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Astron. Astrophys. 331, 925-933 (1998)

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1. Introduction

Studies of the soft X-ray excess were done preferentially with low redshift AGN because this spectral feature will be shifted out of the observed energy windows of the ROSAT, EXOSAT and Einstein satellites for higher redshifts. The existence of the soft X-ray excess was first detected for the [FORMULA] Seyfert 1 galaxy E1615+061 (Pravdo et al. 1981) and Mrk 841, another Seyfert galaxy with [FORMULA] (Arnaud et al. 1985). Among the 58 AGN used for a study of the ultraviolet to soft X-ray bump (Walter & Fink, 1993) the QSO with the highest redshift is 3C 263 with [FORMULA], and only one of the 31 narrow line Seyfert 1 galaxies (NLS1) investigated by Boller et al. (1996), has z higher than that. This object, E1346 [FORMULA] 266 (see also Puchnarewicz et al. 1994), and PHL 1092 (Forster & Halpern 1996) are the most distant narrow line type 1 QSOs reported so far. It is therefore of great interest to examine the properties of further narrow line QSOs with high redshifts and steep soft X-ray spectra. The situation is similar for the study of the time behaviour of AGN. For a statistically unambiguous detection of a short-term variation a minimum of source counts is necessary. Therefore such studies preferred bright nearby AGN.

RX J0947.0+4721 was first detected in the RASS 1 (Bade et al. 1995). It appeared to be a very soft source, but the photon statistics were insufficient to determine the spectral shape. Besides that, RX J0947.0+4721 is the brightest and steepest source in a medium deep ROSAT survey in the field HS 47.5/22 (Molthagen 1996). The field is part of the Hamburg Quasar Survey (Hagen et al. 1995), an objective prism based survey of the northern hemisphere. Due to its brightness, RX J0947.0+4721 can be found in 16 of the 48 pointings which form the survey. Strong variations, already suggested in the RASS data, were clearly seen in the pointed data.

RX J0947.0+4721 shows striking similarity in optical and X-ray behaviour with NLS1 objects such as IRAS 13224-3809 (Boller et al. 1993). Both RX J0947.0+4721 and IRAS 13224-3809 have exceptionally steep soft X-ray spectra, narrow Balmer lines, and strong optical Fe II emission. RX J0947.0+4721 is also an IRAS source (Moshir et al. 1990). Its optical and X-ray luminosities, computed with [FORMULA] and [FORMULA], clearly denote it as quasar. The low Galactic absorption in the direction of this QSO enables high-quality spectral analysis of this quasar-luminosity NLS1.

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© European Southern Observatory (ESO) 1998

Online publication: March 3, 1998