Lightest Neutron Star Ever or Strange Quark Matter?

(Nanowerk News) The lightest neutron star found so far 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 Department of High Energy Astrophysics at 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 previously discovered by the same team. This enabled the astrophysicists to determine the mass and radius of the neutron star with unprecedented accuracy.

Their study was published in the latest natural astronomy (“An oddly light neutron star in a supernova remnant”). Left: False color image of supernova remnant HESS J1731-347. In the center is the neutron star, which emits X-rays and could therefore be observed with the XMM Newtonian X-ray telescope. At the center of the dust shell is the companion star observed by the Gaia telescope. All types of invisible light have been measured, from infrared (orange; Spitzer telescope) to X-rays (green, XMM Newton telescope) to the 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 used to determine the star’s 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 viewed as celestial laboratories for the study of basic physics.

“Neutron stars have still unknown properties of matter, they have a much higher density than atomic nuclei,” says the researcher. Conditions like these could not be reproduced in terrestrial laboratories. “Space-based observations of neutron stars with extreme properties like the one we just found, using X-ray or other telescopes, will allow us to solve the mysteries of superdense matter – at least if we can solve challenges like the inaccuracy of measurements over such distances, that arises from observations. That’s exactly what we’ve managed to do now – to push the knowledge of these mysterious objects a step further.”

Precise calculations

The neutron star at the center of supernova remnant HESS J1731-347 was one of a few objects discovered during gamma-ray measurements with the HESS telescopes in Namibia and subsequently 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 the same team of researchers had previously noted, 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 accurate distance measurements to both objects. The Gaia mission involves a high-precision three-dimensional optical survey of the sky.

“This enabled us to eliminate previous inaccuracies and improve our models,” says Pühlhofer. The mass and radius of the neutron star could be determined much more precisely than before,” explains theoretical astrophysicist Valery Suleimanov.

It is not yet clear how the unusual object came about, he says. 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, says Andrea Santangelo, adding: “This is currently the most promising quark or strange Dull star candidate we know so far, even if its properties match those of a ‘normal’ neutron star.” But even if the object at the center of HESS J1731-347 is a neutron star, it remains an interesting and puzzling object. “It enables us to study the still unexplored part of the parameter space in the mass radius plane of neutron stars. This will allow us to impose valuable constraints on the dense matter equation of state used to describe their properties,” says Santangelo.