Currently, the supermassive black hole Sagittarius A*, located at the center of the Milky Way, appears to be very calm. However, this has not always been the case. Research conducted using the XRISM X-ray telescope shows that it has experienced several powerful explosions in the past.
Sagittarius A* and evidence of a flare
The supermassive black hole in our galaxy is known as one of the faintest in the Universe. Data obtained with a new space telescope suggests that this was not always the case.
Sagittarius A*, located at the center of the Milky Way, appears to have flared dramatically somewhere in the last few hundred or thousand years, judging by X-ray emissions observed by the XRISM space telescope. These amazing discoveries reveal new details about the evolution of supermassive black holes. They also give astronomers insight into the history of our cosmic home.
Michigan State University researcher Stephen DiKerby collaborated with an international team to measure X-ray radiation coming from a giant cloud of gas near the center of the galaxy. The team studied the cloud in detail thanks to XRISM’s ability to distinguish the energy of individual X-ray photons. Their findings provide strong evidence that the cloud is glowing in response to a past flare from Sagittarius A*.
XRISM telescope observation of a black hole
Many supermassive black holes are bright because the gas around them heats up and emits high-energy radiation. In contrast, Sagittarius A* emits almost no light. It is one of the dimmest known black holes in the Universe, visible only because it is so close to Earth.
Several large molecular clouds float around Sagittarius A and may act as cosmic mirrors, reflecting past bursts of X-rays from the black hole. Previous space telescopes could detect these bursts, but did not have sufficient energy resolution to investigate their fine structure or determine what caused them.
XRISM changed that. The telescope was launched in 2023 as part of a partnership between NASA and the Japan Aerospace Exploration Agency. Its first observations are generating a lot of interest because they significantly exceed all existing space telescopes in terms of energy resolution.
Most space telescopes operating in the X-ray range can distinguish photon energy with an accuracy of one-tenth or even one-hundredth. XRISM can distinguish energy with an accuracy of one-thousandth. The new images represent a transition from Polaroid photographs to high-quality color images.
Revealing the long-standing activity of a black hole
DiKerby used this sharp optics to magnify two very narrow lines of X-ray radiation coming from one of the molecular clouds. By measuring their energy and shape with unprecedented precision, he was able to determine the cloud’s motion and compare it with previous radio observations. He also studied subtle features of the spectrum to test two conflicting hypotheses about the cloud’s emission.
These details ruled out the hypothesis of cosmic ray influence and instead showed that the cloud reflects X-ray bursts from Sagittarius A* — essentially a “light echo” from the past. By studying several clouds at different distances from the black hole, astronomers can reconstruct the chronology of these ancient flares, similar to using echo delay to map the shape of a cave.
“These extraordinary measurements show how powerful XRISM is for revealing the hidden history of our galaxy’s center,” say astronomers. “Thanks to such a clear distinction between the iron lines, we can now read the past activity of the galactic center in unprecedented detail.”
The data shows for the first time how XRISM’s energy resolution allows us to measure very fine features of the Universe. The team expects the telescope to provide many new opportunities for discovery.
According to phys.org
