The geometry of the Hercules X-1 accretion disk from X-rays

Hercules X-1 has a consistent 35-day periodic light-curve in X-rays consisting of main high, short high and low states caused by a precessing tilted and twisted accretion disk. The MAXI instrument has recently measured the average 35-day X-ray light-curve with small errors. The current work utilizes the MAXI data and ray-tracing software to derive a better-constrained geometric model of the accretion disk than previous studies. A basic 3-D model based on previous work is iteratively expanded in complexity until it can satisfactorily replicate the 35-day light-curve. We summarize the disk models tested and confirm the requirement for a scattering corona and for a disk atmosphere which is different for the inner and outer disk. We are able to constrain the system inclination to $$85.5\pm 0.3^{\circ }$$ , much improved compared to previous estimates ( $$\sim$$ 82 to $$88^\circ$$ ). The constraint on inclination allows for more accurate determinations of the radius of the companion HZ Her (2.64 to 2.83 $$\times 10^{11}$$ cm), mass of the HZ Her (2.34 to 2.45 $$\hbox {M}_\odot$$ ) and mass of the neutron star (1.52 to 1.69 $$\hbox {M}_\odot$$ ).