Last summer we introduced you to a weird neutron star named Swift J1818.0-1607. This particular bundle of densely packed neutrons was first spotted flickering in X-rays in a way characteristic of a special kind of neutron star called a magnetar, which is a neutron star with an extremely powerful magnetic field. And it also was observed to give off pulses of radio emission consistent with it also being a pulsar. This combination of characteristics is extremely rare, and astronomers from the ARC Centre of Excellence for Gravitational Wave Discovery used the Parkes radio telescope to perform follow-up observations between May and October 2020. 

During this time, the star was observed to change its behavior in completely novel ways. It started out behaving like a pulsar, giving off blasts of radio light that were brighter at the low-frequency end of the dial. Over time, however, this would change, and by July 2020, it was switching between acting like a pulsar and acting like a more normal magnetar, which gives off radio waves with similar power across the frequencies.

With your standard pulsar, you have a fast-rotating star, and the magnetic pole of the star is out of alignment; every time the magnetic pole sweeps past, you see a pulse. With a magnetar, well, we’re still figuring that out, but we thought their magnetic and rotational poles were aligned. This star — this weirdo neutron star that appears to be both a magnetar and a pulsar — this star appears to have a magnetic field that is twisted up in ways that look like nothing we’ve ever seen before.

According to Ph.D. student Marcus Lower: From our observations, we found that the magnetic axis of J1818 isn’t aligned with its rotation axis. Instead, the radio-emitting magnetic pole appears to be in its southern hemisphere, located just below the equator. … This is the first time we have definitively seen a magnetar with a misaligned magnetic pole.

In a new paper in Monthly Notices of the Royal Astronomical Society, with Lower as the first author, the magnetic field is described as having two closely spaced poles that are connected by distorted magnetic field lines. This is like if you shoved a horseshoe magnet into the star, with its north and south poles sticking out the side of the star.

And, if this isn’t weird enough, in August the magnetic field temporarily rearranged itself. Lower goes on to explain: Our best geometric model for this date suggests that the radio beam briefly flipped over to a completely different magnetic pole located in the northern hemisphere of the magnetar.

Exactly what is going on is still being figured out. There are plans to continue observing this wild object to try and understand what behavior is normal and abnormal for it. It’s thought this may be a fairly recently formed neutron star, and it could be this is just the normal misbehavior of a star settling into a new configuration. We don’t know. Hopefully, we’ll be back talking about this object a third time, explaining exactly what is going on, in the not too distant future.

More Information

OzGrav press release

“The dynamic magnetosphere of Swift J1818.0–1607,” M E Lower et al., 2020 December 14, Monthly Notices of the Royal Astronomical Society