Evidence for a binary period in the highly variable X-ray source Aquila X-1.

Ariel V observations of Aquila X-1 (3U 1908+00) during the decay of the June 1975 X-ray flare are reported. A plot of the corrected intensities derived from single-orbit integrations shows an obvious modulation superimposed on the quasi-linear decline in intensity. The data are subjected to both chi-square and power-spectrum analyses; a best-fit modulation of about 1.30 days is estimated. Phase zero is derived from the relative phase of the minimum bin in the power-spectrum analysis and is found to be centered on JD 2442593.63 (+ or - 0.15). The light curve shows evidence for random intensity fluctuations similar to those observed in Cygnus X-1. It is suggested that the period is due to the variation of the X-ray emission from a compact object in a binary system.