HER X-1: Ariel-5 and Ariel-6 observations

Her X-1 was scanned by the Ariel-5 Sky Survey Instrument for a total of 250 days during the period 1974 October to 1980 March, and observed by Ariel-6 for about 10 days in 1981 June. The Ariel-5 flux measurements have a time resolution of 0.07 day and cover the energy range 2 to 15 keV. The Ariel-6 data mainly cover the range 1.5 to 13 keV and have either 0.25 or 0.06 sec resolution and so could be used to measure the pulse period. The 1981 observation spanned the periods MJD 44767.1 to 44774.6 and 44779.9 to 44782.0. Initially in the secondary one, the flux declined over the first two days to the detection limit. When the observation recommenced the turn on had already taken place, and the first feature seen was a pre-eclipse dip, centered on phase, phi_1.7 = 0.84. In the sequence in which cycle 0 turn on occurred at MJD 41325.15 these observations extended over cycles 98 and 99. The light curve for this data set is included in figure 2. From the available data, we conclude that the turn on occurred in the interval MJD 44779.0 + or - 1.0.