Just when astronomers think they’re starting to understand stellar activity, something strange grabs their attention. That’s the case with a newly discovered stellar object called ASKAP J1832-0911. It lies about 15,000 light-years from Earth and belongs to a class of stellar objects called “long-period radio transients.” That means it emits radio waves that vary in their intensity on a schedule of only 44 minutes per cycle. It does the same thing in X-ray intensities, which is the first time anybody’s seen such a thing coupled with long-period radio transits.
Why does it vary in both radio and X-rays like that? Figuring that mystery out is the job of Dr. Ziteng Wang of Curtin University in Australia and a team of astronomers. “Astronomers have looked at countless stars with all kinds of telescopes and we’ve never seen one that acts this way,” said Wang. “It’s thrilling to see a new type of behavior for stars.”
However, ASKAP J1832 (for short) exhibits even weirder behavior. Using Chandra and the SKA Pathfinder, the team found that it also dropped off in X-rays and radio waves dramatically over six months. So, what’s going on there?
A close-up image of ASKAP J1832 in X-ray and radio light. Credit: X-ray: NASA/CXC/ICRAR, Curtin Univ./Z. Wang et al.; Radio: SARAO/MeerKAT; Image processing: NASA/CXC/SAO/N. Wolk
What’s Causing ASKAP J1832’s Emissions?
The big questions about this weird object center around what it is and whether its behavior gives clues to its origin story. Is it typical of long-period radio transients? “We looked at several different possibilities involving neutron stars and white dwarfs, either in isolation or with companion stars,” said Dr. Nanda Rea of the Institute of Space Sciences in Barcelona, Spain. “So far, nothing exactly matches up, but some ideas work better than others.”
The science team is examining a few possibilities, but isn’t completely sure that a pulsar or a neutron star is at the heart of ASKAP J1832. A pulsar does have varying intensity in its emissions. That’s because it’s a stellar remnant, left over from a catastrophic event called a supernova explosion that marks the death of a massive star. The core of the star is all that’s left, and it’s spinning very rapidly. It gives off radiation, which appears as a pulsating signal as the object spins many times per second.
A neutron star, which is also the leftovers from a supernova explosion, isn’t a good explanation either. When such an object exists with a partner star, its gravity will suck material away from the partner star. That action causes variation in emission intensities, too. However, the research team doesn’t think that such a pair explains ASKAP J1832 because the intensities in the radio and X-ray emissions don’t match what these objects typically give off.
The team also doesn’t think it’s a magnetar, which is a neutron star with an intensely strong magnetic field. Magnetars are typically pretty old, and some of the signals from ASKAP J1832 aren’t typical of those, either. The only other possibility might be a white dwarf with a companion star. Such binaries do often give off strong radio and X-ray emissions that could fit the description of what Chandra and the SKA instruments saw. However, to make that work, the white dwarf would need an incredibly strong magnetic field – something that astronomers haven’t yet seen.
ASKAP J1832 does appear in the same field of view as a supernova remnant. It’s not likely to be associated, though, and is probably just a case of coincidental location.
So, What Is It?
Ultimately, the scientists have not figured out what’s causing ASKAP J1832 to feature such changes in its emission intensities. It could be an entirely new version of the objects they’ve already considered. More observations are needed to pin it down.
Beyond observations with Chandra and SKA, this region of space has also been studied by the SWIFT, the Very Large Array, the Australia Telescope Compact Array, the Giant Metrewave Radio Telescope, MeerKAT, and other facilities. Each of these observations has seen the intensity variations and helped establish baseline timings for the outbursts. For the moment, however, astronomers are still trying to fit what they’ve seen into models that will help them assign an origin and explanation for the emissions.
“We will continue to hunt for clues about what is happening with this object, and we’ll look for similar objects,” said team member Dr. Tong Bao of the Italian National Institute for Astrophysics (INAF) – Osservatorio Astronomico di Brera in Italy. “Finding a mystery like this isn’t frustrating — it’s what makes science exciting!”
For More Information
Eccentric ‘Star’ Defies Easy Explanation, NASA’s Chandra Finds
Detection of X-ray Emission from a Bright Long-period Radio Transient