A jet of charged particles moving at almost the speed of light, made from the remnants of a star that was brutally ripped apart by a supermassive black hole, has been found to be one of the most luminous, energetic events astronomers have ever witnessed in the universe.
The jet, triggered by what astronomers refer to as a tidal disruption event (TDE), is so powerful that finding an real world phenomenon to compare it to is difficult. And so, the astronomers led by Yvette Cendes of the University of Oregon have opted to compare it to the estimated energy output of a fictional device: Star Wars’ Death Star, which can blow up entire planets.
“Planets are going to be destroyed for the first few light-years,” Cendes, who is a radio astronomer, told Space.com. “I’m just not sure how far out from the jet this would be the case.”
More specifically, the total energy of this event, which has been officially catalogued as AT2018hyz, depends upon how that energy is being emitted. Relativistic jets from TDEs are very rare, accounting for about 1% of all known cases. The other 99% are a spherical outflow that moves much more slowly. In the latter case we’d be looking at an energy output of 2 x 10^50 ergs (an erg is a unit of energy; the sun outputs 10^33 ergs at its peak) while the jet scenario, which Cendes favors given the immense luminosity of AT2018hyz, would reach 5 x 10^55 ergs.
And the energy output continues to increase. Models suggest that it will peak in 2027 before gradually climbing back down.
“I am hesitant to give a final energy estimate — there are too many things that it will depend on that will become clear once we actually see the peak,” said Cendes. “But we anticipate that it will be about twice as luminous at the peak than what it is now.”
So, how did this immense eruption of energy come about? AT2018hyz was initially detected in 2018 and at the time it seemed like a fairly ordinary TDE, of which just a little more than 100 have been seen.
“There was nothing from that initial discovery that made us think something like this was going to happen years later,” said Cendes.
A TDE occurs when a star wanders a little too close to a supermassive black hole. In the case of AT2018hyz, the black hole resides in an otherwise fairly quiet galaxy 665 million light-years away.
Tidal forces, whereby one side of the star feels a greater gravitational pull from the black hole than the opposite side, begin to stretch and tear apart the star in a vice-like grip, effectively shredding it.
For a few years after its initial discovery, nothing much happened to AT2018hyz. Astronomers are not sure why, but there is often a wait period with TDEs. With that in mind, one hypothesis is that it takes a little time for the shredded stellar material to wrap around the black hole and form an accretion disk.
Some of the stellar material falls into the black hole, but much of it is directed away from the black hole by magnetic fields.
AT2018hyz was seen to come alive again in 2022, when it suddenly grew bright in radio waves probably produced by synchrotron radiation from the jet. This jet is so powerful that Cendes has even nicknamed it “Jetty McJetface” — in reference to the notorious Boaty McBoatface incident — and it is currently 50 times more luminous than upon its original detection. To see a black hole continue to emit so much energy so many years after consuming a star is considered unprecedented.
Another advantage of the jet explanation is that it would solve the mystery of why the energy output is still rising.
When such jets are first produced they are highly collimated with a narrow opening angle, and if the jet wasn’t pointed directly at us, but was at an angle to us, then we wouldn’t have seen its full blast. However, over time jets tend to broaden.
“And now it is entering our line of sight as the jet decelerates,” says Cendes “As to how you get these relativistic jets from a TDE, well no one knows for sure but it’s an active area of research. It probably has something to do with magnetic fields, but you clearly also need some other things to happen or we’d see them more commonly in TDEs.”
Cendes now wants to hunt for more of these exceptionally energetic events. With the Square Kilometer Array (SKA) set to come online in the next decade, astronomers will finally have a tool that can survey the radio sky to great precision and sensitivity, potentially finding many more radio jets not just from TDEs, but also from galaxies that are more regularly active.
Cendes’ team’s findings were published on Feb. 5 in The Astrophysical Journal.
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