“When
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 at the time we
didn’t see any evidence of X-ray emissions,” says Paula Sánchez Sáez, a
researcher at 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 at the Valparaiso University, Chile, began to see bursts of
X-rays from Ansky at nearly regular intervals.
“This rare event provides an opportunity for astronomers to observe 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 known
as a quasiperiodic eruption, or QPE. QPEs are short-lived flaring
events. And this is the first time we have observed such an event in a
black hole that seems to be waking up,” explains Lorena.
“The first QPE episode was discovered in 2019, and since then we’ve
only detected a handful more. We don’t yet understand what causes them.
Studying Ansky will help us to better understand black holes and how
they evolve.”
“XMM-Newton played a pivotal role in our study. It is 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 can
rip it apart. The matter from a captured star, for example, would be
spread into a hot, bright, rapidly spinning disc called an accretion
disc. Current thinking is that QPEs are caused by an object (that could
be a star or a small black hole) interacting with this accretion disc
and they have been linked to the destruction of a star. But there is no
evidence that Ansky has destroyed a star.
The extraordinary characteristics of Ansky’s recurring bursts
prompted the research team to consider other possibilities. The
accretion disc could be formed by gas captured by the black hole from
its neighbourhood, and not a disintegrated star. In this scenario, the
X-ray flares would be coming from highly energetic shocks in 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 at the Massachusetts
Institute of Technology, USA.
“Each of these eruptions is releasing a hundred times more energy
than we have 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 action
Being able 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 point where we have more models than
data, and we need more observations to understand what's happening,”
says ESA Research Fellow and X-ray astronomer, Erwan Quintin.
“We thought that QPEs were the result of small celestial objects
being captured by much larger ones and spiralling down towards them.
Ansky’s eruptions seem to be telling us a different story. These
repetitive bursts are also likely associated with gravitational waves
that ESA’s future mission LISA might be able to catch.”
“It’s crucial to have these X-ray observations that will complement the
gravitational wave data and help us solve the puzzling behaviour of
massive black holes.”
