The Occasion Horizon Telescope collaboration has unveiled new photos 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 light-years away. These photos, unlike the iconic one released in 2019, embody polarized gentle — photons that shimmy at solely sure orientations as they journey by house.
To the informal eye, the distinction may not seem like a lot: We’re nonetheless coping with a glowing doughnut. However 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 think magnetic fields play a crucial role in black holes’ growth, creating turbulence in black holes’ massive fluffy fuel disks that then robs the fuel of its angular momentum and allows it to fall onto the central object. (With out that turbulence, black holes would go hungry.) Magnetic fields additionally energy black holes’ galaxy-scale jets.
However this image is basically theoretical. With a view 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.
As a part of the 2017 marketing campaign that produced the unique black gap shadow picture, the EHT team used a planet-spanning network of radio telescopes to watch synchrotron emission from the fuel enshrouding M87’s supermassive black gap. Synchrotron radiation is emitted by electrons corkscrewing alongside magnetic discipline strains, and it’s extremely polarized by nature.
To acquire the brand new outcomes, the collaboration adopted the same technique as earlier than. First, they painstakingly mixed the totally different telescopes’ information, then they cut up into a number of groups to reconstruct photos utilizing totally different software program codes and strategies. Lastly, they averaged the photographs collectively. The duty 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 a special arc throughout the sky, rotating the polarization angle in a singular method, explains Monika Mościbrodzka (Radboud College, The Netherlands).
Magnetic fields naturally thread the fuel disk round a black gap. Because 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 seem like the left picture within the collection under.
As an alternative, what the EHT workforce 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 to be 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 (College of Colorado, Boulder). “That might be the primary 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, possible as a result of the fuel near the black gap that the photons are touring by is extremely magnetized. The weakened polarization makes it exhausting to see what the magnetic discipline’s precise construction is.
Nevertheless it additionally helps astronomers slim in on what’s happening within the accretion disk.
To unravel the disk’s situations, the EHT collaboration in contrast the info to greater than 100 totally different simulations, encompassing a broad swath of potential fuel densities, magnetic discipline strengths, and temperatures. Their conclusion is that we’re seeing comparatively skinny fuel paired with magnetic fields which can be sturdy sufficient to withstand the fuel’s influx and have an effect on how the fuel strikes. Theorists call this a MAD scenario, for “magnetically arrested disk;” the weaker-field state of affairs is known as SANE, for “customary and regular evolution.” (Lest you suppose 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 whenever you attempt to unwind it. As long as the magnetic fields undergo being dragged together with the fuel, they journey collectively. But when sufficient stubbornly oriented fields accumulate within the accretion disk’s interior elements, they’ll change how the fuel falls onto the black gap and even choke off the movement.
“The conclusion that there are sturdy — or strong-ish — fields within the central accretion disk is nearly actually proper,” says accretion professional Christopher Reynolds (College of Cambridge, UK), who wasn’t concerned with the M87 research. However he’s hesitant about utilizing simulation comparisons to conclude that solely MAD eventualities work. “This method has all the time left me somewhat uncomfortable,” he admits. “What if the info try to inform us one thing that is not within the lexicon of these fashions?” The EHT astronomers agree, acknowledging that their set of eventualities is incomplete.
Notably, in 2015 Michael Johnson (Middle for Astrophysics, Harvard & Smithsonian) and different members of the EHT workforce discovered indicators of the same degree of polarization and orderliness within the emission from our own galaxy’s central black hole, Sgr A*. The work used fewer telescopes, in order that they couldn’t reconstruct a picture. However the comparability is intriguing. “I believe that is undoubtedly pointing to a constant story in these two techniques,” 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 Occasion Horizon Telescope Collaboration. “First M87 Event Horizon Telescope Results VII: Polarization of the Ring.” Astrophysical Journal Letters. March 20, 2021.
The Occasion Horizon Telescope Collaboration. “First M87 Event Horizon Telescope Results VIII: Magnetic Field Structure Near the Event Horizon.” Astrophysical Journal Letters. March 20, 2021.