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 atmaximum light. The flat ultraviolet spectrum in this system requires atruncated inner accretion disk and an unusually flat radial temperatureprofile. This requirement is not satisfied by any non-magnetic steady ornon-steady disk model. We consider a magnetized accretion disk model to explainthe ultraviolet spectrum. The available data on the white dwarf spin andpossible quasi-periodic oscillations constrain the magnetic field, $B_{*}$, andthe disk accretion rate, ${\dot M}$, to lie along a well-definedspin-equilibrium condition $({\dot M}/10^{17} ~ g ~ s^{-1}) \sim100(B_{*}/10^7G)^2$. Our self-consistent treatment of the magnetic torque onthe disk flattens the disk temperature distribution outside the disk truncationradius. This modified temperature distribution is too steep to explain the UVspectrum for reasonable field strengths. X-ray heating is a plausiblealternative 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 diskluminosity of $\sim$ 5--10 $L_{\odot}$. Model spectra of optically thick disksare too blue to match observations. The UV spectrum of an optically thick diskwith 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 theultraviolet luminosity. Radiation drag on material flowing inward along theaccretion column lowers the shock temperature and reduces the X-ray luminosity.Most of the accretion energy is then radiated at extreme ultravioletwavelengths.Comment: 27 pages, AASLATEX, 8 ps figures, Ap