ASKAP and Chandra catch pulses from Long-Period Transient

A new class of astronomical objects is beginning to challenge our understanding of compact stars' behaviour. Known as Long-Period Transients (LPTs), these enigmatic sources emit pulses of radio waves, like pulsars - but with a twist.

Unlike the familiar pulsar population, which typically spin at milliseconds to seconds frequencies, LPTs are much slower, pulsing on timescales of minutes to hours. Their unusual behaviour has astronomers worldwide intrigued, as they may represent a new kind of astrophysical phenomenon - or perhaps even new physics.

To date, only about 10 LPTs have been discovered, thanks largely to the wide-field capabilities of next-generation radio telescopes. Today, scientists from the International Centre for Radio Astronomy Research (ICRAR) and their international collaborators have published their findings in the journal Nature, announcing a significant new addition to this mysterious class: an object called ASKAP J1832-0911, detected using the ASKAP telescope (owned and operated by Australia’s national science agency, CSIRO) on Wajarri Country in Western Australia.

Located around 15,000 light-years away in the Milky Way, ASKAP J1832-0911 emits a two-minute-long radio pulse every 44 minutes - making it far too slow to be a typical pulsar. Based on its properties, astronomers suggest it could be an ultra-magnetised neutron star (a magnetar) or possibly a white dwarf in a binary system. Its true identity, however, remains uncertain.

What sets ASKAP J1832-0911 apart from previously known LPTs is that, for the first time, coincident X-ray emission have also been detected from such an object. The Chandra X-ray Observatory - a NASA space-based telescope - serendipitously observed the same region of sky at the same time as ASKAP, detecting faint X-ray pulses aligned with each radio burst.

“Discovering that ASKAP J1832-0911 was emitting X-rays felt like finding a needle in a haystack,” said lead author Dr Ziteng (Andy) Wang from the Curtin University node of ICRAR. 

“The ASKAP radio telescope has a wide field view of the night sky, while Chandra observes only a fraction of it. So, it was fortunate that Chandra observed the same area of the night sky at the same time.”

Astronomers study the Universe by observing light across the entire electromagnetic (EM) spectrum - from low-frequency radio waves to high-energy gamma rays. In recent years, they’ve even begun detecting cosmic phenomena using gravitational waves, offering a completely new channel of information. 

Each portion of the EM spectrum reveals different types of objects, processes, and interactions - from what’s involved in creating the emission, to how light is absorbed, scattered, or altered as it travels through interstellar space. There is so much data, simply in the light that falls upon us from the cosmos.

That’s why detecting the same object across multiple wavelengths becomes a powerful tool for celestial detectives - it allows astronomers to piece together a more complete picture of its physical nature. In the case of ASKAP J1832-0911, simultaneous detections in both radio and X-ray bands offer critical clues to the origin of its emission.

“This object is unlike anything we have seen before,” Dr Wang said. 

“ASKAP J1831-0911 could be a magnetar (the core of a dead star with powerful magnetic fields), or it could be a pair of stars in a binary system where one of the two is a highly magnetised white dwarf (a low-mass star at the end of its evolution).”

The fact that the radio and X-ray pulses occur in phase, meaning they align in time, suggests that both emissions may be generated in the same region of the star’s magnetosphere, or perhaps along the same magnetic field lines. This coordinated behaviour points toward a linked emission mechanism, rather than two unrelated phenomena.

According to second author Professor Nanda Rea from the Institute of Space Science (ICE CSIC) and Catalan Institute for Space studies (IEEC) in Spain, “Finding one such object hints at the existence of many more. The discovery of its transient X-ray emission opens fresh insights into their mysterious nature,” said Professor Rea.

“What was also truly remarkable is that this study showcases an incredible teamwork effort, with contributions from researchers across the globe with different and complementary expertise,” she said. 

Alongside ASKAP, Andy and his team drew on data from several other radio telescopes to better characterise ASKAP J1832-0911. These included CSIRO’s Australian Telescope Compact Array (ATCA) in Narrabri (NSW), the Giant Metrewave Radio Telescope (GMRT) in India, the MeerKAT array in South Africa, and the Very Large Array (VLA) in the USA.

In addition to Chandra, other space-based X-ray observatories also contributed to the study, including observations from the Einstein Probe, as well as data from XMM-Newton and NASA’s Swift Observatory.

To further broaden the study’s wavelength coverage, the team also reviewed optical and infrared data in hopes of identifying a possible binary companion. However, the results were inconclusive.

ASKAP J1832-0911 now joins a growing population of LPTs, which exhibit these long periods. These range from seven minutes (CHIME J0630+25) up to about three hours (GLEAM-X J0704-37) - which makes them far beyond the typical rotation rates of the known pulsar population.

Just like the other LPTs, J1832-0911 emits highly linearly polarised radio pulses, which indicates that there is an ordered magnetic field associated with the progenitor system. When plotting its location on the transient-phase plot, it also resides in a similar region as other LPTs. The unique difference between this object and the others, is that J1832-0911 is the first and only LPT to be detected simultaneously in both radio and X-ray wavelengths. 

One potential model that was considerwed was that this could be a neutron star in a binary system with a low-mass companion star, such as a spider pulsar system or the low-mass X-ray binary (LMXB) systems that form millisecond pulsars.

The data however suggests this cannot be the case, as quasi-siultaneous position of both the radio and X-ray signals rules out the spider pulsar system, as well typical LMXB behaviour. A big factor lies in the 44-minute periodicity. LMXB systems transfer mass (and momentum) to the neutron star, causing it to spin up rapidly, rotating them at times to hundreds of times per second. This new object is far too slow for this scenario.

Another potential model that was also considered was a white dwarf binary system, similar to two LPT systems (ILT J1101+5521 and GLEAM-X J0704-37) - both of which are associated with M-dwarf stars. However, ASKAP J1832-0911 is up to four orders of magnitude more luminous in the radio regime, and exhibits strong linear polarization, indicating a stable and ordered magnetic field.

Using models, estimates of the inferred White Dwarf’s magnetic field were drawn and indicate that they could be as high as those in the millisecond pulsar range, which is challenging to explain with existing white dwarf physics. Whilst it still remains as a speculative hypothesis, ASKAP J1832-0911 could still be a white dwarf binary system, but in a more active evolutionary stage from what we know of these types of systems. It could be undergoing accretion, which could go towards explaining the X-ray emissions. 

The jury is still out on what it could be. Which means, more exciting science to come.

We acknowledge the traditional owners of the lands in which our instruments are based. ASKAP is located on the traditional lands of the Wajarri Yamatjii people, and the Australia Telescope Compact Array is located on the traditional lands of the Gomeroi people.