May 29, 2023
The lightest neutron star ever or strange quark matter?

The lightest neutron star ever or strange quark matter?

October 26, 2022

(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 High Energy Astrophysics Department of the Institute for Astronomy and Astrophysics of 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 discovered earlier. This allowed astrophysicists to determine the neutron star’s mass and radius with unprecedented precision.

Their study was published more recently Astronomy of Nature (“A strangely light neutron star inside a supernova remnant”). Left: False color image of the supernova remnant HESS J1731-347. In 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 the ultra-high-energy TeV belt (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, massive stars “die” in a supernova explosion, says lead author Victor Doroshenko. He calls them extreme objects that can be thought of as celestial laboratories for studying basic physics.

“Neutron stars have as yet unknown properties of matter; they are much denser than atomic nuclei,” says the researcher. Such conditions could not be reproduced in terrestrial laboratories. “Space-based observations of neutron stars with extreme properties like the one we just found, using X-rays or other telescopes, will allow us to solve the mysteries of extremely dense matter – at least if we can solve challenges such as the inaccuracy of measurements at such distances arising during the observations. Now we’ve managed to do just that – push the knowledge of 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 then studied by X-ray telescopes from space, Doroshenko reports.

“Only then did the cool neutron star become visible,” adds Gerd Pühlhofer. The peculiarity of this object, as the same research team had noted earlier, is that it is physically associated with another star. This star illuminates the cloud of dust around the neutron star, heating it up and causing it to 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 distance measurements to both objects. The Gaia mission involves a high-precision 3D 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,” explains theoretical astrophysicist Valery Suleimanov.

It’s not yet clear how the unusual object formed, he says. There are also doubts about whether it is actually a neutron star or whether the object is a candidate for an even more exotic object than quark strange matter, says Andrea Santangelo, adding: “This is currently the most promising quark or strange matter . candidate star that we know so far, even if 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 neutron star, it remains an interesting and enigmatic object. “It allows us to explore the still unexplored part of the parameter space at the mass radius level 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.

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