The black hole at the center of our galaxy is not so sleepy

The colossus was thought to be asleep, but it turned out to be more gluttonous than previously thought: Sagittarius A*, the supermassive black hole at the center of the Milky Way, recently experienced a powerful burst of activity after devouring cosmic objects within its reach.

The feast occurred 200 years ago, and NASA’s IXPE space satellite recently detected an echo of the event, according to a study published Wednesday in the journal 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 first observed in the 1990s by astrophysicists and its presence was confirmed in images a year ago.

With a mass of around four million suns and 13 billion years old, “it has always been considered an inactive black hole,” Frederic Marin of the Strasbourg Astronomical Observatory, who led the work, told AFP.

Sgr A* is in a state of quiescence, like most of the black holes in the galactic centers that have swallowed all the matter in their radius of attraction.

“Imagine a bear that goes into hibernation after devouring everything around it,” said Marin, a researcher at France’s National Center for Scientific Research (CNRS).

But his team discovered that at the end of the 19th century (a period established by calculating the distance), the monster would wake up from its torpor and gobble up the gas and dust that passed near it, for several months or a year, before back to sleep.

During this period, Sgr A* was “at least a million times brighter than it is today,” Marin noted. This is equivalent to the power of extremely active supermassive black holes that give rise to quasars, such as its sibling M87* in the Messier 87 galaxy, 55 million light-years away.

The moment of Sgr A*’s greatest appetite was revealed by unusual radiation from the molecular clouds in its vicinity: giants made up of icy gas and dust, “by definition cold” and that “should not emit so much light in X-rays”, than invisible to the human eye, according to the researcher.

“The intensity of the X-ray emission between sleep and awakening (from the black hole) can be compared to that of a lurking firefly in a forest that suddenly becomes as luminous as the sun,” the CNRS added in a statement. .

After a million seconds of observations, the IXPE (Imaging X-ray Polarimetry Explorer) satellite was able to detect the polarization of this X-ray light, that is, that its electric and magnetic fields vibrated in a precise direction.

Like a “stellar compass”, the polarization pointed in the direction of Sgr A*, suggesting that it was the source of the radiation reflected by the molecular clouds.

Thus, the black hole “emitted an echo of its past activity, which we were able to observe for the first time,” said the scientist, the French representative in charge 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 boundary (called the event horizon) and is 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*. What causes this return remains to be seen: a cloud that drifted away before falling into the black hole? A star that 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 that covers the origin of supermassive black holes, which remain an enigma of astronomy.