Discovery of the Binary Nature of SMC X-1 from UHURU

The X-ray source SMC X-1 in the Small Magellanic Cloud has been found to undergo periodic intensity changes which have been interpreted as eclipses in a binary system. The phase of the center of occultation is given by UT [1971 January 12.99 0.02 + n(3.8927 I 0.0010)]. No regular pulsations of the source on timescales of seconds have been detected with an upper limit on the percentage pulsed of 10 percent. Intensity fluctuations on timescales of hours and possibly minutes have been observed. Extended nonperiodic intervals of low intensity have also been noted. The energy spectrum is flat and shows low-energy absorption, similar to the behavior of other binary X-ray sources. A possible correlation between low-energy cutoff and orbital inclination in binaries is discussed.