Strange star activity in Milky Way galaxy puzzles astronomers

Astronomers have discovered a peculiar star, exhibiting a unique combination of radio waves and X-rays, unlike anything previously observed. 

This celestial object, located within the Milky Way galaxy approximately 15,000 light-years from Earth in the constellation Scutum, flashes every 44 minutes in both radio waves and X-ray emissions.

This newly identified star has been classified as an exotic member of "long-period radio transients," a class of celestial objects first identified just three years ago. These transients are characterised by bright bursts of radio waves that occur every few minutes to several hours. 

This extended period of emission sets them apart from typical pulsars, which are rapidly rotating neutron stars that blink on and off on timescales of milliseconds to seconds.

"What these objects are and how they generate their unusual signals remain a mystery," said astronomer Ziteng Wang of Curtin University in Australia, lead author of the study published this week in the journal Nature.

In the new study, the researchers used data from the National Aeronautics and Space Administration's (Nasa) orbiting Chandra X-ray Observatory, the Australian SKA Pathfinder (ASKAP) telescope in Australia and other telescopes.

While the emission of radio waves from the newly identified object is similar to the approximately 10 other known examples of this class, it is the only one sending out X-rays, according to astrophysicist and study co-author Nanda Rea of the Institute of Space Sciences in Barcelona.

The researchers have some hypotheses about the nature of this star. They said it may be a magnetar, a spinning neutron star with an extreme magnetic field, or perhaps a white dwarf, a highly compact stellar ember, with a close and quick orbit around a small companion star in what is called a binary system.

"However, neither of them could explain all observational features we saw," Wang said.

Stars with up to eight times the mass of our sun appear destined to end up as a white dwarf. They eventually burn up all the hydrogen they use as fuel. Gravity then causes them to collapse and blow off their outer layers in a "red giant" stage, eventually leaving behind a compact core roughly the diameter of Earth — the white dwarf.

The observed radio waves potentially could have been generated by the interaction between the white dwarf and the hypothesised companion star, the researchers said.

"The radio brightness of the object varies a lot. We saw no radio emission from the object before November 2023. And in February 2024, we saw it became extremely bright. Fewer than 30 objects in the sky have ever reached such brightness in radio waves. Remarkably, at the same time, we also detected X-ray pulses from the object. We can still detect it in radio, but much fainter," Wang said.

Wang said it is thrilling to see a new type of behaviour for stars.

"The X-ray detection came from Nasa's Chandra space telescope. That part was a lucky break. The telescope was actually pointing at something else, but just happened to catch the source during its 'crazy' bright phase. A coincidence like that is really, really rare - like finding a needle in a haystack," Wang said.