With the help of observations made with NASA, JAXA and ESA's X-ray satellites, astronomers revealed that the supermassive black hole at the center of our galaxy, dubbed Sagittarius A*, suffered a massive outburst some three centuries ago. Sagittarius A* is about 4 million times more massive than the Sun, however the energy emitted while 'feeding' is several billion times weaker than the energy emitted by similar supermassive black holes located in the galactic cores of other galaxies.

"We have wondered why the Milky Way's black hole appears to be a slumbering giant. But now we realize that the black hole was far more active in the past. Perhaps it's just resting after a major outburst," said Tatsuya Inui from Kyoto University, leader of the study.

Data was collected in a period of 11 years, between 1994 and 2005, with JAXA's Suzaku and ASCA X-ray satellites, and NASA's Chandra X-ray Observatory and ESA's XMM-Newton X-ray Observatory. Observation revealed that the gas cloud towards the center of the black hole emitted a bright burst of X-ray light, which quickly faded in response to the X-ray pulses emitted from the outer regions of the accretion disk.

X-ray pulses emitted by the central black hole then interacted with the Sagittarius B2 cloud, located 300 light years away from the center of the galaxy. X-ray pulses colliding iron atoms present inside the Sagittarius B2 determined iron ionization by electrons emission. Because electrons ejected from the iron atoms are located closest to the nucleus of the atom, as other electrons take their place, iron emits X-ray.

After the X-ray pulses no longer pass through the Sagittarius B2 cloud, it begins to decrease its X-ray output to normal levels. This process is called light echo.

"By observing how this cloud lit up and faded over 10 years, we could trace back the black hole's activity 300 years ago. The black hole was a million times brighter three centuries ago. It must have unleashed an incredibly powerful flare," said Katsuji Koyama from the Kyoto University.

Last year, a similar study conducted by Michael Muno from the California Institute of Technology, using the same light echo technique, revealed that Sagittarius A* suffered a powerful X-ray burst about 50 years ago. "The outburst three centuries ago was 10 times brighter than the one we detected," said Muno.

Earth is located about 26,000 light years from the supermassive black hole at the center of our galaxy, meaning that, in fact, the X-ray outburst took place 26,000 years ago. However, what determines these bursts remains unknown. Koyama believes that one explanation would be that a supernova in the vicinity of the black hole would have stirred up gas in the accretion disk in order to trigger a temporary feeding.