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

Abstract

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

15 pages, 7 figures, Accepted for publication in MNRAS

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• Pulse Frequency Fluctuations of Persistent Accretion Powered Pulsars
• Torque reversals and wind variations of X-ray pulsar Vela X-1
• AstroSat observations of the Be/X-ray binary XTE J1946+274 during 2018 and 2021 outbursts
• Long-term spin-down and low luminosity regime in the Be/X-ray binary pulsar GX 304-1
• On the origin of super-global spin-up trends of X-ray pulsars in HMXB systems
• Exploring the magnetic field of the ultraluminous X-ray pulsar NGC 4631 X-8