A symbiotic X-ray binary system generated a flare

Artist's concept of binary system with a red giant and a neutron star stealing its materials (Image NASA/Dana Berry)
Artist’s concept of binary system with a red giant and a neutron star stealing its materials (Image NASA/Dana Berry)

An article accepted for publication in the journal “Astronomy & Astrophysics” describes the discovery of a binary system in which one of its two stars emitted an X-ray flare because it’s a neutron star that steals materials from its companion, a red giant. A team of researchers used observations of various telescopes to study the event indicated as IGR J17329-2731 and defined it the birth of a symbiotic X-ray binary.

An X-ray flare was detected on August 13, 2017 by the Integral (INTErnational Gamma-Ray Astrophysics Laboratory) space observatory, an ESA mission in collaboration with Roscosmos and NASA for hard X-ray and gamma-ray observations. That event generated a number of follow-up observations for weeks to identify its origin. The result was that the data collected at various electromagnetic frequencies led to the identification of a neutron star and a red giant that form a binary system.

Neutron stars are one of the possible types of remnants of a star after the end of its normal life and its explosion in a supernova. These objects are incredibly dense and their average mass is about 1.4 solar masses enclosed in a 12-kilometer sphere. Red giants are stars that, after having ended hydrogen fusion, enter a phase of instability that precedes their death.

In this binary system the red giant was stripped of materials in its upper layers of gas that have accumulated on the neutron star’s surface. There, they were compressed to the point that eventually nuclear fusion reactions were triggered and generated the X-ray flare observed in the IGR J17329-2731 event.

The observations following the discovery with ESA’s XMM-Newton NASA’s NuSTAR space telescopes allowed to establish that the neutron star spins almost every two hours. It’s very slow by these objects’ standards as they generally spin very quickly, so much that there are even millisecond neutron stars.

Another surprise came from the measurement of the neutron star’s magnetic field, which turned out to be very powerful. According to the current theoretical models, these stars’ magnetic field decreases over time and this suggests that the one under study is young but there are no certainties.

Enrico Bozzo of the University of Geneva, the article’s first author, also mentioned the possibility that the neutron star is the result of a subsequent evolution in that binary system. In simple words, it’s possible that originally that was a white dwarf that collapsed after stealing materials to its companion for a long time.

This is the first pair of this type discovered by the Integral space observatory. The origin of the neutron star is not certain but it’s likely that we see it right after it stole enough materials to trigger the process that led to the X-ray flare. The pair with a red giant is rare and few such cases have been discovered so far, referred to as symbiotic X-ray binaries, so it will keep on being studied.

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