By studying the GW170817 signal, resulting from the merger of two neutron stars, among other things, in a kilonova, a team of astronomers points out the singular nature of the phenomenon, including a flux of residual X-rays likely to provide clues on the nature of the object resulting from the fusion of these two massive stars.
Detected on August 17, 2017 in the galaxy NGC 4993 using the Ligo and Virgo instruments (two interferometers designed to detect gravitational waves ), GW170817 is a signal attributed to direct observation of gravitational waves . Described as oscillations in the curvature of spacetime that propagate from a source, gravitational waves had been predicted by Albert Einstein as early as 1916, but it took nearly a hundred years for the first observation of gravitational waves to be completed in September 2015.
A unique signal
According to astrophysicists , this signal would have been emitted following the merger between two neutron stars . But what makes it special is the detection of electromagnetic waves associated with it: this is the first time that an astronomical phenomenon has been detected both in the form of gravitational waves, as well as in light form.
Indeed, a gamma-ray burst (GRB170817A), associated with GW170817, was detected by the Fermi Gamma-ray Space Telescope less than two seconds after the start of the gravitational wave signal.
Since then, nearly 70 observatories, on the ground or in space, have taken part in monitoring the phenomenon. The American radio telescopes VLA and VLBA have for example been able to observe remanent radio waves associated with GW170817, making it possible to confirm the scenario of a coalescence of two neutron stars .
A kilonova associated with the phenomenon
Detected almost 11 hours after the gravitational wave observation, the AT 2017gfo event was interpreted as a kilonova (which can be defined as a sub-luminous supernova ).
Due to its spatio-temporal proximity to GW170817, this kilonova has been associated with the same neutron star merger. This phenomenon would have been accompanied by a jet of charged particles, moving at a speed close to that of light and producing an emission of X-rays, measured by the Chandra observatoryfrom NASA.
According to a team of American astrophysicists, the study of these X-rays could be the key to identifying the object resulting from the merger of these two neutron stars.
Shortly after their detection, the X- ray emissions produced by the jet of matter would have gradually decreased, while the jet of matter slowed down. But since 2020, this decline in luminosity would have stopped, giving way to a relatively constant emission of X-rays. According to the astronomers , this would indicate the detection of an additional object, different from the jet of charged particles: another source of X-rays is then necessary to explain these observations.
An afterglow… or even a black hole?
According to the astronomers , this new source of X-rays could come from a shock generated by the rapid expansion of debris resulting from the fusion between the two stars. This shock would have heated the surrounding materials, thus emitting X-rays – this phenomenon would then be associated with a residual luminescence of the kilonova.
The hypothesis of a black hole is also not ruled out, as materials falling into this cosmic giant can similarly generate such X-ray emissions. The idea of the formation of a more massive neutron star n It is little considered, because the associated radiation should be much brighter.
To find the end of the story, the astronomers will continue their observations of GW170817, in X-rays as well as in radio waves: in the case of a residual luminescence of the kilonova, the emissions of X-rays and radio waves are expected to increase over the next few months. On the other hand, if the emission of X-rays decreases and the emission of radio waves is stopped, scientists will lean towards the scenario of the formation of a black hole (it would then be the least massive detected!).