EXOSAT measurements of the pulse period of VELA X-1.

Abstract

Seven new pulse period determinations of Vela X-1 have been made using the existing Exosat data. The measurements span one year and partly fill in the 3.2 yr gap between Tenma and Ginga measurements. The new results show that one pulse period of Vela X-1 continued with an epoch or near-constant period starting in 1981 and lasting up to at least May 1985. Using all the period determinations from 1975 to 1988 (including the new Exosat and Ginga values), the model is confirmed in which white torque noise is responsible for the pulse period changes of the neutron star. It is also shown that the reported TeV gamma-ray period measurements do not fall in the allowed range of this model.