Evidence for 4.4 hour periodic dips in the X-ray flux from 4U 1755-33.

Abstract

The paper reports the discovery by the EXOSAT Observatory of three equally spaced dips in the flux from 4U 1755 - 33 that suggest the binary period of this system to be 4.4 hr. The duration of each dip is of order 30 minutes with considerable variability within each event on time scales between 30 and 600 s. The variations are spectrally independent with a maximum reduction in flux of 40 percent. The similarity of these events to periodic absorption dips seen from 4U 1915 05 and MXB 1659 - 29 suggests that they are caused by the regular occultation of the X-ray source by a thickened region of an accretion disk at the point where it is fed by the gas stream from a binary companion. The spectral independence of the 4U 1755 - 33 events is hard to explain by photoionization of the absorbing medium or partial occultation of an extended X-ray source. It is suggested that the metallicity of the absorbing medium may be at least a factor of 600 below cosmic values.