Spectral Signature of Atmospheric Reflection in Hercules X-1/HZ Hercules During Low and Short-High States

TL;DR

This study analyzes RXTE/PCA X-ray spectra of Her X-1/HZ Her, revealing spectral components modulated by orbital phase and evidence of atmospheric reflection from the companion star during low and short high states.

Contribution

It provides the first detailed spectral analysis distinguishing reflection, accretion disk, and neutron star emissions in Her X-1 during different states.

Findings

Reflection component peaks at orbital phase 0.55

Hard X-ray flux modulated by orbital phase

Asymmetry in X-ray illumination due to disk shadowing

Abstract

We analyse RXTE/PCA X-ray spectra of the binary X-ray pulsar Her X-1/HZ Her during short high state and one binary orbit in the preceding low state, just before short high turn-on. The spectrum is well described by two continuum components (absorbed and unabsorbed). The resulting spectral parameters are modulated with orbital phase. During low state a significant component of the flux, and its spectrum, is consistent with X-ray reflection off the face of the companion star HZ Her. This component has a significantly harder X-ray spectrum than the rest of the flux from the Her X-1 system. A second component in low state is consistent with emission from the accretion disk corona. During short high a third strong component is present with a softer spectrum, which is associated with the neutron star and accretion disk. Due to this direct emission from the neutron star and accretion disk, the reflected emission is less clear, however parameters and fluxes modulations during short high state indicate its presence. In low state, the hard X-ray flux ($h \nu$ $>$ $10 keV$) peaks at orbital phase $\phi_{orb}\simeq$ 0.55, which is expected from a simple model of atmospheric reflection from the companion star. The offset indicates an asymmetry in the X-ray illumination of the companion, which could be due to shadowing of the the inner face of HZ Her by the accretion disk and/or stream.