Ultraviolet continuum variability of the quasar 3C273

Abstract

Several quasars show an excess of radiation in the ultraviolet over the power law defined by their infrared/optical continuum. Several authors^1-4 have fitted the infrared to ultraviolet spectrum of these quasars with a combination of a power law component, line emission, blends of Fe II lines, Balmer continuum and a black-body component which dominates the ultraviolet spectrum. We report here the first observations of variability in the ultraviolet spectrum of the quasar 3C273 (redshift, z = 0.158) as observed by the International Ultraviolet Explorer (IUE)⁵ in the period 1978-84. The flux at λ_observed = 1,675 Å increased by 1.25 between April and June 1982, then decreased by a factor 2 between June 1982 and April 1983. The amplitude of these variations and the constancy of the intensity of the Lyα emission line during the same period are indications that the ultraviolet variations are caused by variations of the black body and/or non-thermal components and not by variations of the continuum or blend of lines originating from the broad-line region. The flux variations are accompanied by variations of the spectral shape. We present an interpretation of the observed variability in terms of a discontinuous and variable distribution of the temperature on the photosphere emitting the ultraviolet continuum.