Swift J1818.0-1607 – The missing link between pulsars and magnetars

Hyperaxion May 1, 2020 11:27 pm

Pulsars and magnetars are types of neutron stars, but so far, researchers have found few examples of objects with characteristics of both.

If it were a pizza, the dying star Swift J1818.0-1607 would be “half pulsar, half magnetar”. Officially classified as a magnetar, the star was observed emitting radio pulses at frequencies similar to those of pulsars. “I think it is safe to call it a potential missing link,” astrophysicist Marcus Lower of the Swinburne University of Technology, told ScienceAlert.

Swift J1818.0-1607 is only the fifth magnetar ever detected emitting pulsed radio waves. Even so, it behaves differently from others detected so far. “There’s still a lot we don’t know about this new magnetar, but there are clear similarities between it and the high-magnetic field pulsars,” explains Lower.

Magnetars

Swift J1818.0-1607 – The missing link between pulsars and magnetars
Artist’s impression of the magnetar in the star cluster Westerlund 1. (Credit: ESO/L. Calçada).

Magnetars are a type of neutron star, like pulsars, but with a much more powerful magnetic field (quadrillions of times more powerful than Earth’s and a thousand times more powerful than a normal neutron star).

Although not much is known about them, magnetars are believed to be a type of neutron star that was created during a Supernova explosion, similar to what happens with a pulsar. So far, only about 24 of these objects have been detected in our galaxy, and of these, only a few have been observed emitting radio waves.

Pulsars

A pulsar is a fast rotating neutron star.
A pulsar is a fast rotating neutron star. (Credit: Wikimedia Commons).

Pulsars, on the other hand, are much more common. They are highly magnetized neutron stars that form when a star more massive than the Sun collapses. After the supergiant phase, the star will eject most of its mass in a supernova explosion, and end its life as a neutron star. If the neutron star has a strong magnetic field, it will emit electromagnetic radiation directed in a cone around the magnetic poles, like a beacon, and it will be a Pulsar.

Since pulsars and magnetars are both neutron stars, it is logical to assume that there is an object with characteristics of both. But this is surprisingly rare, and the reason for this, according to researchers, would be that the magnetic field of the magnetars is too powerful to withstand pulsar-like radio emissions. New research, however, challenges this theory.

“The most likely reason is their radio beams simply don’t cross our line-of-sight,” believes Lower. “This isn’t too surprising, as their slow rotation periods and the high rate at which they’re slowing down over time causes them to have very narrow radio beams when compared to other pulsars.”

On March 12, observations detected an emission of pulsed X-rays under a gamma-ray explosion from Swift J1818.0-1607. Two days later, an initial analysis discovered that the magnetar is also the fastest rotating pulsar found to date – and the youngest, at around 240 years old.

For three hours, Lower and his team recorded the pulsed radio waves, apparently quite similar to those from other objects of the type. “At a glance, the radio pulses emitted by Swift J1818.0-1607 look quite similar to those from the four other radio magnetars. They’re very narrow and sometimes composed of multiple millisecond-long bursts,” said Lower.

However, by looking more closely at the luminosity of the pulses at different radio frequencies, the researchers realized that there was a drastic drop in brightness when switching from low to high frequencies. “While this is similar to many ordinary radio pulsars, it is very different to the pulses seen from other magnetars. They tend to have an almost constant brightness across the radio spectrum,” adds the researcher.

The study is preliminary, but it indicates that the mechanism behind the radio explosions in the two categories of objects could be similar, or suggest that at least some magnetars may evolve from pulsars. As usual, more observations are necessary.

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