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We explore the long-term evolution of the spin period of the High Mass X-rayBinary (HMXB) pulsar Vela X-1 over a period of 46 yr. Our analysis indicatesnearly periodic variations in the spin period of the pulsar on time-scales ofabout 5.9 yr. There is suggestion of an overall spin-down behaviour of thepulsar though it is noticed that the source appears to stay near itsequilibrium period 283.4 s since MJD 51000, with rather erraticspin-up/spin-down episodes around this value. Our study suggests nearly cyclicturnover in the spin behaviour of the pulsar from spin-up to spin-down regimeson time-scales of about 17-19 yr. To our knowledge this is the first report ofperiodic variation in the spin behaviour of a wind-fed accreting pulsar. Wealso observe erratic episodes of spin-up and spin-down behaviour on relativelyshorter time-scales which is a well known archetype of this wind-fed X-raypulsar. We investigate whether nearly periodic long-term spin period changes inthe pulsar can be explained by using known mechanisms of torque reversals inthe accretion powered neutron stars. It appears that changes in the accretionenvironment of the pulsar using current ideas can probably lead to long-termspin period changes in this X-ray pulsar.

Detection of nearly periodic spin period reversals in Vela X-1 on long time-scales: inkling of solar-like cycle in the donor star?