In the center of the Milky Way lies a galactic monster: a supermassive black hole weighing in at 4 million times the mass of the Sun. As gas swirls around the black hole, it heats up and X-ray flares are emitted. While usually quiet – which is black hole speak for minimal flare activity – a trio of space telescopes observed a dramatic increase in the number of X-ray flares produced.
The black hole, dubbed Sagittarius A (Sgr A* for short), typically produces one bright X-ray flare every 10 days. However, within the past year, Sgr A* has become very active, emitting one bright flare every day. What is causing this surge in flare activity? Seems we have a bit of a cosmic mystery, as scientists are trying to determine if this is normal behavior or if the flares were triggered by a recent encounter with a mysterious, dusty object.
Scientists have combined data from NASA’s Chandra X-Ray Observatory and ESA’s XMM-Newton with observations by the Swift Satellite as part of a new long-term monitoring process. The data will help them trace Sgr A*’s activity over the past 15 years.
"For several years, we've been tracking the X-ray emission from Sgr A*. This includes also the close passage of this dusty object,” said Gabriele Ponti of the Max Planck Institute for Extraterrestrial Physics in Germany. “A year or so ago, we thought it had absolutely no effect on Sgr A*, but our new data raise the possibility that that might not be the case."
So what exactly is G2? Originally, astronomers thought it was just an extended cloud of gas and dust; as such, they expected it to be devoured as it moved dangerously close to Sgr A*. However, G2 survived its close encounter with the colossal black hole relatively unscathed. This puzzled astronomers and also led to new theories that G2 might be much more than a gas cloud: It could be a hidden star swaddled in a cocoon of dust.
"There isn't universal agreement on what G2 is," Mark Morris of the University of California at Los Angeles said in a statement. "However, the fact that Sgr A* became more active not long after G2 passed by suggests that the matter coming off of G2 might have caused an increase in the black hole’s feeding rate."
It’s entirely possible that the increase in chatter exhibited by Sgr A* is unrelated to G2 and could even be common among black holes. For example, black holes feed off of material accreted from nearby stars and if the winds produced by those stars strengthen, it could lead to an increase in flares.
"It's too soon to say for sure, but we will be keeping X-ray eyes on Sgr A* in the coming months," said co-author Barbara De Marco, also of Max Planck. "Hopefully, new observations will tell us whether G2 is responsible for the changed behavior or if the new flaring is just part of how the black hole behaves."
A paper on these findings has been accepted by the Monthly Notices of the Royal Astronomical Society. A preprint is available online.