Some 3.5 million years ago, the center of our galaxy experienced a cataclysmic explosion that blasted out radiation across hundreds of thousands of light years.
The giant burst, known as a Seyfert flare, lasted about 300,000 years and occurred relatively recently in cosmic terms. It was triggered by activity near the supermassive black hole at the center of the Milky Way, called Sagittarius A*, which is over four million times as massive as the Sun.
The event reveals that the Milky Way’s core is more dynamic and energetic than expected. It “must have been a bit like a lighthouse beam,” said Joss Bland-Hawthorn, director of the Sydney Institute for Astronomy and lead author of a forthcoming study in The Astrophysical Journal about the flare, in a statement.
The exact origins of the flare are unknown, but it would have required “a lot of gas to suddenly be dumped into the galactic nucleus,” said Bland-Hawthorn in an email. For instance, a giant molecular cloud may have gotten too close to Sagittarius A*, only to be consumed and spit out from both poles of the galaxy in this gigantic flare.
Bland-Hawthorn and his colleagues mapped out the effects of the explosion on the Magellanic Stream, a massive river of gas and dust that orbits the Milky Way at a distance of 200,000 light years.
Despite how far away the stream is from the Milky Way’s core, the team observed that some parts of this gassy river were energized by the pyrotechnic event. Gas in the stream appears to be ionized, or stripped of electrons, signaling that the flare recently influenced it.
Bland-Hawthorn and his colleagues first published a paper about the aftermath of this potential flare event in 2013. The new study includes “new observations and a better understanding” of the flare’s impact on the Magellanic Stream, as well as updates to “the likely timescale for the Seyfert flare,” the team said in the study.
“It's likely that the flare was about as bright as could be expected for a supermassive black hole [of Sagittarius A*’s mass],” said co-author Philip Maloney, an astrophysicist at the University of Colorado Boulder, in an email.
So if you were one of humanity’s early ancestors living at this time, such as the Australopithecines, what exactly would you see in the night sky?
Maloney said that most of the optical light from the blast would be absorbed by the dust and gas surrounding the galactic nucleus. “The flare might have been comparable (say, one-tenth as bright or so) to the full Moon in brightness,” he speculated, “although of course it would be spread over a much larger area of the sky.”
However, if you could transport X-ray observatories back in time 3.5 million years, you would capture a much more dazzling glimpse of the flare’s brilliant jets. This is because about 10% of the flare’s total luminosity was emitted by high-energy X-rays, Maloney explained, which are much less likely to be blocked by gas and dust.
“The flare would be extremely bright to modern X-ray telescopes, unless there was a very large amount of gas lying along our line of sight to it—which can happen, but it takes a lot of gas,” he said. “But we would still be able to see the effects of the flare as it ionized and heated the gas away from the plane, the fading remnants of which we still see today.”