It's probably not realistic 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 image it, and it occasionally gets a bit more active as it rips something nearby to shreds. But supermassive black holes in other galaxies power some of the brightest phenomena in the Universe. The object at the center of the Milky Way, Sgr A^*, is nothing like those; instead, people get excited about the mere prospect that it might wake from its apparent slumber.

There's a chance that it was more active in the past, but any light from earlier events swept past Earth before we had observatories to see it. Now, however, scientists are suggesting they've seen echoes of light that might be associated with an Sgr A^* outburst that took place about 200 years ago.

Looking for echoes

Audible echoes are simply the product of sound waves reflected off some surface. Light travels as a wave, as well, and it can reflect off objects. So, the basic idea of light echoes is a pretty straightforward extrapolation of these ideas. They may seem counterintuitive 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 all gets indistinguishable.

That's not the case at astronomical distances. Here, it can take decades for light to traverse the distances between a source and a reflecting object, allowing us a glimpse of the past. The challenge is that, in many cases, the objects that could be reflecting light from elsewhere often produce light of their own. So we need some way of distinguishing reflected light from other sources.

Sgr A^* is surrounded by a number of clouds of material that both emit light and are a potential source of reflections. But the two sources should be different in their polarization. And we happen to have an instrument in orbit, the Imaging X-ray Polarimetry Explorer, that's capable of (as its name implies) figuring out the polarization of X-ray photons. The researchers combined that with images taken by the Chandra X-ray Observatory, which provided high-resolution images of all the glowing material in the vicinity of our galaxy's core.

The resulting data was a mix of constant sources—the X-ray background, plus the emissions from the clouds of material themselves—plus reflections of any light produced by the nearby Sgr A^*, which could vary over time. So, the astronomers built a model that took all of it into account, including multiple observations over time and the polarization information.

Right place, right time

The net result of the model is a polarization angle that's consistent with one of the sources of X-rays being reflected from a source at Sgr A^*. (You'd expect Sgr A^* to produce an angle of -42°, while the model calls for the source to be between -37° and -59°.) It also provided information on the timing of the flare that was being reflected, indicating it was consistent with an event that happened either 30 or 200 years ago.

But, as the researchers helpfully point out, we had observatories that would have spotted something if it had happened 30 years ago. So, they strongly favor 200 years as the likely timing.

The flare itself was likely to be a short one, in astronomical terms. Based on the limits of how much material was likely to flow into Sgr A^*, the researchers calculate that a low-luminosity event could potentially produce these light echoes given one to two years. If the inflowing material was close to the maximum amount, then Sgr A^* could have output enough energy in just a few hours.

That sort of behavior is consistent with how black holes seem to operate. Their luminosity—technically the luminosity driven by the energy they impart to the material immediately nearby—largely depends on how much material they're ingesting at the time. If the Milky Way's black hole is currently quiet, it's simply because there's nothing around to eat at the moment. But there's no reason to think that has always been the case.

Nature, 2023. DOI: 10.1038/s41586-023-06064-x  (About DOIs).