When it encounters a black hole, a star has very little chance of escaping unscathed. It is even generally a rather tragic end that awaits him. Like the one astronomers have just observed. The story of an unfortunate star transformed into a cosmic donut by a black hole.
Flirting with a black hole is not advisable. Because even if he is not the flirty hunter type, the black hole never refuses the advances of a beautiful young lady who offers herself to him. Thus, when a star approaches, it is quickly trapped by the gravity of the black hole and can no longer move back. It then undergoes what astronomers call a tidal rupture event. The star is literally torn apart and then devoured by the black hole. And the monster then “spits out” intense radiation. A bit like belching after a hearty cosmic meal.
Over the years and using different instruments, astronomers have already been able to observe around a hundred of these tidal rupture events. But the one NASA researchers are talking about today is a bit special. Because it has been observed in the ultraviolet range, “a field that can teach us a lot”. While most tidal rupture events are still observed in the X-ray domain.
It is first the All-Sky Automated Survey for Supernovae – whose mission is precisely to monitor the sky in search of transient events – which spotted, in March 2021, the burp characteristic of tidal rupture events. A high-energy flash of radiation emitted from the core of a galaxy dubbed ESO 583-G004. Its relative proximity — the event occurred some 300 million light-years from Earth — and its intensity allowed NASA astronomers to consider studying it using the ultraviolet spectrograph with which the Hubble space telescope is equipped.
New view of a black hole meal
While usually, tidal rupture events are mainly studied when they are triggered, when they are very bright, the researchers here were able to observe AT2022dsb quite early in its history and over a much longer time. Because the idea of the team, precisely, is to observe the events of rupture by tidal effect over an entire year. What a good way to see the accretion rate evolve – understand, the speed at which the star is engulfed by the supermassive black hole. And then compare the model forecasts with these new observations.
This time, astronomers have therefore seen the accretion rate drop until the black hole looks like a strange bird collecting crumbs. And they interpret today’s Hubble spectroscopic data as coming from a gaseous region that is very bright, hot and… donut-shaped. The remnant of what was once a star. A region now the size of our solar system that swirls around the black hole.
It was from the edge of this donut that astronomers collected data, “from an exciting place, on the edge of what we know and what we don’t know”. Enough to already allow them to confirm that black holes feed a bit like pigs. With star remnants being ejected by stellar winds. These sweep the surface of the cosmic donut at breakneck speeds. About 30 million kilometers per hour, or about 3% of the speed of light.