For centuries, the supermassive black hole at the center of the Milky Way, known as Sagittarius A* (Sgr A*), has remained relatively quiet, showing no significant signs of activity. However, recent observations from NASA”s Imaging X-ray Polarimetry Explorer (IXPE) have uncovered evidence suggesting that this cosmic giant experienced a brief awakening approximately 200 years ago, during which it emitted a powerful burst of X-rays that illuminated its surroundings before returning to dormancy.
The findings, published in the journal Nature, provide an unprecedented glimpse into the recent history of the closest black hole to Earth, located over 25,000 light-years away in the constellation Sagittarius. Sgr A* is estimated to possess a mass millions of times that of the Sun, though its current luminosity is faint compared to active black holes in other galaxies.
Ancient Flare Echoes Detected
The key to this discovery lies in the unexpected brightness of massive molecular clouds near the black hole. Theoretically, these cold, dark clouds should emit only weak X-ray signals. Contrary to expectations, instruments detected intense emissions, suggesting they were illuminated by a past event. “One explanation for why these giant molecular clouds are glowing is that they are echoing a long-lost X-ray flare,” explained Frédéric Marin, an astronomer at the Strasbourg Astronomical Observatory in France and the study”s lead author. “This indicates that our supermassive black hole wasn”t as inactive a few centuries ago as previously thought.”
The IXPE, designed to measure the polarization of X-ray light, allowed scientists to ascertain the direction and intensity of the electric field of the light waves emanating from these clouds. Observations took place in February and March 2022 and were combined with data from NASA”s Chandra X-ray Observatory and the European Space Agency”s XMM-Newton.
“The polarization angle acts like a compass, pointing us toward the mysterious and vanished source of illumination,” noted Riccardo Ferrazzoli, an astrophysicist from the National Institute of Astrophysics in Italy. “And what lies in that direction? None other than Sgr A*.”
Recent Event in Cosmic Timescale
The data confirmed that the light detected in the clouds is a reflection of a brief and intense eruption originating from the black hole, likely caused by its sudden consumption of a large amount of gas or cosmic dust. According to the team”s estimates, this eruption occurred around 200 Earth years ago, placing it in the early 19th century. In astronomical terms, this is a very recent event, providing a unique opportunity to study the fluctuations between calm and active phases of black holes.
Steven Ehlert, a scientist involved with the IXPE project, explained that the mission plans to repeat these observations to reduce uncertainties in the measurements and refine calculations regarding the intensity and duration of the event. These data will also assist in mapping the three-dimensional distribution of the molecular clouds surrounding the black hole. “IXPE is playing a crucial role in helping us better understand the timescales over which the black hole at the center of our galaxy is changing,” Ehlert said. “We know that change can occur in active galaxies and supermassive black holes on a human timescale. We are learning more about their behavior over time and their history of outbursts.”
The IXPE project is a collaboration between NASA and the Italian Space Agency, involving scientists from twelve countries. The mission is managed by NASA”s Marshall Space Flight Center, while Ball Aerospace in Colorado operates the spacecraft alongside the Laboratory for Atmospheric and Space Physics at the University of Colorado Boulder. Thanks to this international collaboration, science has been able to peer—quite literally—into the past of our galaxy. What was once a dark and silent point now reveals traces of intense recent activity. Sagittarius A*, the slumbering heart of the Milky Way, had a fleeting awakening just two centuries ago, reminding us that even the giants of the cosmos can stir in the blink of an eye.
