Pulsations and Line Profile Changes in the Ultraviolet Spectrum of Hercules X‐1: Results from a Multiwavelength Campaign

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Abstract
We report simultaneous X-ray and ultraviolet observations of the X-ray binary pulsar Hercules X-1, obtained over nearly half of a 1.7 day binary orbit using the ASCA X-ray telescope and the Goddard High Resolution Spectrograph (GHRS) aboard the Hubble Space Telescope. The GHRS resolved, for the first time, two moving emission components. One feature in the N V λλ1238.8, 1242.8 doublet is broad (FWHM ≈ 1000 km s–1) and the other is narrow (FWHM ≈ 150 km s–1). We attribute the broad emission line to the X-ray—illuminated surface of a Keplerian accretion disk and the narrow line to the X-ray-heated atmosphere of the companion star, HZ Her. Near orbital phase orb = 0.80 (simultaneous with a pre-eclipse X-ray dip), the N V λ1242.8 component became stronger than the N V λ1238.8 component. Later in the orbit, the blue edge of the broad lines diminished as the approaching side of the accretion disk was occulted by HZ Her. UV continuum pulsations were detected for the first time, with an amplitude ≈ 0.5% of the steady flux. The pulse shapes of the UV continuum and hard X-rays were similar at orb = 0.83 but not at orb = 0.56. We found evidence for a stronger pulsation (amplitude ≈ 15%) within a narrow (0.25 Å) segment of the broad 1242.8 Å N v component, only at W erb = 0.80. We briefly discuss some implications of these observations for the gas flows in the Her X-1 system.

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