An eerie echo coming from the Milky Way's supermassive black hole has been detected. It was emitted about 200 years ago when the dark abyss stirred from a state of dormancy, say scientists.
The sound marks a very intense period of activity as gas and dust were gobbled up. Particles drifted into the event horizon - a point of no return where not even light can escape.
When a black hole feeds on this infalling material, bursts of bright X-ray light bounce and echo. It can be turned into sound waves. Corresponding author Dr Frederic Marin, of Strasbourg University in France, said: "It reveals the past awakening of this gigantic object - which is four million times more massive than the Sun."
It sheds fresh light on the enigmatic and dynamic environment. Supermassive black holes are incredibly dense areas in the centre of galaxies. They act as intense sources of gravity which hoover up dust and gas around them.
Earth's, named Sagittarius A*, is only 26,000 light-years away. Dr Marin said: "Our work presents the missing piece of evidence that X-rays from the giant molecular clouds are due to reflection of an intense, yet short-lived flare produced at or nearby Sagittarius A*.
"These results can further constrain the past activity of the galactic centre." It's one of the very few black holes in the universe where we can actually witness the flow of matter nearby.
Because the area absorbs all surrounding light, it is incredibly difficult to see, and scientists have spent decades searching for hints of black hole activity. Dr Marin said: "To get an idea of the increase in intensity of the X-ray emission when the black hole emerged from its quiescent state, it is as if a single glow-worm hidden in a forest suddenly became as bright as the Sun.
"These findings explain why galactic molecular clouds near Sgr A* are shining more brightly than usual. It is because they are reflecting the X-rays emitted by Sgr A* 200 years ago."
The international team combined data from space telescope IXPE (Imaging X-ray Polarimetry Explorer) and the Chandra X-ray Observatory. Dr Marin said: "The polarisation angle is consistent with Sgr A* being the primary source of the emission.
"The polarisation degree implies that some 200 years ago, the X-ray luminosity of Sgr A* was briefly comparable to that of a Seyfert galaxy." These are galaxies that have very active centres with strong bursts of radiation.
Rather like a compass, the polarised X-ray light points directly to its source - Sgr A*. The scientists are continuing their work on Sgr A* to try to determine the physical mechanisms required for a black hole to switch from a quiescent state to an active one.
Black holes are formed when a dying star collapses inward under the pressure of its own weight. This leads to a supernova, a star's extremely powerful explosion. Black holes are places in space where the pull of gravity is so strong that even light can't escape. This is what makes them invisible.
Supermassive black holes can be billions the size of our Sun. Astronomers believe they can be found at the centre of all large galaxies. The study is in Nature.
