The first picture of a black hole’s shadow just got even more interesting. The Event Horizon Telescope (EHT) collaboration released the first direct image of a black hole in 2019, and while the picture on its own was impressive, it wasn’t the scientific smorgasbord some had hoped for. Now, researchers have added polarised light to the picture, giving us an idea of how magnetic fields around a supermassive black hole create powerful jets of matter.
“It was not a lot of information about the actual physics of the gas around the black hole,” says Sara Issaoun, an EHT team member at Radboud University in the Netherlands. “Looking at it in polarised light told us information about the magnetic field of the black hole.”
The EHT uses a network of eight telescopes around the world to turn Earth into one giant radio telescope, which enabled an unprecedented view of the supermassive black hole at the centre of the M87 galaxy, 55 million light years away. The light that the EHT captures is emitted by electrons as they accelerate along magnetic fields, and the polarisation of the light depends on the direction of the magnetic field.
Using measurements of the polarised light near the M87 black hole, the EHT team found that the magnetic field’s strength is between 1 and 30 gauss. This is up to about 50 times the strength of Earth’s magnetic field as measured at the planet’s poles, where it is strongest.
“The polarised light has these curved swoops like a spiral,” says Issaoun. “This tells us that the magnetic field around the black hole is ordered, and this is really important because only an ordered magnetic field can launch jets – a scrambled magnetic field cannot do that.”
Some black holes, including the one in M87, spew enormous jets of matter, but how exactly they do so has long been a mystery. Researchers think the jets are launched and shaped by magnetic fields, but the evidence is limited.
“This jet process is totally amazing – something the size of our solar system can shoot out a jet that pierces through entire galaxies and even galaxy neighborhoods,” says Issaoun. “Now we’re really seeing the magnetic field close to the black hole for the first time, and that’s connecting it to the jet, which is the most powerful process in the universe.”
Measuring this black hole’s magnetic field with polarised light allowed the researchers to significantly cut down on the number of possibilities for how the black hole and its jet work. They compared the observations with simulations of 120 different theoretical models, and only 15 of the models fit what we actually see.
In all 15 of those models, the black hole’s magnetic fields are relatively strong and divert matter away from the black hole itself, starving it in favour of launching the material into the jet.
It’s not yet clear whether the possibilities are similarly narrowed for all supermassive black holes or if it is specific to this one in particular. “A lot of what we need to do in the next few years is to figure out what lessons we can take from this to other sources as well,” says Andrew Chael, an EHT team member at Princeton University.
So far, it looks like all black holes with strong jets probably behave like the one in M87, he says. Adding just a few more telescopes to the EHT array – which the researchers already plan to do – could help nail down exactly how the black hole is launching its jet.
Journal references: The Astrophysical Journal Letters, DOI: 10.3847/2041-8213/abe71d and DOI: 10.3847/2041-8213/abe4de
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