XMM-Newton's observations highlight the complexities of black hole feeding mechanisms. Predicted by Einstein's theory of general relativity, black holes are immense gravitational entities that capture any matter or energy crossing their event horizons. Matter falling into a black hole forms a swirling accretion disk, which heats up and radiates primarily ultraviolet (UV) light. This UV light interacts with a surrounding plasma cloud, called the corona, and is boosted to X-ray energies detectable by XMM-Newton.

Since 2011, XMM-Newton has monitored the supermassive black hole in galaxy 1ES 1927+654. While initially stable, dramatic changes began in 2018 when the X-ray corona vanished following an outburst. Over time, the corona reformed, and by 2021, conditions appeared to normalize. Yet in July 2022, XMM-Newton observed unusual quasi-periodic oscillations (QPOs) in the X-ray emissions, fluctuating at levels of 10% over 400-1000 seconds. Such phenomena are rarely detected in supermassive black holes.

"This was our first indication that something strange was going on," explained Megan Masterson, a PhD student at the Massachusetts Institute of Technology and leader of the study.

The QPOs suggested the presence of a compact object, likely a white dwarf, orbiting within the accretion disk. Calculations indicated this stellar remnant, roughly 0.1 times the Sun's mass, was orbiting at a staggering speed, completing one orbit every 18 minutes and traveling approximately 100 million kilometers per revolution.

Over the next two years, the oscillations intensified in both frequency and amplitude, defying expectations. The team theorized that orbital energy was being radiated as gravitational waves, as per general relativity. Based on this model, Masterson calculated the object's disappearance into the black hole's event horizon by January 4, 2024. However, observations in March 2024 showed the oscillations persisting, with the object now orbiting every seven minutes at half the speed of light. This led researchers to reconsider their hypotheses.

One alternative explanation involved oscillations originating from the X-ray corona itself, but existing models offered no clear support for this scenario. Revisiting their original theory, the researchers proposed a modification: instead of plunging intact into the black hole, the white dwarf might be gradually disintegrating. Such a process has been observed in white dwarf pairs, where tidal interactions cause one star to strip material from the other, slowing their convergence.

Future observations by ESA's Laser Interferometer Space Antenna (LISA), set to launch in the 2030s, may confirm this scenario. "We predict that if a white dwarf is orbiting this supermassive black hole, LISA should detect its gravitational waves," said Masterson. This discovery could illuminate the intricate dynamics near a black hole's event horizon.

"This is another great example of XMM-Newton's unique abilities," noted Norbert Schartel, ESA XMM-Newton Project Scientist. "The detection relied on its exceptional observation duration, wide X-ray band coverage, and precise timing resolution."

Research Report:Millihertz Oscillations Near the Innermost Orbit of a Supermassive Black Hole

Related Links
XMM-Newton at ESA
Understanding Time and Space