X-RAY CONTINUUM AND EVIDENCE FOR AN IRON EMISSION-LINE FROM THE QUASAR E1821 + 643

The high-luminosity radio-quiet quasar E1821 + 643 has been observed with Ginga. The 2-10 keV spectrum has a power-law energy index, alpha, of 0.9 and the flux is (2.0 +/- 0.3) x 10(-11) ergs cm-2 s-1. The absorption is consistent with the Galactic value of N(H) = 4 x 10(20) cm-2. The spectrum shows strong evidence for the presence of a redshifted iron K emission line of center energy 5.1 +/- 0.2 keV (6.6 +/- 0.3 keV, redshift corrected; z = 0.297), and an observed equivalent width of 210 +/- 80 eV. There is evidence that E1821 + 643 may be embedded within a cluster of galaxies which could contribute part of the observed line. The equivalent width (EW) ascribed to the quasar cannot, therefore, be accurately determined, but lies in the range 60 eV less-than-or-similar-to EW less-than-or-similar-to 380 eV (after redshift correction). The upper limit to the intrinsic absorption of 1 x 10(22) cm-2 rules out production of the line by fluorescence in cold material in the line of sight, but several other possible origins for the line are not constrained by our data. The detection of a redshifted iron line opens the possibility that iron emission lines may be a common feature in the X-ray spectra of quasars. This would have a considerable impact on the study of quasars by future X-ray missions.