We really can’t complain about Sagittarius A, the supermassive black hole at the center of the Milky Way. Many other black holes spew jets of material and disrupt their galaxies, making life there unlikely. But not Sagittarius A – it’s a calm tenant of the Milky Way. So calm that the sudden production of a flare caught astronomers off guard.
Astronomers led by a team at UCLA have been conducting observations of Sagittarius A* using the WM Keck telescope in Hawaii. On May 13, they noticed that the supermassive black hole became brighter than usual and then dimmed over the following 2.5 hours. Details of the observation have been accepted for publication in The Astrophysical Journal Letters and are available on arXiv.
Black holes are by their very nature black. What the researchers saw is a sudden increase in gas swirling around the black hole. Once something gets too close, the incredible forces around a black hole heat the material to incredible temperatures, making it glow. Lead author Tuan Do shared a timelapse on Twitter a few days ago, where the flare is clearly visible.
Researchers are not exactly sure what caused the flare, but a potential culprit is visible in the shared footage. The dot of light just above and to the left of Sagittarius A* that remains visible once the flare disappears is a star called S0-2. The star is one of a handful that orbits close to the black hole, and in May 2018, S0-2 reached its closest point to it at about 20 billion kilometers (12.4 billion miles).
"One of the possibilities," Do told ScienceAlert, "is that the star S0-2, when it passed close to the black hole last year, changed the way gas flows into the black hole, and so more gas is falling on it, leading it to become more variable."
Do and his team have been keeping an eye on S0-2, as the star’s properties have been used over the last few decades to test Einstein’s theory of general relativity. In a recent study, the team showed that Newton’s gravity can’t explain what we see happening around Sagittarius A* and that Einstein’s magnum opus remains the best gravitational theory we currently have.