Astronomers Discover a Mysterious Star That Flashes Every 20 Minutes. But What is it?

Just 4,000 light-years from Earth is a strange, star-sized object. It’s been observed by radio telescopes, but astronomers aren’t sure what it is. They call it a long period transient.

Transients are objects in the sky that change over some period of time. Fast transients are things such as pulsars, which emit a bright flash over a period of seconds or milliseconds. Slow transients are objects such as supernovae, which grow to extreme brightness over days or months. This new object is transient three times an hour. About every 18 minutes, it becomes one of the brightest radio objects in the sky, with its flash lasting anywhere from half a second to nearly a minute. Its long period and extreme brightness are what makes it so unusual.

One idea is that the object is a hypothetical object known as an ultra-long period magnetar. Magnetars are neutron stars, the same as pulsars, but magnetars have much stronger magnetic fields. Most magnetars are thought to rotate as quickly as pulsars, but their strong magnetic fields could interact with surrounding ionized gas in a way that causes it to slow down significantly. This would turn it into a kind of slow rotating pulsar. The problem with this idea is that astronomers have thought ultra-long period magnetars wouldn’t be nearly so bright. Another idea is that the object is a strange type of white dwarf, but it isn’t clear how a white dwarf could become so radio bright.

An artist view of the object as a magnetar. Credit: ICRAR

Based on observations, we do know the object has an intense magnetic field. The radio light we see from the object is highly polarized. Charged particles emit highly polarized light when they interact with a strong magnetic field. We also know the transient can’t simply be a standard pulsar effect. Pulsars emit regular flashes because their rotation sweeps a beam of intense radio light across the sky. We see a radio flash every time the beam sweeps our way, similar to the flash of a lighthouse. This object would flash about every 18 minutes, but these flashes would only happen over the course of a few hours. The team saw the object shift between active and quiet periods during their observation runs. So some strange happenings are going on.

Of course, the most exciting idea is that the transient object is something we don’t expect. Perhaps a newly formed black hole, or a hypothetical quark star. With only one example, it’s difficult to narrow down the possibilities. So the team is searching for similar objects in order to solve the mystery they never expected to find.

Reference: Hurley-Walker, N., et al. “A Radio Transit with Unusually Slow Periodic Emission.” Nature 601 (2022): 526-530.

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