Taking the pulse of a neutron star for almost five decades finds nearly cyclic spin changes on long time-scales

The X-ray binary system Vela X-1 consists of a neutron star (which contains as much mass as our Sun (~10³⁰ kg) but is very compact and has size of a small city having radius of ~10 km or so) and a supermassive star which has mass of ~23 times of our Sun and is ~34 times bigger in size than our Sun! The stars orbit around their common center of mass. The neutron star rotates once every ~283 s and shines brightly in X-rays by accreting gas from the stellar wind emanating from the massive companion star. The captured gas falls in the strong gravitational potential of the neutron star and is funnelled along the strong magnetic field of the neutron star and hits the poles of the neutron star at high velocities making them hot which generates thermal X-rays during the process. The magnetic axis and the rotation axis of the star is misaligned (like our Earth) which generates X-ray pulses akin to a lighthouse beacon.

Figure 1: Schematic of Vela X-1 X-ray binary system. The dotted arrows show the stellar wind emanating from the companion star which is all pervading. Image credit: Amar Deo Chandra, License: Attribution (CC BY 4.0)

Exploring the rotation of Vela X-1 for ~46 long years using different X-ray space observatories since 1975, a team of Indian scientists have constructed the longest known spin period history of an X-ray pulsar. They detect that the neutron star shows cyclic changes in spin period on long time-scales of about 5.9 years and this makes Vela X-1 only the third X-ray pulsar where such a long-term periodicity in spin changes has been detected. The pulsar also shows random spin period changes on shorter timescales which is a hallmark of X-ray pulsars which accrete from the stellar wind from the companion star.

Figure 2: Long-term spin history of Vela X-1 from 1975 February until 2021 June. The general trend of secular spin-down of the X-ray pulsar superposed on random episodes of spin-up and spin-down behaviour is discernible. The dashed curve shows the best fit polynomial to the spin evolution of Vela X-1. A careful look at the long-term pulse period evolution also suggests presence of 5.9 year cyclic trend in the long- term spin evolution of Vela X-1. The different markers indicate spin period measurements from 24 different X-ray space missions.

Slowing or speeding of a bicycle requires external torques–brakes if one wants to slow it
down or paddling harder if one wants to cycle faster. However, what causes intriguing rslowing down or speeding up of a massive neutron star on both short (a few minutes to few days) and long time-scales (a few months to many years) has been a long standing exciting puzzle and many attempts have been made to solve it to some extent in different X-ray binary systems.The scientists explore working of “astrophysical brakes” and “paddles” using various theoretical models which could make Vela X-1 spin-up or spin-down on cyclic time-scales of about 5.9 years. They suggest that the answer to this puzzle is probably blowing in the wind from the companion star! The companion star may iteself undergo cyclic changes on time-scales of about 5.9 years during which the amount of gas lost from the star changes
in a cyclic manner modulating those “astrophysical brakes” and “paddles”!

Figure 3: An artist’s conception of spin-up (left panel) and spin-down (right panel) scenario of a
neutron star. When the external torque acts in the same sense (clockwise) as that of the rotation of
the neutron star (clockwise) it causes the neuton star to rotate faster leading to spin-up of the
neutron star (left panel). But if the external torque acts in the opposite sense (anti-clockwise) as
that of the rotation of the neutron star then it acts like a “brake” and causes the neutron star to
spin-down (right panel). Image credit: Amar Deo Chandra, License: Attribution (CC BY 4.0)

This discovery opens a new paradigm of exploring spin changes on long time-scales in other X-ray pulsars and attempts to understand the underlying phenomena which causes long-term nearly cyclic spin-changes in other X-ray pulsars.

We dedicate our work to the memory of Late Prof. Shashikumar Madhusudan Chitre, who sadly passed away early this year.

arXiv link to the paper: “Detection of nearly periodic spin period reversals in Vela X-1 on long time-scales: inkling of solar-like cycle in the donor star?”, Chandra et al. 2021 (to appear in the Monthly Notices of the Royal Astronomical Society)