Astronomers Unveil Rebel Neutron Star Defying Gravity with its Unusually Slow Six-Hour Spin

In our Milky Way, approximately 13,000 light years distant, a deceased star known as ASKAP J1839-075 is defying all expectations … at a remarkably slow pace.

Deceased stars — or neutron stars — typically rotate at incredible rates, yet a group of astronomers has recorded this recently discovered star completing a leisurely six-and-a-half-hour rotation for just one turn, which is thousands of times slower than anticipated.

“This could significantly alter our understanding of neutron star development,” said Yu Wing Joshua Lee, an astronomer from the University of Sydney and the principal author of the recent investigation.

The finding has been published in Nature Astronomy.

The researchers suspect that ASKAP J1839-075 is a “pulsar”, a high-energy neutron star that emits short bursts of radio waves.

However, the common belief is that as pulsars decelerate, they cease emitting radio waves, suggesting that ASKAP J1839-075 should be undetectable by radio telescopes.

So, what is happening?

What defines a neutron star?

Neutron stars are among the most extreme entities in the Universe.

These compact, dense stars are formed when the nucleus of a massive star implodes, resulting in a fiery detonation known as a supernova.

During the star’s collapse, it can shrink from a million kilometer diameter to a mere 10 kilometers.

This intense compression accelerates the rotation, akin to a figure skater spinning faster as they draw their arms closer to their torso.

Thus, rotating at extreme speeds is intrinsic to the nature of neutron stars. A complete spin usually occurs in mere milliseconds or seconds for these collapsed stars.

Should our Sun undergo the transformation into a neutron star, its current 27-day rotation period could escalate to 1,000 rotations per second.

[neutron star gif]

Through radio telescopes, astronomers can ‘detect’ radio wave pulses from Earth as the neutron star rotates, with this movement often described as a “cosmic lighthouse”.

Later in the life of the collapsed star, it was believed that as it expended energy and started to decelerate, the radio wave bursts observed from Earth would also cease.

“Once they surpass the [speed] threshold, we thought they’d be permanently silenced,” Mr. Lee remarked.

However, in recent years, astronomers have identified pulsars that appear to challenge that theory.

What unexpected findings have researchers encountered?

In early 2022 researchers discovered pulsars that spun on minute-long time frames instead of seconds, and by June of the previous year, researchers had come across an entity that took nearly an hour between pulses. These typically sluggish entities were termed “long-period radio transients”.

However, ASKAP J1839-075’s relaxed 6.45-hour rotation was unprecedented.

“The former record was 54 minutes, so this represented a substantial leap,” Mr. Lee stated.

“The team was genuinely astonished.”

Gemma Anderson, an astronomer at Curtin University not involved in this study but part of the team that discovered the first long-period radio transient, commented that 6.45 hours expands “our comprehension of physics”.

“A typical pulsar couldn’t rotate this slowly and emit radio signals,” she stated. 

“There seems to be some form of extreme particle acceleration … which is leading to its significant radio brightness over these prolonged time intervals.”

A fortunate discovery

Mr. Lee was probing for “unusual radio transients” by sifting through archival data from a sky survey conducted by CSIRO’s ASKAP radio telescope in remote Western Australia.

With scant prior knowledge of where these transients might emerge, the approach was to randomly select a point in the sky to observe if anything noteworthy materializes, Mr. Lee explained.

Within the archived data, the group identified a pulsar-like flicker from early January 2024. By the time the survey commenced, the signal was already beginning to diminish, restricting the team to only analyze the latter portion.

“Had the observation been scheduled a mere 15 minutes later, we would have completely overlooked it,” Mr. Lee remarked. 

It took an additional 14 observations to reveal its repetitive pulses, allowing for a deeper understanding of the type of entity it could signify. 

All discovered long-period radio transients to date involve Australian teams, and according to Dr. Anderson, Australia is especially well-situated to identify them due to our contemporary generation of radio telescopes.

“[The Murchison Widefield Array and the ASKAP telescope] serve as dual discovery mechanisms for these kinds of phenomena,” she noted.

The SKA-Low telescope, anticipated to achieve full operational capability by the conclusion of this decade, is set to be even more potent.

So what might be the reason?

While locating these anomaly creators has taken a few years, deciphering what could be generating these enigmatically slow pulses is proving notably more demanding.

Earlier studies have posited that other star types, such as white dwarfs (formed when stars of lesser mass than our Sun exhaust their fuel and collapse) or unique pulsars termed magnetars may be responsible for the sluggish pulses. 

However, the long-period radio transients identified thus far emit radiation in somewhat varying manners.

“They all possess unique characteristics,” Mr. Lee commented.

“We remain uncertain if they belong to the same family or if they represent a class of objects operating via different mechanisms.”

Dr. Anderson observed that there might be two different categories of objects, one group linked to white dwarfs and another related to magnetars. 

For ASKAP J1839-075, evidence indicates that it is improbable to be a white dwarf.

“This [study] effectively clarifies the various potential scenarios, concluding that in this instance, the isolated neutron star or magnetar scenario is the most plausible,” Dr. Anderson explained. 

The distinct characteristics were evident in the unique radio emissions of ASKAP J1839-075, coupled with the absence of stars observable via optical telescopes, which would typically be identified if the star were a white dwarf.

Even if this star is indeed a magnetar, its slow rotation remains quite unusual, as the majority of magnetars rotate approximately every two to ten seconds, necessitating further investigation to comprehend their function. 

According to Dr. Anderson, this is unlikely to be the final long-period radio transient scientists will encounter, even though the most prominent and apparent ones have likely already been discovered. 

With the simpler discoveries behind them, Dr. Anderson proposes that researchers might focus on unraveling more about how these rule-breaking stars could have emerged.

“Perhaps this opens an even broader discovery field where numerous objects are generating these [transients],” she added.