Simultaneous X-ray and optical observations of the X-ray dip source X1755-338.

Abstract

Simultaneous X-ray and optical observations have been made of the 'dipping' X-ray source X1755-338 which has a dip recurrence period of 4.4 hr. A sinusoidal modulation of amplitude 0.4 mag is found in the V-band light of the 18 mag optical counterpart of the X-ray source. The period of the optical modulation is consistent with the X-ray dip recurrence period. The optical minimum occurs about 0.15 cycles after the centre of the X-ray dip. The optical modulation is probably caused by the changing aspect of an X-ray-heated companion star or an asymmetric disc. The X-ray data confirm that the dips in X1755-338 are shallow and not associated with photoelectric absorption. The most likely explanation for this remains that the elements responsible for absorbing 0.5 - 10.0 keV X-rays have a very low abundance in the material that obscures the X-ray source.