FUSE Observations of a Full Orbit of Hercules X-1: Signatures of Disk, Star, and Wind

TL;DR

This study presents FUSE observations of Hercules X-1's full orbit, revealing signatures of a prograde accretion disk, a bright spot offset from the companion star, and the influence of stellar wind and fluorescence on emission lines.

Contribution

First detection of double-peaked accretion disk line profiles in Hercules X-1 and insights into disk, star, and wind interactions through comprehensive spectroscopic analysis.

Findings

Keplerian accretion disk spinning prograde observed

Bright spot offset from the Roche lobe of the companion star

Absence of disk signatures explained by stellar wind absorption

Abstract

We observed an entire 1.7 day orbit of the X-ray binary Hercules X-1 with the Far Ultraviolet Spectroscopic Explorer (FUSE). Changes in the O VI 1032,1037 line profiles through eclipse ingress and egress indicate a Keplerian accretion disk spinning prograde with the orbit. These observations may show the first double-peaked accretion disk line profile to be seen in the Hercules X-1 system. Doppler tomograms of the emission lines show a bright spot offset from the Roche lobe of the companion star HZ Her, but no obvious signs of the accretion disk. Simulations show that the bright spot is too far offset from the Roche lobe to result from uneven X-ray heating of its surface. The absence of disk signatures in the tomogram can be reproduced in simulations which include absorption from a stellar wind. We attempt to diagnose the state of the emitting gas from the C III 977, C III 1175, and N III 991 emission lines. The latter may be enhanced through Bowen fluorescence.