Ultra-bright neutron star
© Chris Butler/Science Photo Library/Getty Images
An artists impression of an ultra-luminous pulsar, seen from a nearby planet.
Ultra-bright neutron stars are somehow breaking what was thought to be a hard-and-fast law of physics, by collecting matter at a rate that should be impossible.

So far, four of the unfeasibly hungry stars have been detected, with the latest described in a paper published in the journal Nature Astronomy. The small number is not necessarily an indication of rarity; the first was only discovered, by NASA's Nuclear Spectroscopic Telescope Array (NuStar), in 2014.

Or, perhaps more correctly, it was only in 2014 that a neutron star was definitively identified as the cause of a phenomenon, known as ULX, that had been initially observed in the 1980s. ULX stands for "ultra-luminous X-ray source", and characterises an astronomical X-ray source that is less bright than a galactic nucleus but brighter than pretty much everything else.

A ULX is brighter than any known star. Most galaxies seem to sport one, although some have several. The Milky Way, curiously enough, has none.

They key characteristic that makes ULXs fascinating is that they routinely exceed what is known as the Eddington limit for neutron stars and black holes.

The Eddington limit defines the point at which the outward pressure of a star's radiation matches the inward pull of its gravity. Going beyond this limit would be, in theory, immensely destructive, with the luminosity - the outwards radiation - forcefully disintegrating the star's outermost layers until the limit is once more met and equilibrium restored.

Because of this curious quality, debate has raged about what exactly the source of ULXs might be. A study in 2001 cautiously suggested that individual ULXs may contain extremely massive black holes.

Another, in 2003, plumped for intermediate black holes. And a third, in 2017, concluded simply that "bona-fide ULXs must constitute a homogeneous class of objects".

In 2014, however, a team using NASA data identified the true source of at least one ULX - a magnetised pulsar, or neutron star, in the starburst galaxy known as Messier 82.

That research may have solved one mystery, but it threw up many others. The scientists, led by Matteo Bachetti of the Université de Toulouse in France, noted that the ULX neutron star didn't just break the Eddington limit, it smashed it out of the ballpark, recording a luminosity 100 times greater than it should have.

This meant that the pulsar was fully 10 times brighter than any others previously found. The discovery, Bachetti and his colleagues noted, raised the possibility that "neutron stars may not be rare in the ultra-luminous X-ray population". Furthermore, they added, with admirable understatement, the finding "challenges physical models".

Now, thanks to a team led by Murray Brightman at the California Institute of Technology in the US, ULX neutron stars are a little less rare but the physical model is still being challenged.

"In the same way that we can only eat so much food at a time, there are limits to how fast neutron stars can accrete matter," says Brightman

"But ULXs are somehow breaking this limit to give off such incredibly bright X-rays, and we don't know why."

The latest confirmed neutron star ULX is in a spiral galaxy called M51, about 28 million light-years away from the solar system. To identify it, the team used archival X-ray data gathered by NASA's Chandra X-ray Observatory.

The researchers were able to confirm that the source of the ultra-bright X-rays was a neutron star and not any sort of black hole because the Chandra data included an anomalous dip in the ULX's light spectrum.

The dip was identified as a phenomenon known as cyclotron resonance scattering - something that occurs when positively charged protons or negatively charged electrons circle around in a magnetic field. This was a strong identifying signature, because black holes don't have magnetic fields, while neutron stars do.

Like everything else to do with these mysterious objects, however, the detection of the cyclotron resonance scattering might only add to the number of questions arising.

"If the cyclotron line is from protons, then we know that these magnetic fields around the neutron star are extremely strong and may in fact be helping to break the Eddington limit," says Brightman.

If, on the other hand, they are electrons, then the magnetic field would not be particularly strong and would not, therefore, be contributing to the limit-busting - meaning other so far unknown forces must be at work.

To try to resolve the issue, Brightman and his colleagues are now combing again through Chandra's ULX data, trying to find more evidence concerning the cyclotron resonance scattering. They also intend to drill through information regarding the three other confirmed neutron star ULXs, to see if any scattering can be found there as well.

For the moment, thus, ULXs continue to confuse and confound astronomers. However, there is a feeling afoot that the uncertainties might now be one step closer to being resolved.

"The discovery that these very bright objects, long thought to be black holes with masses up to 1000 times that of the sun, are powered by much less massive neutron stars, was a huge scientific surprise," says Fiona Harrison, principal investigator of NASA's NuStar mission.

"Now we might actually be getting firm physical clues as to how these small objects can be so mighty."