Further x-ray observations of Hercules X-1 from Uhuru

Recent data are presented concerning the X-ray source Hercules X-1. Details of the 1.24-second pulse shape and its variability are shown. The 1.24-second period is observed to decrease with time, with an overall change of 4.5 s between 1972 January and July. The X-ray intensity shows regular ON states lasting 11 or 12 days followed by 24-day OFF states. While the source turn-on is not strictly periodic, the shape of the ON state can be used to determine a period of 34.88 1 0.12 days for this cycle. The source turn-ons are found to only occur at one of two well-defined phases in the 1.7-day orbit. In addition to the regular 1.7-day eclipses, regular dips in the X-ray intensity are seen, whose occurrence is related to the orbital phase. These dips are interpreted as absorption by gas in the system. Subject headings: eclipsing binaries - X-ray sources