The source of TeV gamma-rays in Cen X-3 and Vela X-1

An experimentally verifiable model is developed to explain the discrepancy between detected TeV gamma-ray pulsed periods and neutron star spin periods in X-ray binary systems. It assumes that the TeV gamma-rays are produced at the X-ray period and not on the neutron star itself, but somewhere else in the orbital plane. It was successfully applied to both Cen X-3 and Vela X-1. In the case of Cen X-3, an emission region was identified, with a 99.5% significance. This region trails the neutron star by 70 degrees and is situated between the neutron star orbit and the companion. For Vela X-1 an emission region outside the orbit of the neutron star was found, with a probability for chance occurrence of 4 x 10(-5). This region leads the neutron star by 90 degrees. These detections were used to illustrate the characteristics of the model, especially parameters determining its applicability and possible limitations. (C) 1997 Elsevier Science B.V.