ASASSN-22ci: a black hole shreds a star before the eyes of astronomers
Artist's illustration of a tidal rupture event: a star destroyed by the gravity of a supermassive black hole. © ESO, ESA / Hubble, M. Kornmesser
“Assassin” is the right way to pronounce this name for the sky survey initially devoted to supernovae, these cataclysmic explosions of stars. We can therefore say that there are sometimes well thought out acronyms, like that of ASASSN-22ci (for All Sky Automated Survey for SuperNovae), because it is indeed a cosmic assassination.
One of the two flashes observed, notably in X-rays, showing a tidal rupture event. © Jason T. Coudre (Nasa) et al, arXiv, 2024
The ASASSN automatic survey therefore recorded recently (well, in February 2022…) and not too far from us (well, “only” at 400 million light years…) two light flashes which were not supernovae, but two events per tidal break, or tidal event in English. The French physicist specializing in black holes, Jean-Pierre Luminet, with his colleague Brandon Carter, had nicely named this phenomenon “stellar crepe flambée”.
What is a “stellar crepe flambée” or tidal break event?
Curiosity is not a bad fault, unless its object is a black hole and you are a star orbiting it, say at the center of a galaxy. So, if your scientific curiosity pushes you to get close enough to said supermassive black hole, to the point of penetrating a so-called zone Hill spherecorresponding to the region around the black hole where its gravity is intense enough to deform you, you risk rupture. A break in the form of a stellar crepe flambée! For a smaller black hole, the effect is so extreme that it leads to the spaghettification (see box below)!
20 years of observing the movements of stars passing as close as possible to the central black hole of the Milky Way. © ESO / MPE
In the animation above, we can see the movements of stars around the black hole Sagittarius A* at the center of the Milky Way. Here, no stars exhibit flashes characteristic of tidal rupture, although some come very close to the black hole, reaching crazy speeds of several percent of the speed of light. No doubt these extreme speeds allow them to be less deformed by gravity.
Pieces of star carried to more than 30,000 ° C
The astronomers authors of thestudy led by Jason T. Coudre of NASA therefore observed 720 days apart two flaresthat is to say extremely energetic and hot flashes of light, at around 30,000 K, a temperature five times higher than on the surface of the Sun. As the star has not been completely dislocated, they are planning another event around February 4, 2026: we can therefore save the date on our calendars, because it is clear that this star has no way of escaping its next assassination.
This video illustrates the phenomenon very well:
