After a supernova the core of the remnant will be so compressed by the inward force of gravity that it forms a neutron star or a black hole.

If the remnant of the supernova is below the three solar mass threshold it forms a neutron star. Neutron stars consist of matter that is 100 million times denser than white dwarf matter. Neutron stars are also characterised by their strong magnetic fields and rapid rotation. Indeed, they can typically rotate at up to 1 000 times a second.

A neutron star may also be sub-categorised as a pulsar or magnetar. In the case of a pulsar the neutron star rotates extremely rapidly while its magnetic fields channel escaping matter and radiation into intense beams of energy emerging from their poles.

Magnetars are neutron stars with unusually slow rotational periods. They also have an unusually powerful magnetic field which shifts and creates huge outbursts of energy, including gamma ray outbursts. For example, if a star that is 30 times bigger than the sun leaves behind a magnetar, it will be 100 trillion times stronger than the earth’s magnetic field. And one that is 100 times bigger than the sun would create a hypernova, producing gamma ray bursts, and leaving behind a black hole.

There are 24 orbiting neutron star pairs in the Milky Way. If a pair came together it would produce a two-second gamma ray burst equal to the entire energy produced by our Sun in its lifetime.

Another option is an x-ray burster. This is an object that emits strong bursts of x-rays, lasting from a few seconds to a few minutes. The bursts are believed to occur when gas drawn from an orbiting companion star accumulates on the surface of a neutron star and triggers a nuclear-fusion chain reaction.

Pulsar

A pulsar is a rapidly rotating neutron star that emits brief pulses of radiation at short and precisely timed intervals as it spins around its axis. As Nasa explains, “The pulses of high-energy radiation we see from a pulsar are due to a misalignment of the neutron star’s rotational axis and its magnetic axis. Pulsars seem to pulse from our perspective because the rotation of the neutron star causes the beam of radiation generated within the magnetic field to sweep in and out of our line of sight with a regular period, somewhat like the beam of light from a lighthouse.”

The neutron star spins rapidly after a supernova explosion, and emits two beams of radio waves, light, and X-rays. These beams radiate in a circle because the star is spinning, and so it appears to pulsate on and off.

There are 1 800 known pulsars. The Crab Nebula contains a pulsar spinning at 30 times a second. It is the remnant of a supernova explosion in 1054. From Earth we detect it as a pulse of radio waves, X-rays and light, one pulse every rotation. The pulsar is only 16 kilometres in diameter.

Pulsars radiate their energy in pulses because it is “synchrotron radiation” in which swiftly moving electrons gyrate around the magnetic lines of force. The synchrotron axis is inclined to the axis of spin. And only if this synchrotron axis is pointing towards Earth will we be able to see the pulse.

Aside from radio pulsars and X-ray pulsars, there are also gamma ray pulsars, which are mostly magnetars.

Magnetar

A magnetar is a type of neutron star that has an extremely powerful magnetic field, as much as a trillion times that of Earth. It is thought that one in ten neutron stars become a magnetar by losing 90% of their mass.

A magnetar is certainly the strangest and most dangerous object known to astronomers. It is probably also the densest object in the universe, probably double that of a normal neutron star. The theory is that this density coupled with the right conditions of the dynamo mechanism convert heat and rotational energy to bring about an extreme magnetic field. Another theory is that they simply result from the collapse of stars with unusually high magnetic fields.

In any event, if a magnetar was just ten light years away it would be powerful enough to strip away our ozone layer. Evidently they very rare, and there are only 15 known magnetars in the Milky Way.

The first magnetar to be discovered was in 1979 when detectors both on Earth and on various satellites picked up a burst of gamma rays 100 times more intense than any previous known outburst. The source was traced to the Large Magellanic Cloud and named SGR 0525-66. “SGR” means soft gamma ray repeater.

Another magnetar named 1E 2259+586 is located in a supernova remnant CTB109. It is in the Constellation Cassiopeia, approximately 18 000 light years from Earth. According to Nasa research, the rotation of this magnetar is slowing down.

But the mechanism is little understood. These and several other magnetars are continuing to baffle astronomers struggling to understand their true nature. It may well be that there are several ways such an object could arise. There is even a proposal out there that suggests all pulsars began life as a magnetar.

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By Nigel Benetton, science fiction author of Red Moon Burning and The Wild Sands of Rotar.

Last updated: Saturday, 20 March 2021