Constraints on a soft X-ray excess in the quasar 3C 279

We present the results of a three-week daily monitoring campaign of the quasar 3C 279 by the X-ray satellites RXTE and ROSAT. A cross-correlation provides no evidence for any time lag between the very similar soft and hard X-ray light curves, and the source shows no significant spectral, variability over the observing period. There is no evidence to support the presence of a soft excess, with a 99 per cent upper limit on any such component of 25 per cent of the total observed luminosity in the 0.1-2 keV band (<3 x 10(38) W). This fraction (but not the luminosity) is significantly lower than that of the soft excess observed in 3C 273.