RX J0911.4+0551: A new multiple QSO selected from the ROSAT All-Sky Survey

We report from follow-up observations of high redshift AGN candidates from the ROSAT All-Sky Survey (RASS) that RX J0911.4 + 0551 is a gravitationally lensed QSO with z = 2.800. With an X-ray luminosity of L(x) = 4.10(46) ergs s(-1) it belongs to the X-ray brightest radio quiet QSOs. However, we must point out that the existing observations do not exclude the possibility that other objects inside the error circle are responsible for the X-ray flux, in particular a lensing cluster. Careful image analysis discerned three optical images. The separation between the two brighter components of 0.'' 80 is below the seeing value during the observations. The fainter component has a distance of 3.'' 1. The spectra of the brighter joint A, B component and the C component show no measurable differences in continuum and absorption lines. Three strong CIV absorption Line systems with W-lambda(1549) > 1 Angstrom are visible in the spectra. All three have z > 2.3 which makes it improbable that they are caused by the lensing systems. The only spectral difference are larger equivalent widths of the broad emission lines in the brighter QSO components.