTAt being the characteristic time scale of the continuum variations (in this ca se, approximately 2 years). The properties of Lyalpha in 3C 273 are co mpared to those in other intrinsically bright quasars.