Scientists say a glowing molecular cloud is glowing today due to a 200-year glow emitted by our galaxy’s supermassive black hole. The Milky Way’s central supermassive black hole, also known as Sagittarius A* (abbreviated as Sgr A*), is rather undefined. Unlike some of the brighter galactic centers, which are bright beacons visible throughout the universe, Sagittarius A* is dim and quiet.

However, this was not always the case. According to new research published in the journal Nature, Sgr A* flared up just two centuries ago. Echoes of the explosion can still be seen today, with Sgr A* at least a million times brighter than its current value.

ILLUMINATED CLOUDS

In the 1990s, astronomers aimed new X-ray telescopes at the center of our galaxy. They found that Sgr A* is faint, but surrounded by bright clouds of molecular gas and dust that do not normally emit X-rays.

“One hypothesis was that the clouds didn’t shine on their own, they just reflect light from another source that was much, much brighter,” says Frédéric Maren (University of Strasbourg, France), who led the new study. .

According to the estimates, the reflected light should be polarized; that is, its waves should vibrate in the desired direction. But for decades this type of observation was out of reach of X-ray satellites. Recently, astronomers were finally able to observe the galactic center with the Imaging X-ray Polarimetry Explorer (IXPE), a satellite that specializes in observing the polarization of X-ray light.

“We found that the galactic center is not polarized, but the clouds themselves are polarized,” says Marin. “So they really radiate light from something else.”

BRIGHT LIGHT

Since the discovery of glowing molecular clouds, Sgr A* has been the main source of suspect light. The new study claims that the observed polarization, as the signal depends on the direction of the illuminating light, is indeed indicative of Sgr A*.

However, Maika Clavel (University of Grenoble-Alpes, France), who was not involved in the study, says that the current quality of polarization data hardly allows us to make such an explanation. Still, he is confident that Sgr A* is indeed the resource astronomers are looking for. “Based on all the observations collected since the 1990s, this is the only candidate left,” says Clavel.

Supermassive black holes themselves don’t glow, but nearby stars, asteroids, or gas clouds that were torn apart by the black hole’s extreme gravity do. As the material orbits the black hole, it heats up and emits light across the electromagnetic spectrum.

Based on the distance between the black hole and the clouds, estimated from the measured polarization, the team found that Sgr A* glowed about 200 years ago. Previous studies have shown that the possible outbreak lasted less than 1.6 years. How bright it becomes depends not only on its source, but also on its duration: a one-year glow will be much weaker than an hour-long glow from the same food. By the most conservative estimates, Sgr A* became at least a million times brighter than it is today. The emitted light is comparable to that observed in Seyfert galaxies, a special class of galaxies with active power from black holes at their centers.

The team plans to use IXPE to observe the cloud again, hoping to learn more about the past event, including the nature of the shattered object. Clavel notes that more signals could also indicate whether there are significant changes in the polarization properties of the cloud. Perhaps, he suggests, the team could see Sgr A* fire multiple flares in a short period of time.

Only about 10% of nearby galaxies have active central black holes. Although black holes seem to spend most of their time dormant (not feeding), intense phases of activity profoundly affect the evolution of their host galaxies. Thus, the recent explosion in Sgr A* provides a unique insight into the black hole’s activity and its effect on its immediate environment.