In a new study published today in Nature, we report the discovery of a new long-period transient – and, for the first time, one that also emits regular bursts of X-rays.

Long-period transients are a recently identified class of cosmic objects that emit bright flashes of radio waves every few minutes to several hours. This is much longer than the rapid pulses we typically detect from dead stars such as pulsars.

What these objects are, and how they generate their unusual signals, remains a mystery.

Our discovery opens up a new window into the study of these puzzling sources. But it also deepens the mystery: the object we found doesn’t resemble any known type of star or system in our galaxy – or beyond.

An image of the sky showing the region around ASKAP J1832-0911. The yellow circle marks the position of the newly discovered source. This image shows X-rays from NASA’s Chandra X-ray Observatory, radio data from the South African MeerKAT radio telescope, and infrared data from NASA’s Spitzer Space Telescope. Author provided

Watching the radio sky for flickers

There’s much in the night sky that we can’t see with human eyes but can detect when we look at other wavelengths, such as radio emissions.

Our research team regularly scans the radio sky using the Australian SKA Pathfinder (ASKAP), operated by CSIRO on Wajarri Yamaji Country in Western Australia. Our goal is to find cosmic objects that appear and disappear (known as transients).

Transients are often linked to some of the most powerful and dramatic events in the universe, such as the explosive deaths of stars.

In late 2023, we spotted an extremely bright source, named ASKAP J1832-0911 (based on its position in the sky), in the direction of the galactic plane. This object is located about 15,000 light years away. This is far, but still within the Milky Way.

An overhead view of large white radio dishes under a bright blue sky littered with clouds and a red earth underneath. — Some of the ASKAP antennas, located at Inyarrimanha Ilgari Bundara, the Murchison Radio-astronomy Observatory in Western Australia. CSIRO

A dramatic event

After the initial discovery, we began follow-up observations using telescopes around the world, hoping to catch more pulses. With continued monitoring, we found the radio pulses from ASKAPJ1832 arrive regularly – every 44 minutes. This confirmed it as a new member of the rare long-period transient group.

But we did not just look forward in time – we also looked back. We searched through older telescope data from the same part of the sky. We found no trace of the object before the discovery.

This suggests something dramatic happened shortly before we first detected it – something powerful enough to suddenly switch the object “on”.

Then, in February 2024, ASKAPJ1832 became extremely active. After a quieter period in January, the source brightened dramatically. Fewer than 30 objects in the sky have ever reached such brightness in radio waves.

For comparison, most stars we detect in radio are about 10,000 times fainter than ASKAPJ1832 during that flare-up.

A lucky break

X-rays are a form of light that we can’t see with our eyes. They usually come from extremely hot and energetic environments. Although about ten similar radio-emitting objects have been found so far, none have ever shown X-ray signals.

In March, we tried to observe ASKAPJ1832 in X-rays. However, due to technical issues with the telescope, the observation could not go ahead.

Then came a stroke of luck. In June, I reached out to my friend Tong Bao, a postdoctoral researcher at the Italian National Institute for Astrophysics, to check if any previous X-ray observations had captured the source. To our surprise, we found two past observations from NASA’s Chandra X-ray Observatory, although the data were still under a proprietary period (not yet public).

We contacted Kaya Mori, a research scientist at Columbia University and the principal investigator of those observations. He generously shared the data with us. To our amazement, we discovered clear X-ray signals coming from ASKAPJ1832. Even more remarkable: the X-rays followed the same 44-minute cycle as the radio pulses.

It was a truly lucky break. Chandra had been pointed at a different target entirely, but by pure coincidence, it caught ASKAPJ1832 during its unusually bright and active phase.

A chance alignment like that is incredibly rare – like finding a needle in a cosmic haystack.

Artwork of a tube-shaped telescope in space with large solar panel arrays on one end. — NASA’s Chandra X-ray Observatory is the world’s most powerful X-ray telescope, in orbit around Earth since 1999. NASA/CXC & J. Vaughan

Still a mystery

Having both radio and X-ray bursts is a common trait of dead stars with extremely strong magnetic fields, such as neutron stars (high-mass dead stars) and white dwarfs (low-mass dead stars).

Our discovery suggests that at least some long-period transients may come from these kinds of stellar remnants.

But ASKAPJ1832 does not quite fit into any known category of object in our galaxy. Its behaviour, while similar in some ways, still breaks the mould.

We need more observations to truly understand what is going on. It is possible that ASKAPJ1832 is something entirely new, or it could be emitting radio waves in a way we have never seen before.