October 26, 2022 (Nanowerk News) The faintest neutron star yet found is at the center of the supernova remnant HESS J1731-347. Dr Victor Doroshenko, Dr Valery Suleimanov, Dr Gerd Pühlhofer and Professor Andrea Santangelo from the High Energy Astrophysics section of the Institute for Astronomy and Astrophysics at the University of Tübingen discovered the unusual object with the help of X-ray telescopes in space. According to the research team’s calculations, it has only about half the mass of a typical neutron star. As a basis for their calculations, they used new measurements of the distance to a companion star that the same team had previously discovered. This allowed astrophysicists to specify the mass and radius of the neutron star with unprecedented precision. Their study has been published in the latest Nature Astronomy journal (“A strangely light neutron star inside a supernova remnant”). Left: false color image of the supernova remnant HESS J1731-347. At the center is the neutron star, which emits X-rays and could therefore be observed by the XMM-Newton X-ray telescope. In the middle of the dust envelope is the companion star observed by the Gaia telescope. All kinds of invisible light were measured, from infrared (orange; Spitzer telescope) to X-rays (green, XMM-Newton telescope) and ultra-high energy. TeV band (blue; HESS telescopes). Right: High-resolution X-ray spectra of the neutron star from measurements by the XMM-Newton and Suzaku telescopes, which were used to determine the stellar mass. (Image: Institute for Astronomy and Astrophysics, University of Tübingen) Neutron stars are born when normal stars with large masses “die” in a supernova explosion, says lead author Victor Doroshenko. He calls them extreme objects that can be considered celestial laboratories for studying basic physics. “Neutron stars still have unknown properties of matter; they have a much higher density than atomic nuclei,” says the researcher. Conditions like this could not be replicated in terrestrial laboratories. “Space observations of neutron stars with extreme properties like the one we just found, with X-rays or other telescopes, will allow us to solve the mysteries of superdense matter, at least if we can solve challenges such as the inaccuracy of measurements at these distances that emerges during observations. Now we have succeeded: to push the knowledge about these mysterious objects a little further.”
Accurate calculations
The neutron star at the center of the supernova remnant HESS J1731-347 was one of the few objects discovered during gamma-ray measurements with the HESS telescopes in Namibia and later studied by X-ray telescopes from space, reports Doroshenko . “Only then did the cooling neutron star become visible,” adds Gerd Pühlhofer. The peculiarity of this object, as previously pointed out by the same research team, is that it is physically connected to another star. This star illuminates the dust cloud around the neutron star, heating it up and making it glow in infrared light. The companion star was recently observed by the European Space Agency’s Gaia space telescope, which provided the research team with precise measurements of the distance to the two 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,” said Pühlhofer. The mass and radius of the neutron star could be determined much more precisely than was possible before,” explains theoretical astrophysicist Valery Suleimanov. It is still unclear how the unusual object formed, he says. There are also doubts about whether it really is a neutron star or whether the object is a candidate for an even more exotic object made of strange quark matter, says Andrea Santangelo, adding: “This is currently the quark candidate or most promising strange-matter star known to date, although its properties are consistent with those of a ‘normal’ neutron star.” But even if the object at the center of HESS J1731-347 is a neutrons, remains an interesting and puzzling object. “It allows us to investigate the as yet unexplored. part of the parameter space in the mass-radius plane of neutron stars. This will allow us to put valuable constraints on the equation of state of dense matter, which is used to describe its properties,” says Santangelo.