The intermediate polar GK Persei: An unstable relation of the X-ray and the optical intensities in a series of outbursts

Abstract

Context. GK Per is an intermediate polar that has been displaying dwarf nova outbursts since the middle of the twentieth century. Aims. I analyzed a series of such outbursts in the optical and X-ray bands. I pay attention to the relation of intensities of the optical and X-ray emissions, and its reproducibility in a series of these consecutive outbursts. Methods. This analysis uses the data from the BAT/Swift ,A SM/RXTE, AAVSO, and AFOEV databases. It investigates the relation of the time evolution of the profiles of outbursts in the individual bands (hard X-ray, medium/hard X-ray, and optical). Results. This analysis shows that the X-ray intensity steeply rises only in the start of the optical outburst and steeply declines only when the optical outburst comes to its end. However, the 1.5–50 keV band intensity saturates and balances on a plateau during the outburst. (The longer the outburst, the longer its plateau.) The peak X-ray intensities of this series display a significantly narrower range than the optical ones (a factor of about two versus a factor of about eight). This implies a discrepancy between the mass flow through the disk and the production of the X-ray emission via bremsstrahlung at the polar caps of the white dwarf. This discrepancy is the largest in the time of the peak optical intensity when the whole disk (or at least its inner part) is in the hot state and the flow of matter through the disk is the greatest. This study shows that a series of outbursts constitutes more general properties of this discrepancy. I argue that the saturation of the X-ray luminosity in outburst cannot be caused by a dominant increase in X-ray absorption. In the interpretation, large structural changes of the accreting regions at the magnetic poles of the white dwarf occur during the outburst. A buried shock proposed by some authors for polars is also promising for explaining the X-ray light curves of outbursts of GK Per.