Pulsar failure: how physicists are understanding a cosmic mystery | Explained

A schematic diagram of a pulsar. The sphere in the center is the neutron star, the curves indicate the magnetic field lines and the two cones show the emitted radiation. The green line is the pulsar’s rotation axis. | Photo credit: Mysid (CC BY-SA 3.0)

The year was 1967. The Nathu La and Cho La clashes between the Indian and Chinese armies had just ended. A war was going on in Vietnam. The space race was at its peak. Around this time, a group of astronomers from the University of Cambridge assembled a set of antennas to use as a telescope to study radio waves emitted by distant stars.

When they began operating the matrix, two members of the group – Jocelyn Bell Burnell and Antony Hewish – noticed a set of signals that flashed periodically. They did not know their origins.

Today we know that the duo discovered the first pulsar, called PSR B1919+21.

The pulsar and the neutron

The pulsar turned out to be closely linked to a 1932 discovery, when James Chadwick discovered the neutron. When neutrons are in a group, they cannot have the same energies. Each neutron will have to be content with the smallest available energy level. If gravity tries to compress this collection of neutrons inward, their inability to “merge” into a common energy level will resist external pressure.

When heavy stars die, their cores implode. If they are heavy enough, they become black holes; but if not, they collapse just enough to form a ball of neutrons, with gravity not being strong enough to overcome their external pressure. This compact, superdense object is called a neutron star.

When the Cambridge group reported finding a pulsar, other scientists proposed several possibilities for the origin of the unusual pulsating signal. Many of them were also rejected (including extraterrestrial civilizations). Ultimately, the fact that the signals came from a very small area of ​​the sky and that they were repeated frequently led scientists to identify pulsars as spinning neutron stars.

Radio signals emitted near the poles of such a star would form a narrow cone that passes Earth with each rotation – like the light from a lighthouse shining on a ship at sea.

An animation depicting the beacon-like effect of a pulsar. Source: Michael Kramer (CC BY-SA 3.0)

A not-so-eureka moment

Soon, physicists discovered that the rotation of these neutron stars slowed down over time, and then they discovered why. They found that the energy “saved” by reducing the rotation rate was used to accelerate electrical charges outside the star, producing radio signals.

This explanation was satisfactory because it fit perfectly with their theories – until they discovered a problem in 1969. Two research groups, working separately, reported an abrupt and brief increase in the rotation rate of the pulsar PSR 0833-45.

This flaw remains unexplained 44 years later, although physicists have some ideas. To date, they have detected more than 3,000 pulsars and about 700 such failures. The data they accumulated and some physics ideas gave rise to some hypotheses about what these failures are and why they happen.

A curious feature

When scientists graphed the rotation rate of pulsars over time, they saw the familiar decreasing pattern. During a crash, the rate increases briefly before returning to the original value. They found this process quite slow. This is a big clue if we assume the failure is the result of something happening inside the star.

That is, if the neutron star’s interior were made mostly of regular matter, then all internal relative motions would be quickly damped by friction, in a matter of a few milliseconds. Therefore, the slow post-failure relaxation suggested that the neutrons inside the star were in a slippery, frictionless state, which physicists called a superfluid.

An image of the Vela Pulsar captured by the Chandra X-ray Observatory on July 6, 2003. | Photo credit: NASA

Superfluids have peculiar properties. A superfluid set in motion inside a container will continue to move. At a certain speed, a thin cylindrical portion will rotate in a vortex. As the container’s rotation rate increases, more vortices of this type appear. The number of vortices – or their number density – determines how quickly the fluid itself spins. This has intriguing ramifications for the pulsar.

The origin of the failures

The neutron star is a 20 km wide sphere with a solid crust and core. The crust, believed to be a network of iron-like nuclei, is interspersed with neutron superfluid. The core predominantly contains the superfluid and no solid parts.

The presence of superfluid in a rotating system immediately implies the existence of vortices. It turns out that vortices have less energy when they attach to crustal cores than if they don’t. And all natural phenomena prefer to have less energy than more. The phenomenon of vortices “sticking” to the nuclei is called fixation.

As the rotating neutron star loses energy to radiation, the crust slows down. On the other hand, the pinned vortices are not free to rearrange themselves, which means that the density of the vortices remains unchanged and maintains the rotational speed of the superfluid.

The difference in velocity between the crust and the superfluid leads to a force on the vortices that eventually overcomes attachment. At this point, the vortices are thrown outwards, reducing the speed of the superfluid. The angular momentum lost by the superfluid is gained by the crust and this brief increase in rotation is reflected as a gap in the pulsar’s timing data.

Note that several details of the failure mechanism are highly contested, including how they trigger in space and how they evolve over time. The topic is, therefore, fertile ground for scientific research – research that could help us understand the variety of physics that takes place inside a neutron star.

As Ghalib said: “The stars are one thing and look like another, these tricksters openly deceive us.”

• The Nathu La and Cho La clashes between the Indian and Chinese armies had just ended. A war was going on in Vietnam. The space race was at its peak. Around this time, a group of astronomers from the University of Cambridge assembled a set of antennas to use as a telescope to study radio waves emitted by distant stars.

• Radio signals emitted near the poles of such a star would form a narrow cone that passes Earth with each rotation – like the light from a lighthouse shining on a ship at sea.

• Physicists discovered that the rotation of these neutron stars slowed over time, and then discovered why. They found that the energy “saved” by reducing the rotation rate was used to accelerate electrical charges outside the star, producing radio signals.

The author is a fourth-year PhD fellow in the Department of Physics, Ashoka University.