Variations of the ultraviolet spectrum of 3C 273 : 9 years of IUE data.

Abstract

The quasar 3C 273 has been observed with the IUE satellite in the range 1200-3000 A every two weeks from 29 November 1986 to 18 February 1987, with additional observations in May 1986 and 1 June and 17 July 1987. These recent observations are analyzed together with the archived IUE spectra from 1978 to 1985. The flux at 1275 A undergoes variations by a factor ~2 on time scales of a few months. The fastest variations observed occurred between February and June 1987 when the flux at 1275 A, f_nu_, decreased by a factor ~2 in less than ~3.2 months, at a rate of ~1.7 10^30^ erg s^-1^ Hz^-1^ per month (H_0_ = 50). No significant variation of the emission blend Lyα + N Vλ1242 was detected during this period, nor during other periods of rapid variations of the continuum such as June 1982-May 1984. The near constancy of the intensity of Lyα + N Vλ1242 is taken as evidence that the broad line region and in particular the region emitting the Fe II blends and the Balmer continuum is larger than ~5 light years. The changes in spectral shape of the continuum spectrum which accompany the flux variations are therefore not caused by variations of the Fe II blends + Balmer continuum. We interpret the changes of the UV continuum spectrum in terms of variable contributions of optically thick thermal components.