UV Spectra of HZ Her/Her X-1 from HST: Hot Gas During Total Eclipse of the Neutron Star

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Session 7 -- X-Ray Binaries and Gamma-Ray Binaries
Display presentation, Monday, 9:20-6:30, Heller Lounge Room

[7.06] UV Spectra of HZ Her/Her X-1 from HST: Hot Gas During Total Eclipse of the Neutron Star

S. F. Anderson, S. Wachter, B. Margon (U. Wash.), R. A. Downes (STScI)

The Faint Object Spectrograph aboard Hubble Space Telescope has observed HZ~Her, the companion of the prototypical binary X-ray pulsar Her~X-1, in its high spectral resolution mode ($\lambda/\Delta\lambda\sim1200$). The spectra encompass the 1150--3300\,\AA\ range at binary orbital phases 0.5 and 0.0, corresponding to X-ray maximum and mid-X-ray eclipse, respectively. The maximum light spectra are dominated by strong, narrow N\,V, C\,IV, and He\,II emission, confirming and extending previous IUE results. The O\,III Bowen resonance fluorescence line at $\lambda3133$ is particularly prominent, confirming that the Bowen mechanism is most certainly the source of the strong $\lambda\lambda4640, 4650$ emission complex.

Most remarkable, however, are the minimum light spectra, where the object is too faint for reasonable observations from IUE. Despite the total eclipse of the X-ray-emitting neutron star, our spectra show strong emission at N\,V $\lambda$1240, Si\,IV $\lambda$1400, N\,IV] $\lambda1488$, and C\,IV $\lambda1549$. (The situation at L$\alpha$ is ambiguous due to geocoronal contamination). In a little-cited observation almost two decades ago, Koo \& Kron (PASP, 89, 285) reported an incident of He\,II $\lambda4686$ in emission on a glass plate obtained in 1973 at mid-eclipse in a heroic 6~hr exposure by R.~Kraft. However, very few ground-based spectra of HZ~Her have been published in the last decade, and it does not seem commonly appreciated that strong, high excitation emission is indeed present, probably routinely, when the neutron star is occulted. Although the hot gas might be associated with the ``accretion disk corona" (inferred to be present through very weak X-ray emission during eclipse), more likely the source is somewhat cooler blobs above and around the disk, also thought to be responsible for the occasional odd ``X-ray dips," as well as features in the optical light curve.

This work has been supported by NASA Grant NAG5-1630.

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