An article published in the journal “Nature” reports evidence that about 200 years ago, Sagittarius A*, or simply Sgr A*, the supermassive black hole at the center of the Milky Way, had a period of intense activity during which it swallowed considerable amounts of gas and dust. A team of researchers led by Frédéric Marin of the Astronomical Observatory of Strasbourg, France, used in particular data collected by the IXPE space telescope to examine the polarization of X-ray light emitted by bright large molecular clouds close to Sgr A*. The conclusion is that their out-of-normal brightness must have been due to the fact that they are reflected emissions produced by some kind of powerful and short-lived flare of the supermassive black hole that occurred about 200 years ago.
Sagittarius A* has a mass estimated at just over four million times the Sun’s. It’s remarkable yet it’s small and quiet compared to other supermassive black holes, which can have masses billions of times the Sun’s and can be extremely active, such as quasars.
Some studies had already detected X-ray emissions from molecular clouds, giant clouds of gas near Sagittarius A* that are usually cold and dark and therefore have weak emissions. In this case, the emissions had an abnormal intensity and one possible explanation was that they reflect emissions that arrived from the supermassive black hole in the past. To prove it, however, a complex study with the right instrument was needed.
Launched on December 9, 2021, the IXPE (Imaging X-ray Polarimetry Explorer) space telescope is a mission in collaboration between NASA and the Italian Space Agency capable of measuring the polarization of light in X-rays, which means the direction and average intensity of the electric field of light waves. IXPE was used to examine the molecular clouds near Sagittarius A* and the collected data was combined with data collected by NASA’s Chandra X-ray Observatory and archive observations conducted with ESA’s XMM-Newton space telescope.
The data put together made it possible to isolate the X-ray signal reflected by the molecular clouds and identify the real point of origin, confirming that they came from Sagittarius A*. The researchers estimated that those emissions were generated about 200 years ago from the point of view of the Earth observer. According to the reconstruction, at that time, the supermassive black hole swallowed a significant amount of gas and dust and was very bright for a while.
These conclusions of the researchers are not the end of the research on this event. The observations will continue to improve the precision of the event timeline’s estimates and obtain a three-dimensional reconstruction of the distribution of the molecular clouds surrounding Sagittarius A*. All this will help to better understand the supermassive black hole’s past activity and the conditions in which it can enter periods of activity, also useful to try to predict future events of this type.
