The Evolution of the Orbital Light-curve of Hercules X-1 with 35-day Phase

TL;DR

This study analyzes long-term X-ray observations of Hercules X-1, revealing how its orbital light-curve evolves with the 35-day cycle and providing insights into the accretion disc's structure and absorption effects.

Contribution

It provides a comprehensive set of orbital light-curves across six states of the 35-day cycle, highlighting the evolution of dips and absorption features with phase.

Findings

Pre-eclipse dips occur regularly and shift with 35-day phase.

Derived orbital phase dependence of absorption and transmission.

X-ray leads optical by 0.04 to 0.08 in orbital phase.

Abstract

Hercules X-1/HZ Hercules (Her X-1/HZ Her) is an X-ray binary monitored by multiple X-ray missions since last century. With the abundance of long-term observations, we present a complete set of orbital light-curves of Her X-1/HZ Her during the six states of the 35-day cycle in multiple energy bands. These illustrate in detail the changing light-curve caused by the rotating twisted-tilted accretion disc surrounding the neutron star. The orbital light-curves during Main-High (MH) state are analyzed in 0.05 35-day phase intervals. These show the regular occurrence of pre-eclipse dips which march to earlier orbital phase as 35-day phase increase. From the multi-band light-curves we derive time-average orbital phase dependence of column density for photoelectric absorption and energy-independent transmission as a function of 35-day phase. The X-ray light-curves during Low States are similar in shape to the optical Low State light-curve, but X-ray leads optical by $\simeq$0.04 to 0.08 in orbital phase.