The supermassive black hole at the heart of our galaxy has suddenly started flashing brighter than we have ever seen it, and astronomers don鈥檛 know why.
In the space of two hours, the black hole鈥檚 brightness changed 75-fold 鈥� and astronomers believe it was even brighter before this observation began. This is the brightest and most variable that the black hole, Sagittarius A*, has been since scientists first started studying it more than 20 years ago.
Tuan Do at the University of California, Los Angeles, was watching the Keck telescope in Hawaii as it was pointed at the centre of our galaxy on the evening of May 12. He was confused by an unusually bright spot on the readings at first, thinking it was the nearby star S0鈥�2, only to realise as it became brighter that it must be the black hole itself.
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鈥淚t was strange because I had never seen the black hole that bright before,鈥� says Do. This brightness comes from the way the black hole heats the dust and gas around it. While Sagittarius A* is four million times the mass of our sun, it is not as bright as many other black holes. Normally it flickers like a candle, sometimes even too faint to see.
If it becomes even brighter, it will help the team at the Event Horizon Telescope get a clearer picture of Sagittarius A*, to follow up the first ever image of a black hole they released in April 2019. At the moment the flickering of the light makes it difficult to combine the data from several telescopes, says Do.
One possible explanation for the activity is that the black hole is gobbling something up. 鈥淢aybe more gas is falling into the black hole and that leads to higher amounts of accretion, which leads to it being brighter,鈥� says Do.
Last year, the nearby star S0鈥�2 orbited close to the black hole and this could have disturbed the gas around the black hole, prompting more to fall in now and heat it up.
Another possible explanation is that it has something to do with the mysterious gaseous object called G2 that sling-shotted around the black hole back in 2014. At the time, some astronomers expected fireworks and activity, only to be disappointed. Do says that it may have been inexplicably delayed.
While the reason for the unusual behaviour is still a mystery, Do hopes it could be cleared up with more observations. But we have a limited window to observe the black hole before the sun and Earth shift and that part of the galaxy can鈥檛 be seen for another year.
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