Reincarnation of pulsars

[Nature India Special Issue: Lighting the way in physics]

In the paper ‘On the origin of the recently discovered ultra-rapid pulsar’ (Curr. Sci. 51; 1096-99; 1982) Venkatraman Radhakrishnan and I argued that the then recently discovered ‘solitary’ millisecond pulsar must have been born in a low-mass X-ray binary system (LMXB) and spun up to an ultrashort period during mass accretion from a companion. In other words, although the pulsar was solitary when it was observed, it must have been a ‘recycled’ pulsar from an LMXB. Using this hypothesis, we predicted that the magnetic field of the pulsar must be 5x10⁸ G and this was confirmed by observations a few months later.

The scenario for resurrecting a pulsar from its graveyard was first advanced in an earlier publication from the Raman Research Institute (RRI) (G. Srinivasan and E.P.J. van den Heuvel Astron. Astrophys. 108, 143–147; 1982) while discussing the Hulse-Taylor pulsar. The pulsar had anomalous properties: its rapid spin rate suggested that it might be ‘young’, while its low magnetic field suggested that it might be a ‘very old’ star whose field had mysteriously decayed. The Hulse-Taylor pulsar was identified as the ‘first-born’ of the two neutron stars, the one that had been spun up, while the other unseen neutron star must be the ‘second-born’ and would have ‘normal’ properties similar to the Crab pulsar. The discovery of the double pulsar system in 2003 confirmed this prediction.

The discovery of a second millisecond pulsar further confirmed the recycling hypothesis. This one was in a binary system with a low-mass white dwarf companion as would be expected if the progenitors of millisecond pulsars were LMXBs. In a subsequent paper by researchers at RRI (D. Bhattacharya and G. Srinivasan Curr. Sci. 55, 327–330; 1986) this idea was pursued further and several bold predictions were made including that:

• Millisecond pulsars will be in binaries with low-mass white dwarfs as companions;

• There must be a very large population of these pulsars;

• Their magnetic fields will be close to 5x10⁸ G.

Although the recycling scenario quantitatively explained the properties of the Hulse-Taylor pulsar, it was not accepted for a long time, based on two objections:

If the pulsar was ‘spun up’ to its presently observed ultra-short period, then why didn’t we see pulsating X-ray sources with such short periods?

There was no plausible mechanism for the decay of magnetic fields of neutron stars.

For a neutron star to spin up to a short period, its magnetic field must be many orders of magnitude smaller than that of most pulsars. The low fields of recycled pulsars must somehow be the result of decay during the binary evolution. The difficulty was that in theory, it was not expected that the field would decay significantly even over cosmological timescales and there was no plausible connection between binary evolution and field decay.

A paper published by researchers at the RRI in 1990 provided the breakthrough (G. Srinivasan et al. Curr. Sci. 59, 1, 31–38; 1990). Before the discovery of neutron stars, Russian physicists had given compelling arguments for superconducting and superfluid states in the interior of neutron stars. Because the neutron star has a magnetic field and is rotating, both the proton superconductor and the neutron superfluid will be in a quantised vortex state. The novel mechanism for field decay discussed in the paper involved the interpinning of the quantised vortices in the proton superconductor and the quantised neutron vortices. As the first-born neutron star spun down because of the electromagnetic torque exerted by the stellar wind from the companion, the neutron vortices migrated towards the crust and in the process dragged the magnetic vortices with them. The field then decayed in the crust due to ohmic dissipation. To this day, this is the only plausible proposed mechanism for field decay.

One of the predictions made was that there must be two classes of recycled pulsars – those from massive binaries with magnetic fields of about 10¹⁰ G and those from low-mass binaries with fields of about 5x10⁸ G. Today, more than 2,000 pulsars have been discovered. The overwhelming majority of them are solitary and have magnetic fields in the range of 10¹² to 10¹⁴ G. They form a compact island in the magnetic field-period plane. There are two distinct populations of pulsars with binary companions well beyond this island of pulsars. Those whose progenitors were massive binaries – including the Hulse-Taylor pulsar – have fields of about 10¹⁰ G. The second distinct population consists of more than 100 millisecond pulsars, most of them in binaries with low-mass white dwarf companions. Their magnetic fields cluster in a narrow range of around 5x10⁸ G. Thus, the predictions made 40 years ago have been confirmed.

As mentioned above, one of the sustained objections to the recycling theory was the absence of evidence of rapidly spinning neutron stars in accreting X-ray binaries, which would provide definitive proof of the spin-up hypothesis. Dramatic evidence came in 2014 when several transient millisecond pulsars were discovered. Some LMXBs switched off now and then, revealing millisecond radio pulsars during the X-ray quiet phase. The smoking gun had been found.

There was one other prediction made by astronomers at RRI concerning millisecond pulsars. In 1988, when only three millisecond pulsars had been discovered, Srinivasan predicted that millisecond pulsars will be strong sources of gamma rays and, since a very large population of millisecond pulsars is expected to exist in our galaxy, there should a large population of millisecond pulsars that emit gamma rays (G. Srinivasan Adv. Space. Res. 10, 2, 167–178; 1990). More than 20 years after the prediction was made, the Fermi Gamma-Ray Space Telescope discovered over 100 gamma ray emitting millisecond pulsars all with fields of around 5x10⁸ G.

The notion of recycled pulsars was first advanced by RRI astronomers and they identified that the Hulse-Taylor pulsar was one such pulsar. Later, the solitary millisecond pulsar was also identified as a recycled pulsar. They made bold predictions in a series of papers and all of these have now been confirmed by observations. They are part of the many achievements made by researchers at the RRI that are being remembered as it celebrates its platinum jubilee year.