In 2017, the Atacama Large Millimeter/submillimeter Array (ALMA) stared toward our system’s heart of darkness, Sag A*, a 4 million solar mass black hole. There they found a flicker of light, hanging on, in a mind-breaking orbit. Some small radio emitter has been found orbiting at just 0.2 AU from Sag A*. This is half the size of Mercury’s orbit! 

According to team member Tomoharu Oka, “This emission could be related with some exotic phenomena occurring at the very vicinity of the supermassive black hole.” Whatever it is, the emitter is giving us yet one more test of relativity. As the light source orbits Sag A*, relativity says we should see the emission greatly amplified when its motion is moving toward us. It is this amplification they believe is being seen.

This bit of emission probably isn’t anything out of the norm – it is just newly noticed. It is perfectly normal for hotspots to exist in disks of material like the disk around Sag A*. What makes this so cool is that we found it. This is one heck of a challenging observation and analysis, and my hat is off to the team behind this work.

This rapidly orbiting object could be part of what is making the Event Horizon Telescope images so hard to process. Oka goes on to remind us of a problem we’ve all faced in our own photography: “the faster the movement is, the more difficult it is to take a photo of the object.” In this case, we have radio emissions flying across a radio image taken over many hours. I personally can only hope that careful analysis of the time-series images will allow the streaks of emission to be carefully removed. 

More Information

ALMA Observatory news article 

 “Time Variations in the Flux Density of Sgr A* at 230 GHz Detected with ALMA,” Yuhei Iwata et al., 2020 April 2, Astrophysical Journal Letters  (Preprint on arxiv.org)