Atmospheric reflection during an anomalous low state of Hercules X-1

Still, Martin; O'Brien, Kieran; Horne, Keith; Boroson, Bram; Titarchuk, Lev G.; Engle, Kimberly; Vrtilek, Saeqa Dil; Quaintrell, Hannah and Fiedler, Hauke (2001). Atmospheric reflection during an anomalous low state of Hercules X-1. Astrophysical Journal, 554(number 1,) pp. 352–361.

DOI: https://doi.org/10.1086/321337

Abstract

We present RXTE observations of the eclipsing X-ray binary Hercules X-1 during an anomalous low state. Data reduction reveals a light curve over 2.7 orbital cycles remarkably similar to optical and UV light curves 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 that 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.