200 years ago, the Milky Way’s central black hole briefly awoke

Sometime between the American Revolution and the California gold rush, the black hole at the Milky Way’s heart woke up.

The black hole, called Sagittarius A*, has been quiet and dim since it was discovered in the 1990s. It’s thought to have been mostly quiescent for eons. But roughly 200 years ago, the black hole, as seen from Earth, suddenly brightened as it let out a brief flare of X-rays, researchers report June 21 in Nature.

At about 26,000 light-years away, “Sagittarius A* is the closest supermassive black hole to us,” says astronomer Frédéric Marin of the University of Strasbourg in France. “But it’s dormant.” If the black hole is accreting material from its surrounding disk of gas and dust at all now, it’s at a low rate, making the behemoth difficult to observe (SN: 5/12/22).

About 30 years ago, astronomers detected bright X-rays from large gas clouds in the galactic center. One explanation was that at some point, Sagittarius A* shot an X-ray pulse into space after eating some cosmic material, and the clouds recorded the afterglow (SN: 4/24/08). But other X-ray sources were possible.

Marin and colleagues used NASA’s Imaging X-ray Polarimetry Explorer satellite to measure the polarization, or direction, of the X-rays. “This direction can act as a compass, pointing to the source of emission,” Marin says. And it pointed straight at the black hole.

That polarization also suggests that the light came from at least one X-ray pulse roughly 200 years ago, the team says. The X-ray fluorescence indicates that the black hole suddenly grew a million times as bright as it is now. It’s unknown how often such pulses happen.

It’s also not clear what exactly caused Sagittarius A* to flare. “It could be a giant molecular cloud that was slowly accreted, or a very big star that passed by and was accreted all at once,” Marin says. “We don’t know yet.”