SAS 3 and Einstein Observations of the 11 Minute Orbital Period of the Globular Cluster X-Ray Source 4U 1820-30

We confirm the 685 s modulation of the X-ray intensity of 4U 1820-30, a bright X-ray source in the globular cluster NGC 6624. A unique period is determined from 15 observations made with SAS 3 during 1976 and 1977. The amplitude of the modulation (peak-to-peak) is seen to vary between 1% and 5% in the energy range of 1-6 keV, and the average modulation amplitude is 2.6%. In each case, the X-ray source was in a high-luminosity, non-bursting state, and we find no correlation between the modulation amplitude and the source luminosity. The period determined by combining the SAS 3 epochs with those from EXOSAT and the Einstein Observatory is 685.011836(32) s. The upper limit (3 σ) on the period derivative is -0.09 <P <0.18 ms yr ^-1, which implies -1.3 <P/P <2.7 x 10^-7 yr ^-1. This result lowers the upper limit on P by five orders of magnitude (as reported by Stella, Priedhorsky, and White in 1987) and strengthens their interpretation that the modulation represents the orbital period of the binary system.If the neutron star is accreting matter from a small degenerate helium dwarf(M_c ≈ 0.07 M_sun) that is overflowing itsRoche lobe, we expect P/P> 1.2x10^-7 yr^-1. The cause of the X-ray modulation is not uniquely determined, but comparisons with other X-ray light curves favors the partial obscuration of an extended X-ray source by a bulge in the accretion disk.