Discovery of a 176 day period in 4U 1820-30.

Long-term X-ray observations of the X-ray burster 4U 1820-30 (in NGC 6624) reveal a significant 176.4 + or - 1.3 day period. These data were obtained by a 3-12 keV detector on the Vela 5 B satellite during the years 1969-1976. The X-ray flux varies by a factor of 2 over the cycle; the light curve, which has a sawtooth shape with fast rise and slow decline, is quite similar to the 199 day light curve of the burster 4U 1915-05. These long-period variations may be intrinsic luminosity changes, perhaps similar to the quasi-periodic outbursts of dwarf novae, or aspect-dependent effects, such as periodic obscuration by a precessing accretion disk.