The old nova GK Per : discovery of the X-ray pulse period.

Abstract

EXOSAT observations of GK Per during an optical outburst are reported. The hard X-ray emission (greater than 2 keV) shows a strong coherent modulation at a period of 351 s, identifying GK Per as the latest member of the 'intermediate polar' subclass of magnetic cataclysmic variables. The X-ray pulse profile has a quasi-sinusoidal shape with mean amplitude approximately 50 per cent (increasing to about 80 percent at energies below 3 keV). The pulse fraction is remarkably constant, being essentially unaffected by changes in the X-ray flux. The X-ray spectrum is complex with evidence for two components above 2 keV, both of which show large low-energy cut-offs and a third, much softer component below 0.1 keV. The dominant hard X-ray component has an unprecedently flat spectrum for a cataclysmic variable. In addition to the periodic modulation, the hard X-ray emission also varies aperiodically on time-scales of approximately 3000s. These data are discussed in the context of current ideas on accretion on to magnetic white dwarfs. It is shown that the hard X-ray observations constrain the possible accretion geometry, and that GK Per can be understood quite naturally in terms of 'stripped giant' accretion on to a white dwarf, suggesting quite tightly constrained estimates for various system parameters. A brief comment on the likely evolutionary history of the system is made.