4U 1916-05

F. M. Walter, University of California at Berkeley; and N. E. White and J. Swank, Goddard Space Flight Center, write: "We have independently discovered periodic absorption dips in the steady x-ray flux from the burst source 4U 1916-05 in two observations made with the Einstein Observatory. OSO-8 and HEAO-1 A2 data confirm the result and yield a preliminary period of 50 +/- 0.5 min; the data cannot yet be used to predict a ephemeris. The dips are of varying duration and depth, and cannot be due to eclipses by a companion; they are also observed to disappear and reappear on a timescale of a few days. The dips are probably due to photoelectric absorption by a bulge in the accretion disk, where the gas stream intercepts the disk with the rapid variations due to turbulent fluctuations in the height of the bulge, and the slower trends may be attributable either to long-term changes in the bulge height or to precession of a tilted accretion disk. This is the first direct evidence to confirm the binary nature of x-ray burst sources. We urge further efforts to identify an optical counterpart."