Astronomers Detect Strong Magnetic Fields Around M87’s Black Hole – Sky & Telescope

The Event Horizon Telescope collaboration has unveiled new photographs of the black gap shadow on the middle of the elliptical galaxy M87, which sits on the middle of the Virgo Cluster some 55 million mild-years away. These photographs, in contrast to the enduring one launched in 2019, embody polarized mild — photons that shimmy at solely sure orientations as they journey by means of house.

This reconstructed picture reveals the polarized view of the fuel-enshrouded black gap in M87. The traces mark the orientation of polarization, which is said to the magnetic discipline across the shadow of the black gap.Event Horizon Telescope CollaborationTo the informal eye, the distinction won’t 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 gap, data that astronomers have waited many years to get their arms on.

Magnetic fields are cosmic puppeteers. They management the movement of ionized fuel as if they have been advanced marionette strings, serving to (or hindering) the fuel to maneuver. Astronomers assume magnetic fields play an important position in black holes’ progress, creating turbulence in black holes’ huge fluffy fuel disks that then robs the fuel 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 gap, astronomers have turned to polarized emission, which encodes details about the magnetic fields that the photons handed by means of.

As a part of the 2017 marketing campaign that produced the unique black gap shadow picture, the EHT staff used a planet-spanning community of radio telescopes to look at synchrotron emission from the fuel enshrouding M87’s supermassive black gap. Synchrotron radiation is emitted by electrons corkscrewing alongside magnetic discipline traces, and it’s extremely polarized by nature.

To acquire the brand new outcomes, the collaboration adopted the same methodology as earlier than. First, they painstakingly mixed the totally different telescopes’ information, then they cut up into a number of groups to reconstruct photographs utilizing totally different software program codes and strategies. Finally, they averaged the pictures collectively. The job was additional difficult, nonetheless, as a result of not solely are the polarization alerts weaker than the fuel’s general glow, however every telescope additionally noticed the supply shifting by means of a unique arc throughout the sky, rotating the polarization angle in a novel manner, explains Monika Mościbrodzka (Radboud University, The Netherlands).

Magnetic fields naturally thread the fuel disk round a black gap. As the accreting fuel rotates it drags the fields round with it, wrapping them across the black gap and amplifying them. If the one magnetic fields current within the fuel tutu have been these wound up by the fuel, then the polarization sample would appear like the highest left picture within the sequence beneath.

These simulated photographs present what the polarization sample can be in a fuel disk round a black gap for 3 totally different magnetic discipline geometries. Tick mark orientations point out the path of polarization, and tick lengths are energy. The left panel reveals the sample for a magnetic discipline confined to the fuel disk’s aircraft and following the fuel’s rotation. The middle and proper panels present orientations that break from the fuel’s movement. tailored from EHT Collaboration / Astrophysical Journal Letters 2021 Instead, what the EHT staff noticed is that this.

Left: polarization sample within the fuel round M87’s central black gap. Right: Same information, however proven as streaming traces on the full glow.tailored from EHT Collaboration / Astrophysical Journal Letters 2021 The little tick marks point out the path and quantity of polarization. There are two vital issues about this picture:

First, there’s the polarization sample. There’s clearly order to the sample, however the picture appears to be like extra like a mixture 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 gap 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 barely 10-30% polarized. The sign should have been scrambled, doubtless as a result of the fuel near the black gap that the photons are touring by means of is extremely magnetized. The weakened polarization makes it arduous to see what the magnetic discipline’s actual construction is.

But it additionally helps astronomers slim in on what’s happening within the accretion disk.

To unravel the disk’s circumstances, the EHT collaboration in contrast the information to greater than 100 totally different simulations, encompassing a broad swath of attainable fuel densities, magnetic discipline strengths, and temperatures. Their conclusion is that we’re seeing comparatively skinny fuel paired with magnetic fields which might be sturdy sufficient to withstand the fuel’s influx and have an effect on how the fuel strikes. Theorists name this a MAD state of affairs, for “magnetically arrested disk;” the weaker-discipline state of affairs known as SANE, for “standard and normal evolution.” (Lest you assume astronomers don’t have any humorousness.)

Magnetic fields can resist the fuel’s pull as a result of they’ve a strain related to them, Dexter explains. Magnetic fields don’t wish to be squeezed or twisted out of practice; they push again, like a spring once you attempt to unwind it. So lengthy because the magnetic fields undergo being dragged together with the fuel, they journey collectively. But if sufficient stubbornly oriented fields accumulate within the accretion disk’s inside elements, they’ll change how the fuel falls onto the black gap and even choke off the movement.

“The conclusion that there are strong — or strong-ish — fields in the central accretion disk is almost certainly right,” says accretion skilled 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 eventualities 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 eventualities is incomplete.

Notably, in 2015 Michael Johnson (Center for Astrophysics, Harvard & Smithsonian) and different members of the EHT staff discovered indicators of the same degree of polarization and orderliness within the emission from our personal galaxy’s central black gap, Sgr A*. The work used fewer telescopes, in order 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.

Thus the brand new information is perhaps direct proof not solely that magnetic fields present the turbulence that forces fuel to fall into the black gap, but in addition 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 gap. 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.


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