For the first time in the history of astronomy, a team of scientists led by scientists from Northwestern University has been able to see first-hand the relatively fresh traces of a rare type of cosmic explosion, the kilonova explosion. This type of explosion occurs when two neutron stars, known to us as the compact but densest objects in the Universe, collide. And the actual collision generates an explosion that is a thousand times brighter than a fairly conventional supernova explosion.
The object of scientific interest is the notorious space object GW170817, which is a remnant of the Kilonova explosion. Previously, this object had emitted a narrow stream of high-energy particles, the so-called jets. But, three and a half years after the collision, this flux had disappeared and astronomers were able to see a new and rather mysterious source of X-ray radiation at the site of the explosion.
A hypothesis has been proposed as an explanation for what was observed, according to which the expanding cloud of neutron star remains produces a shock wave similar to the sonic boom from an aeroplane passing through a sound barrier. This shock wave heats the surrounding matter, which begins to glow in the X-ray range. There is also an alternative explanation, according to which X-rays are emitted by matter approaching the event horizon of a black hole formed by the collision of neutron stars. Note that either of the two possible scenarios would be the first observation of such a phenomenon.
We remind our readers that in August 2017, event GW170817 made history as the first case of neutron star collision and merger recorded and identified using gravitational waves and electromagnetic radiation (light). And in the years that followed, astronomers studied the phenomenon at different ranges of the electromagnetic spectrum.
Since early 2018, using the Chandra X-ray Observatory, astronomers have observed X-ray emission from a stream of particles and matter travelling at nearly the speed of light. Over time, the flux gradually slowed and became wider, and between March 2020 and the end of this year, the decrease in X-ray brightness virtually stopped and stabilised at a certain level.
The stabilisation of the X-ray brightness is what has pointed astronomers to the presence of another source of this type of radiation. And the nature of this source, whether it is the afterglow from an explosion or a black hole, is yet to be determined by scientists in the future.
Scientists will have to follow object GW170817 in the X-ray and radio bands for a long time to dot all the i's. If all this is an afterglow effect, the streams of X-rays and radio emissions will become brighter over the next few months or years. But if the cause of what is happening is matter falling into the black hole, X-ray levels would either have to remain constant or decrease dramatically in a short time, with little or no radio emission emanating from the black hole region.
"Either answer would have far-reaching implications," the scientists wrote, "In the first case, it would indicate that black holes may not form in all cases of neutron star collisions. And the other option would give astronomers the opportunity to study the peculiarities of matter absorption by a black hole only a few years after its formation."