SMC X-1 variability observed from HEAO 1

Abstract

The 13-70 keV emission from the 0.7 s X-ray pulsar SMC X-l was monitored by the UCSD/MIT instrument aboard HE AO 1 during three ~80 day epochs in 1977 and 1978. The X-ray flux was seen to wander slowly between a maximum value and unobservably small levels with a rough time scale of 60 days. This variability is found to be consistent with a random walk in intensity and could be the result of unsteady mass transfer from the primary. A possible alternate explanation involves regular shadowing of the X-ray emitting neutron star by a tilted, precessing accretion disk, as in the cases of Her X-l and LMC X-4. If so, the precess-ion period instability of AP/P =15% is a factor of 3 larger than for Her X-l. The maximum 13-70 keV luminosity observed was 2.8 x 10" 11 ergs (cm 2 s)-1. The time-averaged spectrum above 15 keV shows a gradual exponential cutoff with a characteristic energy of 10-20 keV. There is no evidence for linelike features such as observed in Her X-l and A0115 + 63. The spectrum appears to soften at times of low intensity.