INTRODUCTION

A soft X-ray excess below 2 keV is a common feature in the X-ray spectra of AGN (Turner & Pounds 1989; Masnou et al. 1992). A popular interpretation is that it represents the high energy tail of the big blue bump for which the models include accretion disks and reprocessing in cold matter. ROSAT provides us with the opportunity to study the parameters of the soft X-ray excess for the first time.

In this paper, we discuss new ROSAT PSPC spectra (covering 0.1-2.4 keV) of four AGN taken from the USS survey which selected the softest sources in the Einstein IPC database (Córdova et al. 1992; Puchnarewicz et al. 1992a, hereafter C92 and P92a): these AGN are E1346+266, E0845+378, E0844+377 and E2034−228.

E1346+266 – A HIGH REDSHIFT ULTRA-SOFT X-RAY AGN

Observationally, soft X-ray AGN are generally found at low redshifts; all of the soft excess AGN in the Turner & Pounds and Masnou et al. samples have redshifts below 0.2, leading to the suggestion that a redshift of 0.5 is sufficient to make any soft X-ray component undetectable (Masnou et al. 1992; P92a). This implies an upper limit to the effective temperature of the soft component in the rest-frame, providing an observational constraint on models for the big blue bump.

E1346+266 has a redshift of 0.92, much higher than the suggested z=0.5 cut-off, yet the ROSAT PSPC spectrum confirms earlier indications from the Einstein data that this object has a strong soft X-ray excess.