Sagittarius A*—the Milky Way’s central black hole—is normally quite subdued, with low levels of activity recorded over years. However, in May, scientists from the U.S. and Europe watched an “unprecedented” bright flash coming from the black hole, before dimming again.
Tuan Do, from UCLA, and colleagues spent four night observing Sagittarius A* at the WM Keck Observatory in Hawaii in May, ScienceAlert reports.
While it is normal for the supermassive black hole to fluctuate slightly in brightness, the researchers found Sagittarius A*—which is four million times the mass of the sun—had gotten 75 times brighter than normal. “I was pretty surprised at first and then very excited,” Do told the news site. “The black hole was so bright I at first mistook it for the star S0-2, because I had never seen Sagittarius A* that bright. Over the next few frames, though, it was clear the source was variable and had to be the black hole. I knew almost right away there was probably something interesting going on with the black hole.”
In their study, which has not yet been peer reviewed or published in a scientific journal, the researchers say the flash happened over a two-hour time span. They said the increase in brightness could suggest a change in activity at the supermassive black hole. Another cause could be a change to its accretion state—how the black hole is drawing matter inwards. The team also suggests current models about the flux levels of Sagittarius A* may be insufficient.
Do and colleagues also propose a few physical mechanisms that could have produced the bright flash. Black holes do not emit radiation that can be detected by observers on Earth. However, the objects and material close to them do—and changes to the black hole can excite matter nearby, allowing scientists to detect changes taking place.
S0-2 is the closest star to Sagittarius A*, orbiting the black hole about once every 16 years and traveling at around three percent of the speed of light. It has been watched by researchers at UCLA and Germany’s Max Planck Institute for over 20 years. A huge dust cloud known as G2 also came extremely close to Sagittarius A*recently.
Researchers suggest either of these encounters could have caused the flash: “Potential physical origins of Sagittarius A*’s unprecedented brightness may be from changes in the accretion-flow as a result of the star S0-2’s closest passage to the black hole in 2018 or from a delayed reaction to the approach of the dusty object G2 in 2014. Additional multi-wavelength observations will be necessary to both monitor Sagittarius A* for potential state changes and to constrain the physical processes responsible for its current variability.”
Do told ScienceAlert they are now waiting for data from other telescopes, including NASA’s Spitzer and Chandra, to better understand what might have happened with Sagittarius A*.
The paper comes as the team with the UCLA Galactic Center Group and WM Keck Telescopes release data showing 21 years’ worth of observations of the stars orbiting the supermassive black hole. The visualization shows S0-2’s close encounter with the black hole, as over a dozen other stars encircle Sagittarius A*.
