Disk instability and the time-dependent X-ray emission from the intermediate polar GK persei

According to the disk instability model for the 1981-1989 outbursts of the intermediate polar GK Per, the mass accretion rate at the inner disk edge (Min) varies from ∼2 × 10¹⁶ g s^-1 in quiescence to ∼3 × 10¹⁹ g s^-1 in outburst, while the mass transfer rate from the companion weakly fluctuates around 10¹⁸ g s^-1. During quiescence, the postshock region of the accretion column is characterized by optically thin bremsstrahlung emission with a small inverse Compton scattering depth (y < 1). During outburst, high Min produces a hot postshock region with substantial inverse Compton scattering effects (y > 1) and increased photoelectric absorption. This transition may account for the secondary spectral component observed during outburst in the ∼2-10 keV band. During outburst, the height of the radiatively cooled shock decreases substantially (H ∞ M^-5/7) while the disk thickness increases (h ∞ M^3/20) with increased local density (ρ ∞ M^11/20). Consequently, most of the hard X-ray flux is geometrically blocked during outburst. The observed flux may be entirely due to scattering from the unshocked, infalling gas. Hence, despite large fluctuations in M, the observed X-ray flux varies only moderately. However, the linear polarization of the X-ray flux should increase dramatically during outburst.