It may be unrealistic to call a supermassive black hole “quiet.” But, as far as these things go, the one at the center of our galaxy is pretty quiet. Yes, it emits enough energy that we can describe, and sometimes it becomes more active as it tears something nearby to pieces. But supermassive black holes in other galaxies drive some of the brightest events in the Universe. The object at the center of the Milky Way, Sgr A*, nothing like that; rather, people are excited about the mere prospect of it waking up from its apparent slumber.
There’s a chance it was more active in the past, but any light from earlier events washed over Earth before we had observatories to see it. Now, however, scientists suggest that they have seen echoes of light that could be associated with an Sgr A.* outburst that happened about 200 years ago.
Looking for echoes
Audible echoes are simply the product of sound waves reflected off certain surfaces. Light travels as a wave, as well, and it reflects off objects. So, the basic idea of light echoes is a straightforward extrapolation of these ideas. This can be inexplicable because, unlike sonic echoes, we never experience light echoes in normal life—light travels so fast that any echoes from the world around us arrive at the same time as the light. itself. It can all be anonymous.
That is not the case with astronomical distances. Here, light can take decades to traverse the distances between a source and a reflecting object, allowing us a glimpse into the past. The challenge is that, in many cases, objects that may be emitting light from elsewhere are often emitting light themselves. So we need some way to distinguish reflected light from other sources.
Sgr A* surrounded by many clouds of light-emitting material and a potential source of reflections. But the two sources must be different in their polarization. And we have an instrument in orbit, the Imaging X-ray Polarimetry Explorer, which can (as its name implies) determine the polarization of X-ray photons. The researchers combined that with images taken by the Chandra X-ray Observatory, which provides high-resolution images of all the glowing material around the core of our galaxy.
The resulting data is a mixture of constant sources—the X-ray background, plus emissions from clouds of material itself—plus reflections of any light produced by nearby Sgr A*, which may vary over time. So, astronomers built a model that took all of this into account, including multiple observations over time and polarization information.
Right place, right time
The result of the model is the polarization angle consistent with one of the X-ray sources visible from the source of Sgr A*. (You would expect Sgr A* to create an angle of -42 degrees, while the model calls for the source to be between -37 and -59 degrees.) It also provides information on the time of the flare shown, indicating that it is consistent with a event that happened 30 or 200 years ago.
But, as the researchers helpfully point out, we have observatories that have seen something when it happened 30 years ago. Therefore, they strongly favor 200 years as a possible time.
The flare is probably a short, astronomical term. Based on the limits on how much material is likely to flow into Sgr A*, the researchers calculated that a low-luminosity event could produce these light echoes given one to two years. If the outflow of material is close to its maximum value, then Sgr A* can output enough energy in just a few hours.
That kind of behavior is consistent with how black holes seem to work. Their light—technically light driven by the energy they give off to the material immediately nearby—depends largely on how much material they consume at the time. If the Milky Way’s black hole is currently quiet, it’s because there’s nothing to eat around right now. But there is no reason to think that this is always the case.
Nature, 2023. DOI: 10.1038/s41586-023-06064-x (Part of DOIs).