A New Long-Period Transient Reveals Its Secrets

In recent years, radio astronomers have been racing to understand a strange new cosmic phenomenon that appears to challenge our understanding of how some compact objects behave. What they’ve detected are signals with some of the hallmarks of a beamed radio emission, like that of a pulsar, but repeating far more slowly than ordinary pulsar models can explain. The emissions are polarised, indicating strong magnetic fields, but scientists aren’t sure if it's like that of a neutron star. Furthermore, of the few observations of this phenomenon already made, there appear to be some inconsistencies across the population.

Enter stage right, the Long-Period Transients  (LPTs).

Unlike the more familiar sources detected by radio telescopes - such as pulsars, which rotate on their axis tens to hundreds of times per second, or distant supermassive black holes powering ancient galaxies - the observed radio emissions from LPTs repeat at timescales of minutes to hours. Some LPTs produce bursts of radio emission that suddenly switch on, pulse for a short period, then disappear again for hours, or in some cases weeks, at a time. Others exhibit complex pulse profile structures that vary over time. All of this points to the emission process remaining poorly understood.

To date, only a small number of LPTs have been discovered, with each new detection adding another piece to the puzzle, whilst simultaneously making the holistic picture a little more muddled.

That’s not to say that there aren’t any ideas of what they may be. In fact, astronomers are closing in on the answer. In some cases, they’ve proposed that LPTs may be ultra-long-period magnetars: highly magnetised neutron stars rotating very slowly. Others have suggested that at least some LPTs may instead be white dwarf binary systems, in which intense magnetic interactions between two closely orbiting stars generate the radio bursts.

Now, an international team led by astronomers from the University of Sydney has reported on a newly discovered object known as ASKAP J174508.9−505149 (‘ASKAP J1745’ hereafter), which may provide one of the clearest clues in this cosmic mystery yet.

“For the first time, we have pinpointed the origin of these signals, confirming the source to be a ‘cataclysmic variable’, or an accreting white dwarf star,” said lead author and PhD candidate Kovi Rose.

Looking across the electromagnetic spectrum with both ground-based and space-based telescopes capable of observing radio, optical, ultraviolet, and X-ray wavelengths, Rose and his team have identified ASKAP J1745 as an accreting white dwarf binary system. The discovery strengthens growing evidence that at least some LPTs are powered not by neutron stars, but by magnetically interacting binary systems involving white dwarfs.

“Long-period radio transients have puzzled astronomers for years,” said Rose. 

“We’ve only found about a dozen, and their origins have been unclear. Now, we’ve been able to show that the source for one of these transients comes from a white dwarf actively pulling material from a companion star.”

What makes LPTs so intriguing is that they pulse far too slowly for the conventional pulsar emission model to comfortably account for, yet they also appear relatively compact. Some emit radio bursts only once every few hours, while others appear to switch off entirely before suddenly returning. Several also exhibit unusual polarisation properties and drifting signals across radio frequencies, hinting at complex magnetic environments and plasma interactions that researchers are still trying to understand. With only a handful of known examples, astronomers are effectively trying to build an understanding of these objects and/or the systems producing them. 

ASKAP J1745 was first discovered using the ASKAP radio telescope (owned and operated by Australia’s national science agency, CSIRO) during a search for circularly polarised radio sources in the 1.365 GHz RACS program. Follow-up observations with the MeerKAT radio telescope in South Africa refined the object’s position, allowing Rose and the team to identify an optical counterpart in data from the Gaia spacecraft. But it was when the team turned optical telescopes toward the object that the real picture began to emerge. Spectroscopy revealed strong, narrow hydrogen and helium emission lines - the signatures of a compact binary system involving a highly magnetised white dwarf actively accreting material from a nearby companion star.

“These emissions are all tied to the orbital motion of the system,” said Rose. “But interestingly, the radio and X-ray signals don’t peak at the same time, which tells us they’re being produced in different regions of the system.”

At the heart of ASKAP J1745 is a compact binary system locked in an extremely close orbit. One member of the pair is the white dwarf - the stellar remnant left behind after a Sun-like star reaches the end of its life and sheds its outer layers. White dwarfs are approximately the size of Earth and packed with a lot of mass, making them very dense objects. They can also possess very strong magnetic fields.

Orbiting ASKAP J1745 is a much smaller companion star, likely a low-mass red dwarf or a brown dwarf, according to the findings, which have been published in the journal Nature Astronomy. Both objects circle around a centre of mass within the system roughly once every 1.4 hours. This means that they’re very close to each other - so close that the White dwarf is actively pulling material away from its companion through gravity, causing it to heat up and emit X-rays as it falls towards the more massive compact remnant. Astronomers refer to these systems as cataclysmic variables because eventually the infalling material reaches pressures and temperatures to ignite a hydrogen fusion reaction - a thermonuclear explosion.

However, ASKAP J1745 does not appear to be a typical cataclysmic variable. The combination of strong helium emission lines, narrow Balmer lines, and the flat optical spectrum suggests that it indeed possesses a powerful magnetic field, which is likely funnelling some of the infalling material along magnetic field lines rather than the more traditional model of mass transfer via a conventional accretion disk. Systems like this are known as magnetic cataclysmic variables.

“The white dwarfs in non-magnetic cataclysmic variables can still have extremely strong magnetic fields,” said Rose. “In magnetic systems, the magnetic field is so strong that it can synchronise the orbit to the white dwarf's rotation and, in some cases, prevent the formation of an accretion disk.”

“These systems are natural laboratories. They allow us to test our understanding of how matter behaves in strong magnetic fields and under intense gravitational forces.”

Whilst the optical observations revealed the nature of the binary system itself, the radio and X-ray observations really highlighted to Rose and the team just how unusual ASKAP J1745 really is.

The radio pulses from the system repeat on a period of approximately 1.4 hours - closely matching the orbital period of the binary system. However, unlike the regular ticking of a pulsar, the radio behaviour of ASKAP J1745 appears dynamic. The observed pulses would occasionally disappear for several hours before returning, and the emission itself drifted up and down in radio frequency over time.

The bursts are also highly polarised, indicating that the radio waves are being generated within an intense magnetic environment.

Additionally, the pulses themselves are not identical from one to the next. Some appear as narrow-band structures confined to small regions of the radio spectrum, whilst others evolve and change shape during observations.

“The changes in frequency and pulse morphology, as well as the intermittency of the bursts, were extremely puzzling at first,” said Rose. “But our simulation of the stars' interacting magnetic fields was able to reproduce nearly all of these phenomena, helping us constrain the dynamics and magnetic properties of the system.” 

To better understand the system, the team also observed ASKAP J1745 at X-ray wavelengths using both the Neil Gehrels Swift Observatory and the Einstein Probe space telescopes. They found that the X-ray emission also varied periodically on roughly the same 1.4-hour timescale, strongly suggesting that the radio and X-ray emission are both tied to the orbital motion of the binary system itself.

The X-ray observations also revealed that the system varies dramatically in brightness, changing by more than an order of magnitude between observations. This variability provides evidence that the rate at which material is being accreted onto the white dwarf is not steady, but fluctuates over time.

Taken together, the observations paint a picture of a highly active magnetic environment, where material transferred from the companion star is interacting with the white dwarf’s magnetic field in complex and rapidly changing ways. 

The team also noticed an interesting feature in ASKAP J1745’s data: narrow structures embedded within the radio pulses themselves, with some pulses displaying fine-scale intensity variations confined to small regions of the radio spectrum. However, the team realised that this type of behaviour has only ever been clearly observed in one other kind of system: the interaction between Jupiter and its volcanic moon Io.

In the Jupiter-Io system, charged particles that are spewed out from Io’s volcanoes into space move through Jupiter’s enormous magnetic field, generating powerful radio emission in the process. As this emission travels through the plasma near Jupiter, interference can create intricate patterns in the radio signal.

This research suggests that something similar might be occurring with the ASKAP J1745 system. In this case, plasma associated with the companion may be interacting with the strong magnetic field surrounding the white dwarf, producing the observed radio emission and the unusual modulation patterns embedded within it. 

“In the past, the Jupiter-Io system has been used as a model for radio emission from stars,” said Rose. “Our discovery is the first binary system that shows similar patterns in the intensity and frequency of the emission, supporting this model and setting the foundation for future discoveries of interacting binaries.”

For astronomers studying LPTs, the discovery of ASKAP J1745 may represent one of the strongest pieces of evidence yet that at least some members of this mysterious population originate from interacting white dwarf binary systems.

Prior to this discovery, several LPTs had already hinted at possible links to white dwarfs. Objects such as ILT J1101+5521 and GLEAM-X J0704−37 were both suspected to be associated with white dwarfs coupled with M dwarfs in binary systems, whilst the unusual system AR Scorpii had long been discussed as a possible evolutionary analogue to LPTs. However, the physical mechanism responsible for generating the bright radio bursts remained uncertain.

ASKAP J1745 now changes that picture.

“Some similar objects had been linked to binary systems before, but this is the first one where we can clearly see both stars and the accretion process in action,” said Professor Tara Murphy, Head of School at the University of Sydney School of Physics and Chief Investigator at the ARC Centre of Excellence for Gravitational Wave Discovery (OZGRAV).

The optical spectroscopy revealed the characteristic signatures of an accreting magnetic cataclysmic variable, whilst the radio and X-ray observations showed periodic emission tied closely to the orbital motion of the binary system. Together, these observations provide the clearest evidence so far that magnetically-driven accretion and binary interactions can generate the strange radio behaviour observed in at least part of the LPT population.

However, it still remains unclear if the recent findings across the LPT population represent the same kind of object. Some may still turn out to be slowly rotating neutron stars or magnetars, whilst others could involve entirely different physical systems. Instead, the emerging picture suggests that “long-period transient” may ultimately describe a broader observational class containing multiple kinds of astrophysical engines.

“This system gives us a way to decode these signals. It could help us determine whether other long-period transients are more like pulsars or like white dwarf systems, acting like a stellar Rosetta stone,” said Mr Rose.

That is what makes LPTs exciting to researchers like Rose.

“Each new discovery is helping us piece together the bigger picture,” he said. “We’re only just beginning to understand this new class of cosmic events.”

A decade ago, radio astronomers did not even know these objects existed. Now, as the sensitivity of instruments improves, sky surveys are conducted more rapidly, and datasets grow longer, we are starting to observe an increasingly dynamic and transient radio sky, filled with phenomena that challenge existing ideas and force researchers to rethink how compact objects behave.

For now, ASKAP J1745 represents another important piece of that puzzle - one that brings astronomers a little closer to understanding how some of the strangest radio pulses in the Universe are formed.

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. 

J1745-5051 video credit: C. Knox (OzGrav/Swinburne) & J.P. Pritchard (CSIRO).
AR Scopii video credit: ESO/L. Calçada/University of Warwick