These days, the supermassive black hole at the center of our galaxy appears to be pretty docile. This wasn't always the case. New data from NASA's Chandra X-ray Observatory reveal that Sagittarius A* (Sgr A*, for short) once had spurts of high activity. To look at our local gravity trap's slightly more distant past, Chandra observed the x-ray light coming from gas clouds 30 light-years away from it. Fluorescence from these clouds is called a “light echo.”
When a black hole sucks in matter, it produces x-ray waves. Like sound waves in a canyon, the x-ray light bounces off stuff that's nearby. The waves then make their way to Earth. Because the echoed light takes a detour on its way here, it offers information about Sgr A* that's older than the picture compiled from x-rays that travel directly from the black hole to us. Researchers compiled 12 years of Chandra's observations—shown above, with Sgr A* at right and the echoed x-ray emissions in blue—to find two major, distinct flare-ups of light echoes.
They report this month in Astronomy & Astrophysics that signals from Sgr A* would have been, at times, 1 million times brighter than usual had we been observing it for the past couple hundred years. Further research will reveal what the increased activity levels say about Sgr A*'s eating habits—whether they are the last-gasps of a planet, a piece of a star, or maybe just some clumps of gas.
ScienceNOW, the daily online news service of the journal Science