A BBXRT OBSERVATION OF THE HIGH-LUMINOSITY QUASAR H1821+643

We present the results of an observation of the high-luminosity quasar H1821 + 643 with the Broad-Band X-Ray Telescope (BBXRT) performed in 1990 December. The 2-10 keV flux and power-law spectral index are foun d to be consistent with previous values obtained by EXOSAT and Ginga w hich indicates that the medium-energy continuum has remained steady fo r at least approximately 6.5 yr. Contrasting with this is the result t hat the Fe Kalpha line present in Ginga observations of this source (o bserved equivalent width 210 +/- 80 eV) is not detected by BBXRT. We e stimate that a narrow iron line, if it had been present during the BBX RT observation at the same strength as it was in the Ginga observation s, should have been detected with a probability of 88%. We argue again st variability of the line between the BBXRT and Ginga observations an d suggest the possibility that the line is broad.