The UV emission lines of Hercules X-1 , resolved with the HST GHRS and STIS , can be divided into broad ( FWHM \approx 750 km s ^ { -1 } ) and narrow ( FWHM \approx 150 km s ^ { -1 } ) components .
The broad lines can be unambiguously identified with emission from an accretion disk which rotates prograde with the orbit .
The narrow lines , previously identified with the X-ray illuminated atmosphere of the companion star , are blueshifted at both \phi = 0.2 and \phi = 0.8 and the line flux at \phi = 0.2 is \approx 0.2 of the flux at \phi = 0.8 .
Line ratio diagnostics show that the density of the narrow line region is \log n _ { e } = 13.4 \pm 0.2 and T _ { e } = 1.0 \pm 0.2 \times 10 ^ { 5 } K. The symmetry of the eclipse ingress suggests that the line emission on the surface of the disk is left-right symmetric relative to the orbit .
Model fits to the \ion O5 , \ion Si4 , and \ion He2 line profiles agree with this result , but fits to the \ion N5 lines suggest that the receding side of the disk is brighter .
We note that there are narrow absorption components in the \ion N5 lines with blueshifts of \approx 500 km s ^ { -1 } .