Hercules X-1: Empirical Models of Ultraviolet Emission Lines

Abstract

The UV emission lines of Hercules X-1, resolved with the Hubble Space Telescope (HST) Goddard High Resolution Spectrograph and Space Telescope Imaging Spectrograph, can be divided into broad (FWHM~750 km s^-1) and narrow (FWHM~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 φ=0.2 and φ=0.8, and the line flux at φ=0.2 is ~0.2 of the flux at φ=0.8. Line ratio diagnostics show that the density of the narrow-line region is logn_e=13.4+/-0.2, and T_e=1.0+/-0.2×10⁵ 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 O V, Si IV, and He II line profiles agree with this result, but fits to the N V lines suggest that the receding side of the disk is brighter. We note that there are narrow absorption components in the N V lines with blueshifts of ~500 km s^-1.