Black hole at centre of our galaxy 'woke up' 200 years ago

The supermassive black hole at the centre of our Milky Way galaxy 'woke up' 200 years ago and devoured the cosmic debris around it.

Sagittarius A*, which lies at the centre of the Milky Way, is more than 25,000 light years from Earth and is our nearest supermassive black hole, with an estimated mass millions of times that of our Sun.

It is normally far less luminous than other black holes at the centres of galaxies we can observe, which means the Milky Way's central black hole has not been actively gobbling up material around it.

But new evidence from Nasa's IXPE (Imaging X-ray Polarimetry Explorer) telescope suggests the ancient sleeping giant woke up about 200 years ago.

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Scientists called on IXPE for a closer look when previous X-ray studies detected relatively recent X-ray emissions of giant clouds of gas in its vicinity.

Given that most cosmic clouds, called molecular clouds, are cold and dark, the X-ray signatures of these clouds should have been faint. Instead, they shone brightly.

Frederic Marin, astronomer at the Astronomical Observatory of Strasbourg in France, said: "One of the scenarios to explain why these giant molecular clouds are shining is that they are, in fact, echoing a long-gone flash of X-ray light, indicating that our supermassive black hole was not that quiescent some centuries ago."

"The polarisation angle acts like a compass, pointing us toward the mysterious, long-gone source of illumination,” said Riccardo Ferrazzoli, astrophysicist at the Italian National Institute of Astrophysics in Rome. "And what lies in that direction? None other than Sagittarius A*."

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Analysing the data, the team figured out that the X-rays from the giant molecular clouds were reflected light from an intense, short-lived flare produced at or near Sagittarius A*, possibly caused by the black hole abruptly consuming nearby material.

The data also helped researchers estimate the luminosity and duration of the original flare – suggesting the event occurred some 200 Earth years ago, or roughly around the start of the 19th century.

The team's next goal is to repeat the observation and reduce the uncertainties of the measurement, said Steven Ehlert, IXPE project scientist at Nasa's Marshall Space Flight Center in Huntsville, Alabama.

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