The Accretion Flows in the X-Ray Binary Pulsars VELA X-1 and Hercules X-1 Inferred from Time-Resolved Multiwavelength Spectroscopy

Bram Seth Boronson

We inferred the acception flows in the X-ray binary pulsars Vela X-1 and Hercules X-1 from simultaneous observations of X-ray and ultraviolet spectra. We used Ginga and ASCA to observe the X-ray spectra, and the Faint Object Spectrograph and the High Resolution Spectrograph aboard the Hubble Space Telescope to observe the UV spectra. In Vela X-1, we observed the SiIV lines at 1400 A respond to changes in X-ray illumination on timescales of ~30 minutes. Both the SiIV lines and the NV P-Cygni line at 1240 A pulsate at the neutron star rotation period, 283 seconds. We draw conclusions about the structure of the X-ray illuminated wind from these P-Cygni line variations. In Hercules X-1, the UV emission lines are seen for the first time to arise in two distinct regions. The narrower emission lines originate in the X-ray heated face of the normal star (HZ Her), while the broad lines arise from Keplerian rotation of the accretion disk. The narrow lines provide evidence for a tangential velocity field on the surface of HZ Her. As the system progresses towards X-ray eclipse, the broad emission lines show the eclipse of the approaching side of the accretion disk. We model the emission from the disk and find a better fit if the disk shape is twisted as suggested by X-ray and optical light curves. At orbital phase 0.8, we find a reversal of the expected strengths of the components of the N V doublet. This suggests overlapping P-Cygni profiles, arising from at outflow near the disk. We investigate a possible correlation between this outflow and the pre-eclipse X-ray dip. At orbital phases 0.56 and 0.83, the UV continuum pulsates with an amplitude of ~0.4-0.7% of the steady flux. A narrow portion of the NV emission line pulsates at orbital phase 0.8; this pulsation may result from X-ray ionization of a disk wind (in a manner similar to the P-Cygni line pulsations in Vela X-1.) We only observed pulsations in the NV line during the period when P-Cygni line profiles were seen.

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