CONSTRAINING THE SOFT-X-RAY SPECTRUM OF 3C-273 WITH ROSAT

We present below a study of the soft X-ray emission (0.1-2.4 keV) of the quasar 3C 273 with the Position Sensitive Proportional Counter (PSPC) instrument on the ROSAT satellite. The overall count rate shows variations of approximately 20 per cent in approximately 2 d, a considerably higher degree of short-term X-ray variability than had been expected from previous studies. The count rates, when split into hard (1.5-2.4 keV) and soft (0.1-0.3 keV) energy bands, are not correlated, showing that two physically distinct emission components are present. The spectra are modelled best by a combination of two power laws with absorption at the galactic value. The photon number spectral index, GAMMA(hard), of the harder power law is fixed at 1.5, the value determined by observations in the 2-10 keV band. The spectral index of the softer power law, GAMMA(soft), is then determined to be approximately 2.7. There are small but significant variations in GAMMA(soft) which are not correlated with the soft count rate. We discuss the nature of this variable soft component in the light of present theories.