Astronomers were keeping tabs on data from the Zwicky Transient Facility, an all-sky survey based at the Palomar Observatory in California, when they detected an extraordinary flash in a part of the sky where no such light had been observed the night before. From a rough calculation, the flash appeared to give off more light than 1,000 trillion suns.
The team, led by researchers at NASA, Caltech, and
elsewhere, posted their discovery to an astronomy newsletter, where the signal
drew the attention of astronomers around the world, including scientists at
MIT. Over the next few days, multiple telescopes focused in on the signal to
gather more data across multiple wavelengths in the X-ray, ultraviolet,
optical, and radio bands, to see what could possibly produce such an enormous
amount of light.
Now, the MIT astronomers along with their collaborators have
determined a likely source for the signal. In a study appearing in Nature
Astronomy, the scientists report that the signal, named AT 2022cmc, likely
comes from a relativistic jet of matter streaking out from a supermassive black
hole at close to the speed of light. They believe the jet is the product of a
black hole that suddenly began devouring a nearby star, releasing a huge amount
of energy in the process.
Astronomers have observed other such “tidal disruption
events,” or TDEs, in which a passing star is torn apart by a black hole’s tidal
forces. AT 2022cmc is brighter than any TDE discovered to date. The source is
also the farthest TDE ever detected, at some 8.5 billion light years away —
more than halfway across the universe.
How could such a distant event appear so bright in our sky?
The team says the black hole’s jet may be pointing directly toward Earth,
making the signal appear brighter than if the jet were pointing in any other
direction. The effect is “Doppler boosting” and is similar to the amped-up
sound of a passing siren.
AT 2022cmc is the fourth Doppler-boosted TDE ever detected
and the first such event that has been observed since 2011. It is also the
first TDE discovered using an optical sky survey.
As more powerful telescopes start up in the coming years,
they will reveal more TDEs, which can shed light on how supermassive black
holes grow and shape the galaxies around them.
“We know there is one supermassive black hole per galaxy,
and they formed very quickly in the universe’s first million years,” says
co-author Matteo Lucchini, a postdoc in MIT’s Kavli Institute for Astrophysics
and Space Research. “That tells us they feed very fast, though we don’t know
how that feeding process works. So, sources like a TDE can actually be a really
good probe for how that process happens.”
Lucchini’s MIT co-authors include first author and Research
Scientist Dheeraj “DJ” Pasham, postdoc Peter Kosec, Assistant Professor Erin
Kara, and Principal Research Scientist Ronald Remillard, along with
collaborators at universities and institutions around the world.
Feeding frenzy
Following AT 2022cmc’s initial discovery, Pasham and
Lucchini focused in on the signal using the Neutron star Interior Composition
ExploreR (NICER), an X-ray telescope that operates aboard the International
Space Station.
“Things looked pretty normal the first three days,” Pasham
recalls. “Then we looked at it with an X-ray telescope, and what we found was,
the source was too bright.”
Typically, such bright flashes in the sky are gamma-ray
bursts — extreme jets of X-ray emissions that spew from the collapse of massive
stars.
“This particular event was 100 times more powerful than the
most powerful gamma-ray burst afterglow,” Pasham says. “It was something
extraordinary.”
The team then gathered observations from other X-ray, radio,
optical, and UV telescopes and tracked the signal’s activity over the next few
weeks. The most remarkable property they observed was the signal’s extreme
luminosity in the X-ray band. They found that X-ray emissions from AT 2022cmc
swung widely by a factor of 500 over a few weeks,
They suspected that such extreme X-ray activity must be
powered by an “extreme accretion episode” — an event that generates a huge
churning disk, such as from a tidal disruption event, in which a shredded star
creates a whirlpool of debris as it falls into a black hole.
Indeed, the team found that AT 2022cmc’s X-ray luminosity
was comparable to, though brighter than, three previously detected TDEs. These
bright events happened to generate jets of matter pointing straight toward
Earth. The researchers wondered: If AT 2022cmc’s luminosity is the result of a
similar Earth-targeting jet, how fast must the jet be moving to generate such a
bright signal? To answer this, Lucchini modeled the signal’s data, assuming the
event involved a jet headed straight toward Earth.
“We found that the jet speed is 99.99 percent the speed of
light,” Lucchini says.
To produce such an intense jet, the black hole must be in an
extremely active phase — what Pasham describes as a “hyper-feeding frenzy.”
“It’s probably swallowing the star at the rate of half the
mass of the sun per year,” Pasham estimates. “A lot of this tidal disruption
happens early on, and we were able to catch this event right at the beginning,
within one week of the black hole starting to feed on the star.”
“We expect many more of these TDEs in the future,” Lucchini
adds. “Then we might be able to say, finally, how exactly black holes launch
these extremely powerful jets.”
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