In space, a supermassive black hole is a complex phenomenon of unparalleled power, whose activities have profound implications for the evolution of a galaxy. At the center of the Milky Way is one such colossi, Sagittarius A* (Sgr A). A groundbreaking study, published today in Nature, reveals a period of intense activity of the latter some 200 years ago. Through innovative use of NASA’s IXPE satellite, researchers have detected for the first time the polarization of X-ray light emitted by Sgr A. This discovery not only led to a better understanding of the behavior of supermassive black holes, but also opened new perspectives for studying these fascinating cosmic objects and their impact on the universe.

About 200 years ago, Sgr A* emerged from its space slumber. An international team, led by Frédéric Marin, a CNRS researcher at the Strasbourg Astronomical Observatory, managed to prove it.

For a year, it swallowed passing cosmic objects, before returning to a state of rest. This event had no effect on Earth, since the distance between Sgr A* and our planet was too great. It is approximately two billion times greater than the distance between the Earth and the Sun.

However, it is an X-ray echo that was perceived, a kind of remnant of the light emitted by Sgr A* two centuries ago. It reveals an original intensity at least a million times more intense than that currently emitted by the supermassive black hole.

For the authors, the intensity of the X-ray emission between sleep and awakening can be compared to “a firefly lurking in a forest that would suddenly become as bright as the sun.” These results explain why the galactic molecular clouds around Sgr A* shine brighter than usual. They reflect X-rays emitted by Sgr A* 200 years ago.

To carry out their observations, the researchers used a state-of-the-art technological tool: the Imaging X-ray Polarimetry Explorer (IXPE) satellite, as mentioned above. This satellite is specially designed to detect and analyze X-rays. This is a form of high-energy radiation emitted by various cosmic objects, including black holes.

The IXPE satellite was pointed at the center of the Milky Way, directly at Sagittarius A*. The objective was to detect the X-rays that it could emit. Then analyze them to get information about the past activity of that black hole.

One of the main characteristics of the X-rays that the IXPE satellite can analyze is their polarization. It is a property of electromagnetic waves, like light, that describes the direction of the oscillations of the electric field. In the case of X-rays emitted by Sgr A*, their polarization provides valuable information about the origin and nature of these rays.

Thanks to the sensitivity of the IXPE satellite, the researchers were able to detect with great precision the polarization of the X-rays emitted by Sgr A. They determined and confirmed that he was the source of these X-rays. Information that until now was impossible to obtain with certainty. As the authors explain: Like a compass, polarized X-ray light points directly at its source, Sgr A. Even if the latter is almost extinct today.”

Scientists continue their work at Sgr A*. They want to determine the physical mechanisms required for a black hole to transition from an inactive state to an active state. This research may provide answers to fundamental questions about black hole activity. Thus, it will shed light on its interaction with the environment and its influence on the evolution of galaxies.

This unprecedented use of the IXPE satellite to detect X-ray polarization offers a method for studying cosmic objects. This will help to map the space, determining the precise position of the X-ray sources.

In addition, the success of this research highlights the potential of instruments capable of detecting X-ray polarization. It paves the way for new technologies for space observation. Scientists can now study these cosmic phenomena with unprecedented precision.