Over the last two decades or so, astronomers have noticed changes in Sagittarius A, the supermassive black hole at the center of the Milky Way. Intense flares have been reported, with a particularly spectacular one last May. A new study now confirms its activity is indeed changing.

The research by French and Belgian astronomers, accepted for publication in Astronomy & Astrophysics, builds on previous work published a few years back by the same lead authors. In the prior study, they detected 107 flares from Sagittarius A* between 1999 and 2015. The new work adds 14 more flares observed between 2016 and 2018.

No difference in the flaring rate of the black hole was found with statistical analysis. However, they did find that the number of bright flares increased. This change has been ongoing since the end of August 2014, according to analysis based on X-ray data from the Chandra, XMM-Newton, and Swift observatories.

Why the change in activity? That's exactly what the team are working to find out. After a bright flare in 2019, a possible explanation was posited that the motion of certain stars around the black hole might have changed the amount of gas reaching it. The close passage of star S2 in 2018 could have potentially pushed some gas inwards and created last year's flare. The newly discovered S62, the closest star orbiting the black hole, had a close passage in 2013, so these stars might be affecting the supermassive black hole's feeding. While this influence is a possibility, the researchers want to move away from speculations. Observations in X-rays from 2019 are currently missing, so the team can’t yet compare it with what was observed last year in near-infrared. 

“Since 2014, the activity of Sgr A* thus increased in several wavelengths. Additional multiwavelength data are required to conclude on the persistence of this increase and to obtain clues on the source of this unprecedented activity of the supermassive black hole,” the team wrote in the paper.

Sagittarius A* weighs over 4 million times the mass of the Sun and is located 26,000 light-years away from Earth.