Multiwavelength studies of Hercules X-1 during short high and anomalous low states: on-again, off-again

Vrtilek, S.D; Quaintrell, H.; Boroson, B.; Still, M.; Fiedler, H.; O'Brien, K. and McCray, R. (2001). Multiwavelength studies of Hercules X-1 during short high and anomalous low states: on-again, off-again. Astrophysical Journal, 549(number 1,) pp. 522–536.

DOI: https://doi.org/10.1086/319071

Abstract

We present results from multiwavelength observations of the Hercules X-1 system during a short high state (SHS) and during an anomalous low state (ALS). The magnitude of deviation from spin-up appears to be positively correlated with duration of the ALS. Such a correlation is consistent with an interpretation of the ALS in terms of a change in mass accretion rate that causes the disk to tilt and twist beyond the normal deviations that cause the 35 day cycle. A larger deviation from the average results in a stronger disruption of the disk and causes the disk to take longer to settle back to its "normal" 35 day behavior. Our modelwhich includes X-ray heating of the disk and companion star, shadowing of the X-ray flux by the disk, and a contribution to the continuum emission from the accretion stream or hot spotcan consistently explain the observed changes in X-ray, ultraviolet (UV), and optical continuum light curves for both the SHS and ALS. The Hubble Space Telescope (HST) and Space Telescope Imaging Spectrograph (STIS) observations presented here are the first UV observations of sufficient spectral and temporal resolution to construct Doppler tomograms of the line emission. Doppler tomograms of the UV emission lines during SHS and ALS show the majority of the emission coming from the surface of the companion star rather than from the accretion disk. Tomograms made after separating the N V emission lines into broad and narrow components suggests that while the narrow component is associated with emission from the companion star, the broad component may be associated with emission from a distorted disk. The Doppler maps also show that heating over the inner face of HZ Her is not uniform and imply partial eclipse of the UV line emission by an accretion stream and/or hot spot.