DISCOVERY OF A LONG-TERM PERIODIC VARIATION IN LMC X-3

The highly variable X-ray luminosity of LMC X-3 is found to be strongly modulated with a period of approximately 198 (or possibly approximately 99) days. Observations from both Ginga and HEAO 1 satellites show this periodic variation. For energies < 13 keV, the X-ray intensity and hardness are positively correlated; for higher energies, there appears to be no correlation. Available optical photometry indicates the mean V brightness also varies by > 1 mag with this same long-term period. The regularity of this "clock" in LMC X-3 suggests that it may be related to an accretion disk precession, similar to that seen in LMC X-4, Her X-1, SS 433, or possibly periodic variations in the mass transfer rate.