/ 98 11 40 4 v 1 2 5 N ov 1 99 8 A persistent ∼ 1 Hz quasi - periodic oscillation in the dipping low - mass X - ray binary 4 U 1323 – 62

We have discovered a ∼1 Hz quasi-periodic oscillation (QPO) in the persistent-emission, the dips, and the type I X-ray bursts of the low-mass X-ray binary 4U 1323–62. The rms amplitude of the QPO is approximately 9%, only weakly depending on photon energy. The amplitude is consistent with being constant throughout the persistent-emission, the dips and the bursts in all but one observation, where it is much weaker during one dip. These properties suggest that we have observed a new type of QPO, which is caused by quasi-periodic obscuration of the central X-ray source by a structure in the accretion disk. This can only occur when the binary inclination is high, consistent with the fact that 4U 1323–62 is a dipping source. The quasi-periodic obscuration could take place by partial covering of an extended central X-ray source by a near-opaque medium, or by covering of a point source by a medium of suitable characteristics to produce the relatively energy-independent