Multi-Wavelength Views of New Galactic Radio Astronomy Mysteries

There’s a new, emerging class of astrophysical objects that have astronomers scratching their heads and challenging the physics rulebooks. They’re called Long-period transients (LPTs) and unlike the familiar pulsars, neutron stars or white dwarfs (referred to as compact remnants), these enigmatic sources exhibit properties that defy traditional categorisations.

What sets LPTs apart is their period - which is on the scale of minutes to hours, unlike the typical compact remnant periods typically in the milliseconds to seconds range, such as pulsar rotation periods. 

Adding to this mystery is their polarised emissions suggest complex magnetic field behaviour, and yet the origin of these emission processes remains poorly understood. To date, only a handful of LPTs have been found, with the majority only over the last few years.

However, advances in radio telescope sensitivity, with wider sky coverage at regular cadence are starting to reveal more of these objects, and pique the interests of astronomers. As a result, they are expanding their views beyond the radio regime into multi-wavelength studies (incorporating other parts of the electromagnetic spectrum), to try and unveil more about these peculiar sources and probe their nature.

After all, these could be an entirely new class of object that humans have never detected before, or potentially lead to some new understanding of physics that we are yet to learn about. 

Recently, two compelling multi-wavelength study cases of LPTs have been reported. The first is a 2.9-hour radio transient (GLEAM-X J0704−37) with an optical counterpart, and the second is an exceptionally bright 44.2-minute transient (ASKAP J1832−0911) with coincident X-ray emission. In both these cases, radio astronomers - who first made the detections - are starting to showcase how multi-wavelength observations will be crucial in unravelling the secrets of these strange new objects.

We know a lot about radio-emitting compact remnant objects due to decades of observations, which have produced a decent catalogue of sub-categorised classes (e.g., classical pulsars, millisecond pulsars, magnetars, etc.). We also have observed (outside the radio frequency regime) many white dwarfs, thermally cooling neutron stars, and X-ray binary systems that feature compact remnants using both ground and space-based instruments.

LPTs however challenge a few of the assumptions that are core to our understanding of radio-emitting compact objects. The defying feature - these very long periodicities - places them in uncharted territory. For example, pulsar radio emissions are considered to arise from their rapid rotation (milliseconds to seconds) and their powerful magnetic fields (in the realm of billions of times that of your fridge magnet). LPTs however, rotate much slower, with their periods stretching over minutes to hours. This slow rotation is not thought to generate the right conditions to cause the types of emissions we see from pulsars.

Which is why it has radio astronomers very excited - are LPTs a whole new class of astrophysical object, or are they just neutron stars or white dwarfs exhibiting behaviour that we’ve never seen before (both cases are equally exciting). Their unique emissions, and polarisation point to well-structured and organised magnetic fields and non-thermal coherent emission mechanisms, which suggests that this behaviour arises in an environment of intense magnetism and dynamic plasma physics -  and yet, they don’t neatly fit into the known models for pulsars, magnetars or double white dwarf systems.

Deeping this mystery is that these objects are rare (so far), likely due to our observation biases with historical radio sky surveys not tuned to detect their slow periodicities. This is why we are now only starting to see the rise of this population - as our instruments have become more sensitive, we’ve been able to repeatedly observe large portions of the sky at regular occurrence and computational hardware and techniques have become more efficient.

Associate Professor Natasha Hurley-Walker, from Curtin University and the International Centre for Radio Astronomy Research (ICRAR)) last week published a new paper in The Astrophysical Journal Letters, describing the discovery of GLEAM-X J0704-37, a remarkable member of the LPT family.

Working with Csanád Horváth, a Curtin undergraduate student at the time, the source was detected in the archival data of the Murchison Widefield Array (MWA) telescope - an odd-looking telescope that features many groups of smaller, spider-looking antennas that are spread across the red Earth at Inyarrimanha Ilgari Bundara, the CSIRO Murchison Radio-astronomy Observatory. These antennas, which are arranged in a 4 x 4 grid per station all work together to look at the whole sky at lower radio frequencies.

What they found was that GLEAM-X J0704-37 exhibits a periodicity of 2.9 hours - the longest yet found for any radio transient

Yet, another opportunity arose with the discovery of this object. Other LPTs so far have been found to mostly exist in the busy region of the sky that correlates to the central, Galactic Plane of the Milky Way. This is where the large majority of stars, gas, dust and other objects all reside - which can sometimes make it difficult to localise the object when there is another source also sitting on or near the direct line of sight. GLEAM-X J0704-37, however, was found away from this busy region and in a much quieter part of the sky around 5,000 light-years away.

“The long-period transients are very exciting, and for astronomers to understand what they are, we need an optical image,” said Hurley-Walker. “However, when you look toward them, there are so many stars lying in the way that it’s like 2001: A Space Odyssey. ‘My god, it’s full of stars!’.”

Seizing this opportunity, the team was able to then use another telescope, located in South Africa (called MeerKAT) to zone in on the location of the radio waves from a specific star. 

The radio observations revealed pulsations that exhibited complex microstructures that were highly linearly polarised, which suggests a compact radio emitter. The most plausible scenario would be a binary system with a low-mass, M dwarf star coupled with either a neutron star or a white dwarf. What could be causing the radio emissions might be the interaction of the M dwarf’s stellar wind as it collides with the magnetosphere of the compact object, causing particles to accelerate and produce coherent radio waves.

Driven by intrigue, the team then also used the SOAR observatory - a 4.1m optical/infrared telescope in Chile, to look at this very specific location and found a low-mass, M dwarf-class star.

“Our new discovery lies far off the Galactic Plane, so there are only a handful of stars nearby, and we’re now certain one-star system, in particular, is generating the radio waves.”

“An M dwarf alone couldn’t generate the amount of energy we’re seeing,” said Hurley-Walker.

“The M dwarfs are low-mass stars that have a mere fraction of the Sun’s mass and luminosity. They constitute 70 per cent of the stars in the Milky Way, but not one of them is visible to the naked eye.”

“Our data suggests that it is in a binary with another object, which is likely to be a white dwarf, the stellar core of a dying star. Together, they power radio emission.”

Another breakthrough in the emerging LPT scene was announced last week when Curtin University’s Post-Doc Dr. Ziteng Wang published his pre-print paper on ASKAP J1832-0911 - the first LPT object with a coincident X-ray and radio emission. Discovered in ASKAP’s Variable and Slow Transient (VAST) survey, this LPT has a 44.2-minute periodicity, placing it firmly in the population of this emerging population.

"This discovery was very serendipitous. The data was collected by another project, which is not targeting this source, but covering this source," said Dr. Wang. "The observation just happened to be scheduled during the same time the source was extremely bright in radio - so totally a coincidence!" 

What makes Dr Wang’s paper fascinating is that along with the radio frequency detections already made, for the first time, X-ray detections were also made and shown to correlate with the radio signal. The X-ray data was obtained through the space-based Chandra X-ray Observatory. Both the radio and X-ray luminosities and spectral signatures vary together, confirming the compact nature of the source. 

"As many previously discovered LPTs do not emit bright X-ray emission, when we reduced the data we did not expect to see it at all. But later on, we found that there is a detection, and there is also a period. We just felt like our dream had come true. It is really exciting," he said.

From the radio signals, and using a relatively new tool called CRACO, the team were able to constrain the distance to this object to about 14,700 light-years away, however, it is located deep in the Galactic Plane, in a region that is densely filled with stars, gas and dust making it challenging to try and find an optical companion, as was the case with GLEAM-X J0704-37.

Instead, serendipitous observations made with Chandra’s ACIS instrument in February 2024 were able to associate an uncatalogued X-ray source that was positionally coincident with ASKAP J1832-0911. When running a search on the periodicity of the X-ray signal they found that it was a match with the radio periodicity, thus concluding that the same source of the signal (just in different parts of the electromagnetic spectrum).

This makes ASKAP J1832-0911 the only LPT detected with an X-ray counterpart signal, which is crucial for understanding the nature of this object. The X-ray pulses offer direct evidence of compact object characteristics, while the radio polarisation points to an organised and structured magnetic field.

The most likely scenarios to produce such a system would involve a highly magnetised neutron star (a magnetar) or an ultra-magnetised white dwarf in a binary system. Both explanations challenge existing models of formation and emission mechanisms.

"Both magnetar and white dwarf in a binary system can produce X-ray and radio emission," said Dr Wang. "Its radio properties align with the magnetar population, the pulsed X-ray emission is also expected from a magnetar. But its low quiescent X-ray luminosity infers that this source might be an old magnetar, which is believed to be radio-quiet." 

"A white dwarf binary is also possible, but having such a high radio luminosity, you need a very strong, stronger than the most magnetised WD observed, magnetic field for the WD."

"Follow-up observations in infrared and X-ray will help us confirm the nature of this object. Deep infrared (e.g., JWST) can help us either detect the companion or put a strong limit on the companion stellar type. If the source becomes X-ray bright again, you can follow it up with XMM to get a nice spectrum, and the spectrum will tell you the mechanism of the X-ray emission, and its physical parameters. We can compare these parameters with the known population, and therefore understand its nature."

"Correlated X-ray and radio emissions are not unexpected from magnetars. Given the correlation between X-ray and radio luminosity, also coming at almost the same phase, it implies that the X-ray emission must originate to the same field lines tied to the radio pulsations. This will be helpful in its further interpretations but its current properties cannot yet be fully explained by any known models. Some new models will likely need to be raised to understand them."

Both GLEAM-X J0704-37 and ASKAP J1832-0911 represent significant milestones in the exciting journey of uncovering what these LPTs are, in particular, that both cases present not only the radio wavelength observations but a counterpart electromagnetic signal that now gives astronomers like Associate Professor Hurley-Walker and Dr Wang extra tools to try and decode what these enigmatic objects might be.

These sources are challenging astrophysicists' existing models for radio-emitting compact objects and hint at tantalising new physics that involve magnetic fields, plasma interactions and coherent emission processes. These small encouraging steps now help the astronomy community to work towards more surveys, more analysis of archival data, and more multi-wavelength approaches to try to model and solve.

Important key questions remain, such as what is driving the emissions of these objects, are there any evolutionary links between the known classes of compact objects and LPTs, and how many more remain hidden due to our observational biases or instrumental limitations.

As we get closer to the next generation of extremely powerful radio telescopes, like the SKA being fully functioning, we are going to want to learn more about the LPTs, so that new surveys with these instruments can find more of them.

In the meantime, the LPTs are giving us a glimpse of the “unknown unknowns” aspects that comes with astrophysics, and offering a potential and exciting new way to take a look at some of the most extreme environments in the Universe.

First Video Credit: ESO/L. Calçada/University of Warwick
Second Video Credit: ICRAR