An international team of astronomers has discovered how a giant star transmits matter to a moribund neutron star evolving around it, giving it life.
It is very rare to find a neutron star binding to a red giant: only a dozen of these symbiotic X-ray binary systems are known throughout the Galaxy.
Satellites continually scan space and trigger an alert when an exceptional event is spotted. That’s what happened when the ESA (European Space Agency) Integral satellite seized the moment when a binary system involving a red giant emitted a huge X-ray flash. By reaching a moribund neutron star, the stellar wind lost by the red giant has brought new life to the dying companion, creating a rare binary system, under the eyes of an international team of astronomers led by the University of Geneva (UNIGE). The results of the study were published in the journal Astronomy & Astrophysics.
On August 13, 2017, the screens of the satellite Integral, located in Ecogia near Geneva, trigger the alarm: an exceptional event has just been spotted. A burst of X-rays is emitted from the densely populated center of the Milky Way, our galaxy. At once, several instruments in the world and in space turn focus to the unknown source. This is notably the case for XMM satellites from ESA and NASA’s NuStar and Swift, all three X-ray sensitive, as well as SOAR ground-based telescopes in Chile, Faulkes in Australia and Las Cumbres in California.
Stars that are between one and eight times the size of the Sun end their lives by inflating and ejecting their outer layers as a stellar wind. The largest, some as big as thirty solar masses, end their lives with a gigantic explosion called supernova, leaving bare an extremely compact heart, named a neutron star. At this stage, the neutron star is barely 6 miles in diameter, while its mass is one and a half times that of the Sun.
The simultaneous observation campaign triggered by the integral satellite alarm showed how a red giant has given back matter to a strongly magnetized neutron star, spinning weakly on itself. However, it is very rare for a neutron star to bind to a red giant: only a dozen of these symbiotic X-ray binaries — as they are called — are known throughout the Galaxy.
“Integral has recorded the unique moment when the material of the red giant falls onto the neutron star,” enthuses Enrico Bozzo, a researcher in the Department of Astronomy of the Faculty of Science of UNIGE and lead author of this study . “We have been able to observe that the falling of the material on the surface of the red giant feeds the neutron star which accompanies it and brings it back to life, emitting X-rays.”
X-ray measurements have allowed astronomers to see that the magnetic field of the neutron star is particularly strong, a feature generally associated with a recent neutron star, the magnetic field being assumed to decrease with time. But how could this relatively “young” neutron star associate itself with an older red giant to form this binary system? “Two scenarios can explain this apparent contradiction,” says Enrico Bozzo, “either the magnetic field does not decrease with time and the neutron star is older than the theory suggests, that is, the system was initially composed of a white dwarf and a red giant which, by feeding the white dwarf, turned it into a neutron star.”
In fifteen years of operations, this is the first time that the Integral satellite has detected an X-ray emission from this source, suggesting that the red giant has just started to feed its companion because of a stellar wind more intense or an exceptional episode of ejection of matter.