An extended X-ray low state from Hercules X-1

Abstract

Hercules X-1 exhibits a 35-day cycle in its X-ray intensity^1,2 in addition to its pulsar rotational and orbital periodicities of 1.24 s and 1.7 days respectively. The effects of X-ray heating on the companion are visible throughout the 35-day cycle³, suggesting that the observed intensity modulation is caused by periodic blocking of the line of sight to the pulsar. We report here observations made with the EXOSAT Observatory between 1983 June and August that failed to detect the expected 35-day variation in X-ray intensity, although low-level extended X-ray emission was seen. Models where the obscuring of the pulsar is caused by a processing tilted accretion disk have been proposed^4-9, although the physics of the tilted disk are not clear¹⁰. Alternatively, wave-like thickenings of the accretion disk have been proposed¹¹ to account for the observed modulation. Our EXOSAT observations suggest that a temporary change in the disk structure may have occurred such that the disk was in the line of sight throughout.