A LABORATORY FOR MAGNETIZED ACCRETION DISK MODEL - ULTRAVIOLET AND X-RAY-EMISSION FROM CATACLYSMIC VARIABLE GK PERSEI

We analyze the ultraviolet spectrum of the cataclysmic variable GK Per at maximum light. The flat ultraviolet spectrum in this system requir es a truncated inner accretion disk and an unusually flat radial tempe rature profile. This requirement is not satisfied by any nonmagnetic s teady or nonsteady disk model. We consider a magnetized accretion disk model to explain the ultraviolet spectrum. The available data on the white dwarf spin and possible quasi-periodic oscillations constrain th e magnetic held, B, and the disk accretion rate, M, to lie along a we ll-defined spin-equilibrium condition (M/10(17) g s(-1)) similar to 10 0(B/10(7) G)(2). Our self-consistent treatment of the magnetic torque on the disk flattens the disk temperature distribution outside the di sk truncation radius. This modified temperature distribution is too st eep to explain the UV spectrum for reasonable held strengths. X-ray he ating is a plausible alternative to magnetic heating in GK Per. We est imate that the disk intercepts similar to 5% of the accretion energy i n outburst, which results in an extra disk luminosity of similar to 5- 10 L.. Model spectra of optically thick disks are too blue to match ob servations. The UV spectrum of an optically thick disk with an optical ly thin, X-ray heated corona resembles the observed spectrum. The X-ra y luminosity observed during the outburst indicates M < 10(18) g s(-1) , which is a factor of 10 lower than that required to explain the ultr aviolet luminosity. Radiation drag on material flowing inward along th e accretion column lowers the shock temperature and reduces the X-ray luminosity. Most of the accretion energy is then radiated at extreme u ltraviolet wavelengths.