The Milky Way’s Supermassive Black Hole had a Burst of Activity 200 Years Ago. We Just Saw the Echo.

We in the Milky Way Galaxy are pretty lucky to have a fairly quiet central supermassive black hole in Sgr A*. It’s not loud and bright like an active galactic nucleus. It appears to be active for brief periods before going to sleep. Two hundred years ago, it “woke up” for about a year and a half and had a bite to eat.

How do astronomers know this happened? They recently looked at specific molecular clouds in the neighborhood of the supermassive black hole. Those clouds seem brighter than usual. That’s because they’re glowing from X-rays emitted when Sgr A* had a brief period of activity at the beginning of the 19th century.

Most molecular clouds are dark and cold, so when one lights up like these are doing, it grabs astronomers’ attention. “One of the scenarios to explain why these giant molecular clouds are shining is that they are, in fact, echoing a long-gone flash of X-ray light, indicating that our supermassive black hole was not that quiescent some centuries ago,” said Frédéric Marin, an astronomer at the Astronomical Observatory of Strasbourg in France and lead author of a new study published in Nature.

Interestingly, astronomers think Sgr A* black hole accretion disk was also actively snacking about 150 years earlier for a time before it quieted down again. They tracked another nearby cloud bathed in gamma rays from that event. That hints at periods of wakefulness and quiescence that they want to explain.

Tracking the Glow of a Supermassive Black Hole Snack

Marin and a team of colleagues studied the X-ray-brightened clouds from Sgr A*’s most recent snack using NASA’s Imaging X-ray Polarimetry Explorer (IXPE). It measures the polarization of X-rays (or the average direction and intensity of the electric field of light waves). The polarization angle acts very much like a compass, pointing toward the source of the illumination. It turns out that Sgr A* is right on the path, making it the source of the emissions.

The team combined their data with images from the Chandra X-Ray Observatory and compared them to data from the XMM-Newton mission. That helped them really pinpoint the origin of the emissions brightening the nearby clouds. Basically, when the black hole accretion disk gobbled up nearby material, there resulting X-ray emissions were more than a million times greater than usual. That made it as luminous as an active galactic nucleus in a Seyfert galaxy.

What Happens When Our Supermassive Black Hole Feasts

The Sgr A* black hole and its accretion disk lie embedded at the center of our galaxy. It lives in a busy place. Stars circle the black hole in various orbits. Occasionally a cloud of gas and dust flits by. Just as occasionally, Sgr A*’s intense gravitational pull sweeps up some of that material into its accretion disk. The material gets heated and eventually, that energizes it enough to give off X-ray emissions.

This snacking happens fairly often, but usually, the flares are very short-lived—some on the order of only minutes. Those may take place in very small regions of the accretion disk. But some, like the one 200 years ago, are longer and brighter. For that event, the cloud of material that got sucked into the accretion disk must have been fairly large to trigger the brilliant, long-term blast of X-rays.

Learning More About Sgr A*

Now that Marin and the team have pinpointed the source of those emissions, they want to know more about the large flare that emitted them. For example, when exactly did it occur? How intense was it at its peak? How long did it actually last? Follow-up observations should give some answers and also provide astronomers a three-dimensional view of the glowing clouds.

One of the biggest unanswered questions about such flares is: what causes them? Obviously, the infall of material plays a role. But, what else is going on with Sgr A* that would cause it to “wake up” and start eating its neighbors? “IXPE is playing a key role in helping us better understand the timescale on which the black hole at the center of our galaxy is changing,” said Steven Ehlert, Steven Ehlert, IXPE project scientist at NASA. “We know change can happen to active galaxies and supermassive black holes on a human timescale. We’re learning more about this one’s behavior over time, its history of outbursts, and we’re eager to observe it further to determine which changes are typical and which are unique.”

Sonifying Sgr A*’s X-ray Emissions

Obviously, Sgr A* went back to sleep after its year-and-a-half-long cloud snack at the beginning of the 19th century. But, the echoes of its “burp” toward the nearby molecular clouds provided astronomers with a unique opportunity. They combined images of Sagittarius A* from NASA’s Imaging X-ray Polarimetry Explorer and Chandra X-ray Observatory and adapted them to sound. Essentially, they took the digital data from all the observations and translated it into musical notes and sounds. The original data would be too low for humans to hear, so the sounds were transposed into a region of sound that we can hear.

In this video, we see an arched line ripple across the image. As it passes over the orange-tinted IXPE data, sounds like digital winds are triggered, particularly where those orange areas are brightest. When the traveling line passes the blue-tinted Chandra data, the resulting notes resemble steel drums.

Aside from this sonification and the X-ray echo astronomers detected from Sgr A‘s outburst, there was no effect felt on Earth. That’s because Sgr A* lies some 26,000 light-years away from us. At that distance, we get a nice view of the long-ago action without any of the danger associated with a huge burst of X-rays.

For More Information

Detection of an echo emitted by Sagittarius A* 200 Years Ago
X-ray Polarization Evidence For a 200 years-old Flare of Sgr A*
Milky Way’s Central Black Hole Woke Up 200 Years Ago, NASA’s IXPE Finds