Using X-ray telescopes in space, astronomers may have discovered the lightest neutron star found so far.

It was discovered by Dr Victor Doroshenko, Dr Valery Suleimanov, Dr Gerd Pühlhofer, and Professor Andrea Santangelo from the High Energy Astrophysics section of the University of Tübingen’s Institute of Astronomy and Astrophysics.

Located at the centre of the supernova remnant, HESS J1731-347, the star has only about half the mass of a typical neutron star.

The findings, published in Nature Astronomy, used new measurements of the distance to a companion star that the team had discovered earlier. This meant that they were able to specify the mass and radius of the star with unprecedented accuracy.

What are neutron stars?

Neutron stars are formed when normal stars with large masses ‘die’ in a supernova explosion, said the study’s lead author, Victor Doroshenko. According to Doroshenko, these stars are extreme objects that can be regarded as celestial laboratories for studying basic physics.

He explained: “Neutron stars have yet unknown properties of matter, as they have much higher density than atomic nuclei. Conditions like that could not be replicated in terrestrial laboratories.

“Space-based observations of neutron stars with extreme properties such as the one we’ve just found, using X-ray or other telescopes, will allow us to solve the mysteries of super-dense matter. At least if we can solve challenges such as the inaccuracy of measurements over such distances, that arise during observations. We have now succeeded in doing just that, pushing the knowledge about these mysterious objects a bit further.”

Making precise calculations to improve space models

The neutron star at the centre of the supernova remnant HESS J1731-347 was just one of a handful of objects discovered during gamma ray measurements with the telescopes in Namibia. According to Pühlhofer, this was the moment when the cooling neutron star finally became visible.

What makes this object unusual, is that it is connected to another star, which illuminates the dust cloud around the light star, heats it, and makes it shine in the infrared light. The accompanying star was recently observed by the European Space Agency’s Gaia Space Telescope, which provided the research team with accurate distance measurements of both objects.

The Gaia mission involves a high-precision three-dimensional optical survey of the sky. “This allowed us to resolve previous inaccuracies and improve our models,” Pühlhofer said.

“The mass and radius of the neutron star could be determined much more precisely than was previously possible,” added Suleimanov, who is a theoretical astrophysicist.

It is not yet clear how this unusual star formed. There are also doubts as to whether it is actually a neutron star, or whether the object is a candidate for an even more exotic object made of strange quark matter.

Santangelo explained: “This is currently the most promising quark or strange-matter star candidate we know of so far, even if its properties are consistent with those of a ‘normal’ neutron star.” Even if the object at the centre of the supernova remnant is a neutron star, it remains an interesting and puzzling object.

“It allows us to probe the yet unexplored part of the parameter space in the mass-radius plane of neutron stars. This will enable us to put valuable constraints on the equation of state of dense matter, which is used to describe its properties,” Santangelo concluded.

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