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

TL;DR

This study analyzes 46 years of data on Vela X-1, revealing nearly periodic long-term spin period variations and suggesting a possible solar-like cycle in the donor star, marking a first in wind-fed pulsar behavior.

Contribution

It reports the first observation of nearly periodic long-term spin period variations in a wind-fed accreting pulsar, proposing a potential solar-like cycle in the donor star.

Findings

Nearly periodic spin period variations on 5.9-year cycles.

Cyclic spin-up and spin-down behavior on 17-19 year timescales.

Erratic short-term spin episodes consistent with known accretion dynamics.

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.