A Model for the Accretion Disk of Hercules X-1 from 35-Day Cycle Lightcurves

The binary Hercules X-1 exhibits a 35-day brightness X-ray cycle, including a Main High state and a Short High state which are separated by two Low states. The cycle is due to blockage of the neutron star by a rotating twisted disk. The best available 35-day average lightcurves are from Swift/BAT observations in the 15–50 keV band and from MAXI observations in the 2–20 keV band. The current work fits both Swift/BAT and MAXI 35-day X-ray lightcurves using a disk plus corona model. The parameters of the disk and its corona from the two data sets are shown to be consistent with each other. The fit of two different data sets with the same model verifies the reliabilty of the disk plus corona model and its parameter values. We derive the scaling of the model disk radius to physical disk radius and use that to find that the physical corona inner radius is very small ∼100 km, and consistent with the estimate from pulse shape analysis. The system inclination of 85.12±0.24° is consistent with the recent value derived from MAXI observations alone, and much improved compared to earlier estimates (∼82 to 88°). This results in significantly improved binary system parameters, including neutron star mass, companion mass and companion radius.