Secular variation and short-term fluctuations of the pulse period of VELA X-1

Abstract

Data from new measurements (December 1981 to December 1982) of pulse period of Vela X-1 were joint-fitted with previously obtained measurements in order to determine the degree of secular variation and short-term fluctuation in the pulse period. The pulse period was found to increase at an average rate of 2.7 x 10 to the -4th per year between March 1979 and December 1981, and to stay at a constant value of 282.93 sec between December 1981 and December 1982. Fluctuations in the pulse period change were observed within the time scale of an orbital period superimposed on the secular trend of pulse period change. A correlation was found between the long-term period change and the orbital phase dependence of the X-ray spectrum, but no conspicuous correlation with X-ray luminosity was observed. No evidence was found for a change in orbital period or for the advance of periastron.