A supermassive black hole radiation ‘echo’ is spreading across the galaxy

Around 200 years ago Sagittarius A* (Sgr A*), the black hole that lies at the centre of the Milky Way, suddenly burst into life. It began consuming as much nearby cosmic material it as it could and released radiation a million times stronger than it does today before going back to sleep.

That's according to a new discovery made by researchers at the Astronomical Strasbourg Observatory in France using measurements taken by NASA's IXPE (Imaging X-ray Polarimetry Explorer)satellite.

They wanted to know why the energetic X-rays emitted by galactic molecular clouds located near Sgr A* shine so brightly.

It turns out it is because Sgr A*, which is four million times more massive than the Sun, emerged from a long period of dormancy in the early 19th century to begin a year-long feeding frenzy causing it to emit massive amounts of radiation.

It is this light that is now being reflected by the galactic molecular clouds.

Although black holes suck in and trap all light that enters their event horizons, the enormous forces they generate causes nearby matter to heat up to millions of degrees and results in the emission of radio waves and X-rays.

The researchers say that the effect was so dramatic that it was comparable to a lone glow-worm buzzing around a forest at night suddenly becoming as bright as the Sun.

“This is a very unique observation, we have never seen such dramatic changes in other supermassive black holes,” research leader Dr Frédéric Marintold BBC Science Focus.

“Such a discovery can potentially tell us a lot about the accretion cycles of supermassive black holes and help us to understand how they can shape a whole galaxy that surrounds them by pushing gas around and suppressing star formation.”

The team now plan to investigate the exact mechanisms that allowed Sgr A* to switch from its dormant state into a voracious one.

“Our next goal is to understand how the black hole woke up for just a year. Maybe a star, or a group of stars, passing by, caused the accretion of interstellar dust clouds,” said Marin. "We intend to continue our investigation.”