GK Per : X-ray observations in quiescence.

Abstract

Two X-ray observations of GK Per in quiescence made with Exosat are presented. The X-ray light curve obtained on both occasions shows clear evidence for the 351-s pulse period first discovered during an earlier outburst. The pulse profile in the first of the two quiescent observations is similar to that observed during outburst, albeit with lower modulation depth. In the second quiescent observation the average pulse profile is different with some indication of a double minimum. There is evidence for substantial variability of the modulation depth, and possibly the pulse profile, within the observation on time-scales of hours. The X-ray spectra obtained with the ME instrument show that in quiescence GK Per has a less-absorbed spectrum with a less intense FE line, but a continuum similar to that observed in outburst. The origin of the differences in the X-ray properties of GK Per between outburst and quiescence is commented on.