Most neutron stars spin rapidly, completing a rotation in seconds or even a fraction of a second. But astronomers have found one that takes its time, completing a rotation in 54 minutes. What compels this odd object to spin so slowly?
When a massive supergiant star explodes as a supernova, it leaves a collapsed core behind. The extreme pressure forces protons and electrons to combine into neutrons. Since they’re made almost entirely of neutrons, we call them neutron stars. These stellar remnants are extremely small and extremely dense. Only black holes have greater density.
Due to the conservation of angular momentum, neutron stars start to spin rapidly, often rotating as fast as several hundred times per second. Astronomers have found more than 3,000 radio-emitting neutron stars, and out of all of them, only a very small number rotate slowly.
We usually detect neutron stars by their electromagnetic radiation and call them pulsars. Astrophysicists also call the ones with slow rotations long-period radio transients. There’s uncertainty around their slow rotation speeds and if they’re even neutron stars, and the most recently discovered one isn’t helping remove the uncertainty.
In new research in Nature Astronomy, a team of researchers presented the discovery of ASKAP J1935+2148, a long-period radio transient about 16,000 light-years away. The paper is “An emission-state-switching radio transient with a 54-minute period.” The lead author is Dr. Manisha Caleb from the University of Sydney in Australia.
“Long-period radio transients are an emerging class of extreme astrophysical events of which only three are known,” the paper’s authors write. “These objects emit highly polarized, coherent pulses of typically a few tens of seconds duration, and minutes to approximately hour-long periods.”
Researchers have proposed different explanations for these long-period objects, including highly-magnetic white dwarfs and highly-magnetic neutron stars called magnetars. But the research community hasn’t reached a consensus.
ASKAP J1935+2148 has an extremely long period of 53.8 minutes and three distinct emission states. Its bright pulse state lasts between 10 and 50 seconds, and its weaker pulse state, 26 times dimmer, lasts about 370 milliseconds. It also exhibits what’s called a “quenched state” with no pulses.
Astronomers discovered the puzzling object accidentally while observing an unrelated gamma-ray burst with the Australian Square Kilometre Array Pathfinder (ASKAP) telescope in October 2022. The observations revealed ASKAP J1935+2148’s bright pulses of radio emissions. In about six hours of observations, the object emitted four bright pulses lasting from 10 to 50 seconds. Light curve inspections and follow-up observations with the MeerKAT radiotelescope revealed the object’s entire pulsing pattern.
“This discovery relied on the combination of the complementary capabilities of ASKAP and MeerKAT telescopes as well as the ability to search for these objects on timescales of minutes while studying how their emission changes from second to second! Such synergies are allowing us to shed new light on how these compact objects evolve,” said Dr. Kaustubh Rajwade, paper co-author and an Astronomer at the University of Oxford.
The three emission states, each different from the others, are puzzling. The researchers needed to verify that each signal from each state came from the same point in the sky. The fact that each signal had the same time of arrival (TOA), as determined by both ASKAP and MeerKAT observations, indicates a single source.
“What is intriguing is how this object displays three distinct emission states, each with properties entirely dissimilar from the others. The MeerKAT radio telescope in South Africa played a crucial role in distinguishing between these states. If the signals didn’t arise from the same point in the sky, we would not have believed it to be the same object producing these different signals.”
ASKAP detected the object’s strong, bright pulse mode, while MeerKAT detected its fainter, weak pulse mode. Both telescopes detected the quiescent mode.
“In the study of radio-emitting neutron stars, we are used to extremes, but this discovery of a compact star spinning so slowly and still emitting radio waves was unexpected,” said paper co-author Ben Stappers, Professor of Astrophysics at the University of Manchester. “It is demonstrating that pushing the boundaries of our search space with this new generation of radio telescopes will reveal surprises that challenge our understanding.”
The nature of the emissions and the rate of change of the spin periods strongly suggest that ASKAP J1935+2148 is a neutron star. However, the researchers say they can’t rule out a highly magnetized white dwarf. Since astrophysicists think that white dwarfs become highly magnetized as binaries, and there are no other white dwarfs nearby, the neutron star explanation is more likely.
The object’s radius also doesn’t conform to our understanding of white dwarfs. “However, the implied radius is ~0.8? solar radii, leading us to conclude that this source cannot be expected by standard white-dwarf models,” the researchers explain. White dwarfs are only slightly larger than Earth, which seems to eliminate one as the potential source.
Only follow-up observations and more dedicated studies can reveal the object’s true nature. Either way, whether it’s a white dwarf or a neutron star, the object will open another window into the extreme physics of either type of object. Our understanding of both objects is only decades old, so there’s bound to be lots left to discover.
“It is important that we probe this hitherto unexplored region of the neutron-star parameter space to get a complete picture of the evolution of neutron stars, and this may be an important source to do so,” the authors conclude.