The Event Horizon Telescope collaboration has unveiled new photographs of the black hole shadow on the heart of the elliptical galaxy M87, which sits on the heart of the Virgo Cluster some 55 million light-years away. These photographs, unlike the iconic one released in 2019, embrace polarized gentle — photons that shimmy at solely sure orientations as they journey by space.
To the informal eye, the distinction may not appear like a lot: We’re nonetheless coping with a glowing doughnut. But the polarization information include key details about how magnetic fields behave close to the black hole, data that astronomers have waited a long time to get their arms on.
Magnetic fields are cosmic puppeteers. They management the circulation of ionized gas as if they have been advanced marionette strings, serving to (or hindering) the gas to maneuver. Astronomers think magnetic fields play a crucial role in black holes’ growth, creating turbulence in black holes’ large fluffy gas disks that then robs the gas of its angular momentum and allows it to fall onto the central object. (Without that turbulence, black holes would go hungry.) Magnetic fields additionally energy black holes’ galaxy-scale jets.
But this image is basically theoretical. In order to catch magnetic fields pulling the strings shut round M87’s black hole, astronomers have turned to polarized emission, which encodes details about the magnetic fields that the photons handed by.
As a part of the 2017 marketing campaign that produced the unique black hole shadow picture, the EHT team used a planet-spanning network of radio telescopes to watch synchrotron emission from the gas enshrouding M87’s supermassive black hole. Synchrotron radiation is emitted by electrons corkscrewing alongside magnetic discipline traces, and it’s extremely polarized by nature.
To receive the brand new outcomes, the collaboration adopted the same methodology as earlier than. First, they painstakingly mixed the completely different telescopes’ information, then they break up into a number of groups to reconstruct photographs utilizing completely different software program codes and strategies. Finally, they averaged the pictures collectively. The process was further difficult, nonetheless, as a result of not solely are the polarization indicators weaker than the gas’s total glow, however every telescope additionally noticed the supply transferring by a unique arc throughout the sky, rotating the polarization angle in a novel way, explains Monika Mościbrodzka (Radboud University, The Netherlands).
Magnetic fields naturally thread the gas disk round a black hole. As the accreting gas rotates it drags the fields round with it, wrapping them across the black hole and amplifying them. If the one magnetic fields current within the gas tutu have been these wound up by the gas, then the polarization sample would appear like the left picture within the sequence under.
Instead, what the EHT group noticed is that this.
The little tick marks point out the course and quantity of polarization. There are two essential issues about this picture:
First, there’s the polarization sample. There’s clearly order to the sample, however the picture seems extra like a mix of the middle or proper panels of the sooner diagram. That tells us that there’s a reasonably sturdy magnetic discipline current that’s oriented in another way than what would exist if it’s merely wrapped across the black hole by the accretion disk, explains Jason Dexter (University of Colorado, Boulder). “That would be the main science takeaway,” he says.
Second, there’s the polarization fraction. Synchrotron emission should be extremely polarized (roughly 70%), however what we see right here is simply 10-30% polarized. The sign will need to have been scrambled, doubtless as a result of the gas near the black hole that the photons are touring by is very magnetized. The weakened polarization makes it laborious to see what the magnetic discipline’s precise construction is.
But it additionally helps astronomers slender in on what’s occurring within the accretion disk.
To unravel the disk’s circumstances, the EHT collaboration in contrast the info to greater than 100 completely different simulations, encompassing a broad swath of potential gas densities, magnetic discipline strengths, and temperatures. Their conclusion is that we’re seeing comparatively skinny gas paired with magnetic fields which are sturdy sufficient to withstand the gas’s influx and have an effect on how the gas strikes. Theorists call this a MAD scenario, for “magnetically arrested disk;” the weaker-field state of affairs is named SANE, for “standard and normal evolution.” (Lest you suppose astronomers haven’t any humorousness.)
Magnetic fields can resist the gas’s pull as a result of they’ve a strain related to them, Dexter explains. Magnetic fields don’t prefer to be squeezed or twisted out of practice; they push again, like a spring while you attempt to unwind it. So lengthy because the magnetic fields undergo being dragged together with the gas, they journey collectively. But if sufficient stubbornly oriented fields accumulate within the accretion disk’s internal components, they’ll change how the gas falls onto the black hole and even choke off the circulation.
“The conclusion that there are strong — or strong-ish — fields in the central accretion disk is almost certainly right,” says accretion knowledgeable Christopher Reynolds (University of Cambridge, UK), who wasn’t concerned with the M87 research. But he’s hesitant about utilizing simulation comparisons to conclude that solely MAD situations work. “This approach has always left me a little uncomfortable,” he admits. “What if the data are trying to tell us something that isn’t in the lexicon of those models?” The EHT astronomers agree, acknowledging that their set of situations is incomplete.
Notably, in 2015 Michael Johnson (Center for Astrophysics, Harvard & Smithsonian) and different members of the EHT group discovered indicators of the same stage of polarization and orderliness within the emission from our own galaxy’s central black hole, Sgr A*. The work used fewer telescopes, so that they couldn’t reconstruct a picture. But the comparability is intriguing. “I think this is definitely pointing to a consistent story in these two systems,” Johnson says. Infrared observations of Sgr A* with the GRAVITY instrument in Chile have additionally indicated sturdy fields are at play.
Thus the brand new information could be direct proof not solely that magnetic fields present the turbulence that forces gas to fall into the black hole, but additionally that the fields act just like the nozzle on that influx, controlling the speed of infall — and maybe, this image is true for extra than simply M87’s supermassive black hole. The outcomes seem in two papers in Astrophysical Journal Letters.
The Event Horizon Telescope Collaboration. “First M87 Event Horizon Telescope Results VII: Polarization of the Ring.” Astrophysical Journal Letters. March 20, 2021.
The Event Horizon Telescope Collaboration. “First M87 Event Horizon Telescope Results VIII: Magnetic Field Structure Near the Event Horizon.” Astrophysical Journal Letters. March 20, 2021.