February 21, 2024


Lurking at the center of our galaxy is a supermassive monster that’s currently sleeping — but it hasn’t always been so quiet. New observations reveal the X-ray “echo” of the Milky Way’s central black hole awakening 200 years ago, a million times brighter than it is today.

Like most galaxies, the Milky Way has a supermassive black hole at its core. Known as Sagittarius A* (Sgr A*), the object has a mass of about 4 million suns and is relatively quiet compared to some of its counterparts we see in other galaxies.

But we know that’s not always the case. Evidence suggests that Sagittarius A* fired a huge flare of radiation about 6 million to 3.5 million years ago, blasting matter and leaving behind massive shock waves that are still visible today in certain wavelengths of light. But now, astronomers have found solid evidence of a more recent outburst that occurred 200 years ago.

Several X-ray space observatories, including IXPE, Chandra, and XMM-Newton, had previously spotted giant molecular clouds near Sgr A* that were surprisingly bright in X-rays. Astronomers used the IXPE satellite to measure the polarization of light coming from these clouds to find the culprit.

Above: A panorama of Sagittarius A*, the supermassive black hole at the center of the Milky Way galaxy, taken by the Chandra X-ray Observatory.Bottom: A composite close-up image of the region surrounding a black hole, with orange spots representing molecular clouds bright in X-rays
Above: A panorama of Sagittarius A*, the supermassive black hole at the center of the Milky Way galaxy, taken by the Chandra X-ray Observatory.Bottom: A composite close-up image of the region surrounding a black hole, with orange spots representing molecular clouds bright in X-rays

NASA/CXC/SAO/IXPE

When light is polarized, its waves are all directed along the same plane. In this case, the angle of polarization points towards Sagittarius A* as the source of X-ray radiation, while the degree of polarization reveals how far the cloud has traveled since the black hole’s ejection. This in turn allowed the researchers to time the lightning strikes – about two centuries ago.

From these details, astronomers were able to estimate the brightness of the original flash. It turns out that our local supermassive black hole briefly glowed in X-rays about a million times brighter than usual. That would make it comparable to the Seyfert Galaxy, whose core is as bright as all the stars in the galaxy. Before you ask how 19th-century astronomers missed such a show, that was long before the X-ray telescope was invented and pointed skyward.

A better understanding of the activity history of Sgr A* can help us predict its future. After all, it has emitted new flares of X-ray and near-infrared light in recent years, which could be a harbinger of a new uptick in activity, or just some fireworks from falling objects.

The study was published in the journal nature.

source: University of Strasbourg