This paper presents optical and soft X-ray observations of a QSO discovered within the RASS and present in 16 pointings from a medium deep ROSAT survey in the field HS 47.5/22 (Molthagen et al., 1997).
The object is similar to narrow-line Seyfert 1 galaxies, but both its optical and X-ray luminosity are about an order of magnitude higher than most of the objects in Boller et al. (1996), thus clearly extending the object class into the quasar domain.
It shows an extremely soft X-ray spectrum. The usual description with a power law and low energy absorption gives only very poor fits. Single component thermal models underestimate the flux above 1 keV. A two component model with the soft excess modeled by a blackbody, and a power law as hard component, gives a better fit. Strong and - at the high luminosity - remarkably fast variations can be seen in the light curve of RX J0947.0+4721.
Warm absorbers could be an explanation of NLS1 phenomena. Applied to RX J0947.0+4721, such models conflict with the lack of observed spectral variability. However, the faintness of RX J0947.0+4721 at harder X-rays may have prohibited the detection of changes close to 1 keV.
Models including reprocessing might be another explanation for the X-ray emission of RX J0947.0+4721, but again, the lack of spectral variations may be a problem.
At present, the hard X-ray data are not sufficient to distinguish between the above models, or others like a filamentary or spot-like emission region, or the tidal disruption of stars. A thorough investigation of intensity and spectral changes would be highly desirable.
© European Southern Observatory (ESO) 1998
Online publication: March 3, 1998