X-ray ‘echoes of light’ signal as the Milky Way’s central black hole erupts – Ars Technica

Zoom in / This is the first image of Sgr A*, the supermassive black hole at the center of our galaxy. It is the first direct visual evidence of the existence of this black hole. It was taken by the Event Horizon Telescope (EHT).

EHT collaboration

Perhaps it is not realistic to call a supermassive black hole “quiet”. But in terms of these things, the ones in the center of our galaxy are pretty quiet. Yes, it emits enough energy that we can picture it, and sometimes it gets more energetic 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*Nothing like these; Instead, people get excited at the mere prospect of being awakened from their apparent slumber.

There is a possibility that it was more active in the past, but no light from past events traversed Earth before we would have had observatories to see it. Now, however, scientists are suggesting they saw echoes of light that may be associated with Sgr A.* The eruption that occurred about 200 years ago.

I’m looking for echoes

Audible echoes are simply the product of sound waves reflecting off some surface. Light travels as a wave too, and it can reflect off things. So, the basic idea of ​​light resonance is a very direct extrapolation of these ideas. They may sound inconsequential because, unlike acoustic echoes, we never feel a light echo in normal life — light travels so quickly that any echoes from the world around us arrive at the same time as the light itself. Everything is indistinguishable.

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This is not the case at astronomical distances. Here, light can take decades to traverse the distances between the source and the reflecting object, giving us a glimpse into the past. The challenge is that in many cases, objects that can reflect light from somewhere else often produce their own light. So we need some way to distinguish reflected light from other sources.

Sergeant A.J* It is surrounded by a number of clouds of material that emit light and are a possible source of reflections. But the two sources must be of different polarities. And we happen to have an instrument in orbit, which is Polarizing X-ray Imaging Explorer, this is capable (as its name suggests) of detecting the polarization of x-ray photons. The researchers combined that with the photos you took Chandra X-ray Observatorywhich provided high-resolution images of all the glowing material found near the core of our galaxy.

The resulting data was a combination of stationary sources – background X-rays, as well as emissions from the clouds of the material itself – plus reflections of any light produced by nearby Sgr A.*, which may vary over time. So, the astronomers built a model that took all of that into account, including multiple observations over time and polarization information.

Right place right time

The net result of the model is a polarization angle corresponding to one of the X-ray sources reflected from a source in Sgr A*. (You would expect Sgr A* to produce an angle of -42 degrees, while the model calls for the source to be between -37 and -59 degrees.) It also provided information about the timing of the glow that was reflecting, suggesting that it was consistent with an event that occurred 30 or 200 years ago.

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

The flare is likely to be astronomically short. Based on the limits of the amount of material that is likely to flow into Sgr A*, the researchers calculate that a low-luminosity event could produce a potential photoresonance within one to two years. If the flowing substance is close to the maximum amount, then Sgr A* It can produce enough power within a few hours.

This kind of behavior is consistent with the way black holes work. Their luminosity — technically the luminosity is driven by the energy you impart to the material directly nearby — depends largely on how much material they’re ingesting at the time. If the black hole in the Milky Way is currently quiet, it is simply because there is nothing around it to eat at the moment. But there is no reason to believe that this is always the case.

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

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