Although we all know that supermassive black holes (tens of millions of times the mass of our Sun) lurk on the centre of most galaxies, their very nature makes them difficult to identify and study. In contrast to the favored idea of black holes always ‘gobbling up’ matter, these gravitational monsters can spend long periods of time in a dormant, inactive phase.
This was true of the black hole at the guts of SDSS1335+0728, a distant and unremarkable galaxy 300 million light-years away within the constellation of Virgo. After being inactive for a long time, it suddenly lit up and recently began producing unprecedented flashes of X-ray light.
The primary signs of activity appeared in late 2019, when the galaxy unexpectedly began shining brightly, attracting the eye of astronomers. After studying it for several years, they concluded that the weird changes they saw were probably the results of the black hole suddenly ‘switching on’ — entering an energetic phase. The intense, compact, central region of the galaxy is now classified as an energetic galactic nucleus, nicknamed ‘Ansky’.
“After we first saw Ansky light up in optical images, we triggered follow-up observations using NASA’s Swift X-ray space telescope, and we checked archived data from the eROSITA X-ray telescope, but on the time we didn’t see any evidence of X-ray emissions,” says Paula Sánchez Sáez, a researcher on the European Southern Observatory, Germany, and leader of the team that first explored the black hole’s activation.
Ansky wakes up
Then, in February 2024, a team led by Lorena Hernández-García, a researcher on the Valparaiso University, Chile, began to see bursts of X-rays from Ansky at nearly regular intervals.
“This rare event provides a chance for astronomers to look at a black hole’s behaviour in real time, using X-ray space telescopes XMM-Newton and NASA’s NICER, Chandra and Swift. This phenomenon is generally known as a quasiperiodic eruption, or QPE. QPEs are short-lived flaring events. And that is the primary time we have now observed such an event in a black hole that appears to be waking up,” explains Lorena.
“The primary QPE episode was discovered in 2019, and since then we have only detected a handful more. We do not yet understand what causes them. Studying Ansky will help us to raised understand black holes and the way they evolve.”
“XMM-Newton played a pivotal role in our study. It’s the only X-ray telescope sensitive enough to detect the fainter X-ray background light between the bursts. With XMM-Newton we could measure how dim Ansky gets, which enabled us to calculate how much energy Ansky releases when it lights up and starts flashing.”
Unravelling puzzling behaviour
The gravity of a black hole captures matter that gets too close and may rip it apart. The matter from a captured star, for instance, can be spread right into a hot, brilliant, rapidly spinning disc called an accretion disc. Current pondering is that QPEs are attributable to an object (that might be a star or a small black hole) interacting with this accretion disc they usually have been linked to the destruction of a star. But there isn’t a evidence that Ansky has destroyed a star.
The extraordinary characteristics of Ansky’s recurring bursts prompted the research team to contemplate other possibilities. The accretion disc might be formed by gas captured by the black hole from its neighbourhood, and never a disintegrated star. On this scenario, the X-ray flares can be coming from highly energetic shocks within the disc, provoked by a small celestial object travelling through and disrupting the orbiting material, repeatedly.
“The bursts of X-rays from Ansky are ten times longer and ten times more luminous than what we see from a typical QPE,” says Joheen Chakraborty, a team member and PhD student on the Massachusetts Institute of Technology, USA.
“Each of those eruptions is releasing 100 times more energy than we have now seen elsewhere. Ansky’s eruptions also show the longest cadence ever observed, of about 4.5 days. This pushes our models to their limits and challenges our existing ideas about how these X-ray flashes are being generated.”
Watching a black hole in motion
Having the ability to watch Ansky evolving in real time is an unprecedented opportunity for astronomers to learn more about black holes and the energetic events they power.
“For QPEs, we’re still at the purpose where we have now more models than data, and we’d like more observations to grasp what’s happening,” says ESA Research Fellow and X-ray astronomer, Erwan Quintin.
“We thought that QPEs were the results of small celestial objects being captured by much larger ones and spiralling down towards them. Ansky’s eruptions appear to be telling us a distinct story. These repetitive bursts are also likely related to gravitational waves that ESA’s future mission LISA might have the opportunity to catch.”
“It’s crucial to have these X-ray observations that may complement the gravitational wave data and help us solve the puzzling behaviour of massive black holes.”
