Hercules X-1: Empirical Models of UV Emission Lines

The UV emission lines of Hercules X-1, resolved with the HST GHRS and STIS, can be divided into broad (FWHM 750 km/s) and narrow (FWHM 150 km/s) 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 0.2 of the flux at phi=0.8. Line ratio diagnostics show that the density of the narrow line region is log n=13.4+/-0.2 and the temperature is T=1.0+/-0.2x10^5 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 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.Comment: To be published in the Astrophysical Journa

Discovery of Millihertz ULTRAVIOLET Quasi-periodic Oscillations in Hercules X-1

Observations of the ultraviolet continuum of the X-ray binary system Her X-1/HZ Herculis with the Space Telescope Imaging Spectrograph on the Hubble Space Telescope (HST) show quasi-periodic oscillations (QPOs) at frequencies of 8 ± 2 and 43 ± 2 mHz, with rms amplitudes of 2% and 4% of the steady flux. Observations with the Keck Telescope confirm the presence of the higher frequency QPO in the optical continuum, with a rms amplitude of 1.6% ± 0.2%. The QPOs are most prominent in the HST data near φ = 0.5 (where φ = 0 is the middle of the X-ray eclipse), suggesting that they arise not in the accretion disk but on the X-ray-heated face of the companion star. We discuss scenarios in which the companion star reprocesses oscillations in the disk which are caused by either Keplerian rotation or a beat frequency between the neutron star spin and Keplerian rotation at some radius in the accretion disk

RXTE Observations of Hercules X-1 during the 1998 July Short High State

We present RXTE monitoring of the eclipsing X-ray binary Hercules X-1 conducted over the short high state of 1998 July. This was one of the last major short high states before the source entered an anomalous low state of activity. A comparison with previous epochs finds no evidence for special behavior during these observations. We determine orbital and pulsar spin periods to facilitate measurements of Ṗspin and Ṗorb during the subsequent anomalous low state and the next epoch of high-state activity. Spectrally, the decay of the short high state and concurrent pre-eclipse dips are consistent with obscuration of a central X-ray source by a cloud of nonuniform column density. The standard model of a warped accretion disk of finite vertical scale height fits the characteristics of this absorber well. Pre-eclipse dips have durations a factor of a few longer than the characteristic durations of dips during main high states. Pulse profile structure increases in complexity toward the tail of the short high state, suggesting changes in accretion curtain geometry

Atmospheric Reflection during an Anomalous Low State of Hercules X-1

We present RXTE observations of the eclipsing X-ray binary Hercules X-1 conducted during an anomalous low state. Data reduction reveals a light curve over 2.7 orbital cycles remarkably similar to optical and UV light curves which are dominated by the companion star. Count rates are modulated close to the orbital period, attaining a maximum when the inner face of the companion star, irradiated by X-rays from the compact source, is most-visible. Cold reflection provides an acceptable fit to the energy spectrum. Employing binary geometry to scale the model and assuming companion star reflection, we are able to reconstruct the incident X-rays which are removed from our direct line-of-sight (presumably by the accretion disk). We find the flux of the hidden source to be identical to the observed flux of Her X-1 at the peak of its main-high state. Consequently, Her X-1 is emitting a reflected spectrum, largely uncontaminated by direct X-rays in the anomalous low-state. The spectral energy distribution, period, amplitude and phasing of the modulation are all consistent with a companion star origin. Since this source occurs in a well-understood binary environment, it provides an excellent case study for more sensitive experiments in the future.Comment: 18 pages, 7 figures. ApJ, in pres