When a flash of light appears somewhere in the sky, astronomers notice. When it appears in a region of the sky not known to host a stellar object that’s flashed before, they really sit up and take notice. In astronomical parlance, objects that emit flashing light are called transients.
Earlier this year, astronomers spotted a transient that flashed with the light of a trillion Suns.
In this case, it was the Zwicky Transient Facility (ZTF) that spotted the flash. The ZTF is an all-sky survey aimed at the northern night sky. It’s hosted at the Palomar Observatory, and it’s a systematic study using an extremely wide-field optical light camera to scan the entire northern sky every two days. It’s part of what’s known as Time-Domain Astronomy, the study of astronomical objects that change over time.
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When the ZTF spots a new transient in the sky, other astronomers are alerted. The ZTF isn’t suited to studying objects in detail. It just finds them and then passes the baton to other facilities that are better suited for observing astronomical objects in greater detail. In this case, a whole group of facilities took part.
Hubble Space Telescope observations in optical and infrared combined with data from the Jansky Very Large Array pinpointed the flash’s precise location. The European Southern Observatory’s (ESO) Very Large Telescope (VLT) determined that it was 8.5 billion light-years away. Observational data from other facilities followed, giving astronomers a picture of the flash across a wide swath of the electromagnetic spectrum.
The results of all those observations, and the analysis that followed, are published in a new paper in Nature Astronomy. The paper is “The Birth of a Relativistic Jet Following the Disruption of a Star by a Cosmological Black Hole.” The first author is Dheeraj Pasham, a research scientist at the Kavli Institute for Astrophysics and Space Research at MIT.
As the title tells us, the transient light source was a jet of matter emitted from a supermassive black hole (SMBH) at 99.9% of the speed of light. The light signal has a name, AT 2022cmc, and the SMBH responsible for it is halfway across the Universe. What caused it? Something extraordinary, according to lead author Pasham.
A behemoth supermassive black hole (SMBH) at the heart of a distant galaxy is responsible. The SMBH is swallowing a star that got too close. This is called a Tidal Disruption Event (TDE) and it’s the first one observed since 2011. It’s also the first one spotted in optical light, and the 78th one that ZTF has detected.
AT 2022cmc is the most distant TDE ever seen, and also the brightest. Gamma Ray Bursts (GRB) are the brightest objects in the Universe, second only to the Big Bang. So it’s natural to assume that the event was a GRB. But it wasn’t. The jet’s high x-ray luminosity helped rule that out.
“This particular event was 100 times more powerful than the most powerful gamma-ray burst afterglow,” lead author Pasham said in a press release. “It was something extraordinary.”
The TDE just happened to point its searing jet of material directly at Earth, like a flashlight shone right in our eyes. Rough calculations showed that the jet was as bright as a trillion Suns.
The Universe is full of transient events, but observing TDEs is still rare. It helps when the jet is aimed right at Earth, as it was in this case. But when a SMBH consumes a star that got too close, it doesn’t always emit jets. TDEs like this one give astronomers an opportunity to learn more about the SMBHs that cause them.
“The last time scientists discovered one of these jets was well over a decade ago,” said Michael Coughlin, an assistant professor of astronomy at the University of Minnesota Twin Cities and co-lead on the paper. “From the data we have, we can estimate that relativistic jets are launched in only 1% of these destructive events, making AT2022cmc an extremely rare occurrence. In fact, the luminous flash from the event is among the brightest ever observed.”
Supermassive Black Holes are, obviously, extraordinarily huge. The most massive ones are several billions of times more massive than the Sun. Even in astronomy, a subject known for large numbers, something several billion times more massive than our star is almost incomprehensible.
But as it turns out, even something that large can’t eat a star in one bite. It’s taking its time devouring the star. The jet was probably emitted during an intermittent ‘feeding frenzy,’ according to Pasham. “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.”
Astronomers can’t yet see the galaxy that emitted it. The jet’s light is so powerful that it’s outshining its host galaxy. But astronomers think that once the jet dims they’ll be able to spot the galaxy with the Hubble and the James Webb Space Telescope.
That might lead them partway to answering an important question: All SMBHs are bound to eat stars, why do so few of them emit jets? Observations show that the ones that emit these types of jets are likely spinning rapidly. The rotation helps power these ultraluminous jets.
The rapid rotation might be only one factor, perhaps the factor that’s easiest to observe. But it does bring researchers one step closer to understanding the awesome forces at work in SMBHs.
“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.”
What astrophysicists need, is to find more of these jets, TDEs, and SMBHs. They’ll probably get their wish in the near future.
With facilities like the Vera Rubin Observatory coming online soon, we’re bound to spot more transients like AT2022cmc. The Vera Rubin should see first light in 2023, and will perform a synoptic survey that will image the entire visible night sky every few nights. One of its four science goals is to find transients and notify other observatories for follow-up observations. And it should find a lot of them.
“Our new search technique helps us to quickly identify rare cosmic events in the ZTF survey data. And since ZTF and upcoming larger surveys such as Vera Rubin’s LSST scan the sky so frequently, we can now expect to uncover a wealth of rare, or previously undiscovered cosmic events and study them in detail,” says Igor Andreoni, a postdoctoral associate in the Department of Astronomy at UMD and NASA Goddard Space Flight Center.
“Astronomy is changing rapidly,” Andreoni said. “More optical and infrared all-sky surveys are now active or will soon come online. Scientists can use AT2022cmc as a model for what to look for and find more disruptive events from distant black holes. This means that more than ever, big data mining is an important tool to advance our knowledge of the universe.”
Gone are the days when professional astronomers spend long cold nights looking into the eyepiece of their telescopes. If we still relied on those efforts, we’d likely never even see a TDE. Automated sky surveys are becoming more and more prevalent, covering larger swaths of the sky than astronomers can, and doing it more diligently. They never get tired, get sick, or take holidays.
But facilities like them generate an enormous amount of data, which Andreoni alluded to. The Vera Rubin Observatory is expected to take 200,000 pictures each year of its ten-year run. That means that it’ll generate 1.2 petabytes of data each year, far more data than astronomers will be able to handle. It’ll be up to AI and machine learning to deal with all that data.
The Zwicky Transient Facility served as a prototype for the Vera Rubin. But while the ZTF found 78 TDEs since its inception, the Vera Rubin will dwarf those results. Nobody’s certain how many TDEs it’ll find, but the observatory is expected to generate hundreds of alerts per second, and each one will be a transient of some sort.
Some of those will be TDEs, and as more detections roll in, astronomers will do follow-up observations with other facilities.
“We expect many more of these TDEs in the future,” said Lucchini. “Then we might be able to say, finally, how exactly black holes launch these extremely powerful jets.”