Scientists find slowest spinning 'radio neutron star' — it breaks all the dead-star rules

Astronomers have discovered the slowest spinning radio wave-blasting neutron star ever seen; it takes almost an hour to complete a full rotation. 

That may sound rather fast, but these dead stars are known to spin so rapidly that some experience 700 full turns every second. Even the most leisurely of the about 3,000 radio-emitting neutron stars, or "pulsars," discovered so far complete a full rotation in a second or so.

This ultra-leisurely neutron star, however, designated ASKAP J1935+2148 and located 16,000 light-years from Earth, is emitting radio light at a rate too slow to even fit with current theories describing the behavior of these dense stellar remnants.

"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," Ben Stappers, a member of the team behind the  "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."

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Neutron stars grow old gracefully

Neutron stars like ASKAP J1935+2148 are born when massive stars with about eight to 10 times the mass of the sun run out of the fuel needed for nuclear fusion within their cores. This ends the outward radiation pressure that, for millions (sometimes billions) of years, has supported the star against the inward push of its own gravity.

Once this outward flow of energy ceases, the star's core collapses, triggering a supernova explosion that blows away its outer layers and the majority of its mass. The net result is a stellar remnant with between one and two times the mass of the sun compressed to a width of just around 12 miles (20 kilometers).

The extreme birth of this neutron star forces electrons to crush into protons, creating a sea of neutrons so dense that if a tablespoon of the object's material were brought to Earth, it would weigh as much as 1 billion tons — about the same weight as Mount Everest. But that's not all. The collapse has another extreme consequence, too.

Just as an ice skater here on Earth draws in their arms to increase the speed at which they twirl due to the conservation of angular momentum, the rapid reduction in the width of a stellar core means young examples of these dead stars can spin faster than the blades of a blender.

Young neutron stars also possess some of the strongest magnetic fields in the known universe, which causes them to beam out highly collimated radio waves from their poles. As these neutron stars spin, the beams sweep across the cosmos, making pulsars almost akin to celestial lighthouses.

However, as neutron stars age, their rotation slows, and they can no longer power their lighthouse-like radio emissions. That is what makes ASKAP J1935+2148, first spotted with the ASKAP radio telescope located at the Murchison Radio-astronomy Observatory in Western Australia, so challenging to decode. This neutron star's slow rotation indicates an advanced age, but somehow, it is still beaming out radio waves.

"It is highly unusual to discover a neutron star candidate emitting radio pulsations in this way. The fact that the signal is repeating at such a leisurely pace is extraordinary," team leader Manisha Caleb of the University of Sydney Institute of Astronomy said in a statement. "What is intriguing is how this object displays three distinct emission states, each with properties entirely dissimilar from the others." 

The scientist added that the 64 radio antennas of the MeerKAT radio telescope in South Africa played a vital role in distinguishing between these emission 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," Caleb continued.

The team still has pressing questions to answer about ASKAP J1935+2148, including the outside possibility that it might not be a neutron star at all!

There is still the chance that ASKAP J1935+2148 could actually be a white dwarf, the type of compact stellar corpse left over when the core of a smaller star, like the sun, dies. To produce a signal of the type observed using the ASKAP and MeerKAT radio telescopes, however, this isolated white dwarf would have to possess an extraordinarily strong magnetic field. 

Such objects have never been seen in the region of space occupied by ASKAP J1935+2148. That means this explanation just doesn't seem to fit ASKAP J1935+2148's emissions as well as a slow-spinning neutron star with extreme magnetic fields does.

Nonetheless, more research will be needed to confirm the true nature of ASKAP J1935+2148 and determine whether it is an outlaw white dwarf or a rule-breaking neutron star. Whatever the outcome, the results will challenge our understanding of the latter stages of stellar evolution.

"It might even prompt us to reconsider our decades-old understanding of neutron stars or white dwarfs, how they emit radio waves, and what their populations are like in our Milky Way galaxy," Caleb concluded.

The team's research was published on Wednesday (June 5) in the journal Nature Astronomy.