A Laboratory for Magnetized Accretion Disk Model: Ultraviolet and X-ray Emission from Cataclysmic Variable GK Persei

Abstract

We analyze the ultraviolet spectrum of the cataclysmic variable GK Per at maximum light. The flat ultraviolet spectrum in this system requires a truncated inner accretion disk and an unusually flat radial temperature profile. This requirement is not satisfied by any non-magnetic steady or non-steady 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 the magnetic field, $B_{*}$, and the disk accretion rate, ${\dot M}$, to lie along a well-defined spin-equilibrium condition $({\dot M}/10^{17} ~ g ~ s^{-1}) \sim 100(B_{*}/10^7G)^2$. Our self-consistent treatment of the magnetic torque on the disk flattens the disk temperature distribution outside the disk truncation radius. This modified temperature distribution is too steep to explain the UV spectrum for reasonable field strengths. X-ray heating is a plausible alternative to magnetic heating in GK Per. We estimate that the disk intercepts $\sim$ 5% of the accretion energy in outburst, which results in an extra disk luminosity of $\sim$ 5--10 $L_{\odot}$. Model spectra of optically thick disks are too blue to match observations. The UV spectrum of an optically thick disk with an optically thin, X-ray heated corona resembles the observed spectrum. The X-ray luminosity observed during the outburst indicates ${\dot M}<10^{18} ~ g ~ s^{-1}$, which is a factor of 10 lower than that required to explain the ultraviolet luminosity. Radiation drag on material flowing inward along the accretion column lowers the shock temperature and reduces the X-ray luminosity. Most of the accretion energy is then radiated at extreme ultraviolet wavelengths.