一个国际科学家小组发现了that Sagittarius A* (Sgr A*)1, the supermassive black hole at the centre of the Milky Way, emerged from a long period of dormancy some 200 years ago. The team, led by Frédéric Marin2, a CNRS researcher at the Astronomical Strasbourg Observatory (CNRS/University of Strasbourg), has revealed the past awakening of this gigantic object, which is four million times more massive than the Sun. Their work is published inNatureon 21 June. Over a period of one year at the beginning of the 19th century, the black hole gobbled up cosmic objects that got a little too close to it, before once more entering a state of quiescence.
No effect was felt on Earth, as the distance between Sgr A* and our planet is too great (about two billion times the distance from the Earth to the Sun). However, the X-ray echo detected, which was emitted about 200 years ago, reveals that the original intensity was at least a million times greater than that currently emitted by Sgr A*. To get an idea of the increase in intensity of the X-ray emission when the black hole emerged from its quiescent state, it is as if a single glow-worm hidden in a forest suddenly became as bright as the Sun. These findings explain why galactic molecular clouds near Sgr A* are shining more brightly than usual: it is because they are reflecting the X-rays emitted by Sgr A* 200 years ago.
To carry out their research, the scientists used NASA's IXPE (Imaging X-ray Polarimetry Explorer)satellite3, which was for the very first time able to detect the polarisation of this X-ray light with great precision and also determine its source, something that had previously proved impossible. Rather like a compass, the polarised X-ray light4points directly to its source, Sgr A*, even though the latter is now virtually extinct. The scientists are continuing their work on Sgr A* to try to determine the physical mechanisms required for a black hole to switch from a quiescent state to an active one.
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