We thought the colossus was asleep, but it turns out to be more gluttonous than expected: Sagittarius A*, the supermassive black hole lurking in the center of the Milky Way, experienced a powerful upsurge in activity in the recent past after devouring cosmic objects in his reach.
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The feast took place 200 years ago, and NASA’s IXPE space satellite recently detected an echo of it, according to a study published Wednesday in Nature.
Sagittarius A* (Sgr A*), which owes its name to its detection in the constellation Sagittarius, is located 27,000 light years from Earth, in the heart of our galaxy. It was observed in the 1990s by astrophysicists, and its presence was proven in images a year ago.
With a mass of around four million suns, 13 billion years old, it has “always been seen as a dormant black hole”, Frédéric Marin of the Strasbourg Astronomical Observatory told AFP. led the work.
Sgr A* is in a state of quiescence, like most supermassive black holes in galactic centers that have swallowed up all the matter within their radius of attraction. “You can imagine a bear going into hibernation after having devoured everything around it,” adds this CNRS researcher.
AFP
But his team discovered that at the end of the 19th century (a period established via distance estimation), the monster came out of its torpor and engulfed the gas and dust that passed too close to it, during several months or even a year. Before going back to sleep.
During this period, Sgr A* was “at least a million times brighter than it is today”, explains Frédéric Marin. That is a power equivalent to that of the extremely active supermassive black holes at the origin of quasars, such as its congener M87* in the galaxy Messier 87, 55 million light-years away.
Sgr A*’s appetite spike was betrayed by unusual radiation from molecular clouds in its vicinity: giants made of frozen gas and dust, “by definition cold” and which “shouldn’t emit as much light in rays X (invisible to the human eye, Ed)”, according to the researcher.
“The intensity of the X-ray emission between sleep and awakening (from the black hole, editor’s note) can be compared to a firefly lurking in a forest which would suddenly become as luminous as the sun”, completes the CNRS in a press release. .
After a million seconds of observations, the IXPE (Imaging X-ray Polarimetry Explorer) satellite succeeded in detecting the polarization of this X-ray light, that is to say that its electric field and its magnetic field were vibrating in a specific direction.
Like a “stellar compass,” the polarization pointed in the direction of Sgr A*, suggesting that it was the source of the radiation reflected from the molecular clouds. The black hole thus “emitted an echo of its past activity, which we managed to observe for the first time”, welcomes the scientist, French representative of the international consortium of the IXPE mission.
The density of a black hole is such that nothing can escape, not even light. But before matter crosses the final frontier (called the event horizon) to be swallowed up forever, it swirls, heats up and emits light.
“It’s like a swan song”, transmitted indirectly by the molecular clouds around Sgr A*. It remains to be seen what caused this resurgence: a cloud that would have drifted before falling into the black hole? A star that has ventured too close?
Additional observations, planned for September with IXPE, should help to better understand the activity cycle of Sgr A*, and perhaps lift a corner of the veil on the origin of supermassive black holes which remains an enigma of astronomy. .