Sky & Telescope - December 2022 - 18

The Big Black Hole Next Door
20
15
10
5
-5
-10
-15
-20
-15 -10 -5
5 10 15 20
Distance from black hole
(in black hole radii)
-15 -10 -5
5 10 15 20
Distance from black hole
(in black hole radii)
-15 -10 -5
5 10 15 20
Distance from black hole
(in black hole radii)
GAS INFALL A black hole might eat from a thin, weakly magnetized stream of gas (left) or a flow choked with magnetic fields (center and right).
Many of the EHT team's simulations used a MAD torus to feed the accretion disk, but a trickle from stellar winds (right) might be smooth enough to
explain the calm glow from Sgr A*.
Tchekhovskoy says that, on reflection, maybe it makes
sense that a MAD feeds Sgr A*. The key factor that determines
the flow's nature is whether magnetic fields can dominate the
gas motions.
As gas streams into the black hole in a MAD, the black
hole binges on the magnetic fields in the material, stuffing
itself. There are so many field lines threading the black hole
that they're like a handful of uncooked spaghetti. And like
uncooked spaghetti, magnetic fields don't like to be squeezed
or shoved into place: Loosen your fingers, and strands will
pop out of the bunch. In an accretion disk, the only thing
keeping the fields stuck in the black hole's throat is the gas
pouring in. Concentrate enough fields together, though, and
they'll fight back, buoyant against the gas. They'll form a dam
or even shoot out and rip through the disk, creating hotspots
and flares.
Given how little Sgr A* eats, even a relatively small magnetic
buildup could control the gas flow, Tchekhovskoy says.
" Maybe this is not such a big surprise, that it is MAD, " he
says. " What's surprising is that it pretends to be not MAD. "
For example, if the accretion flow is MAD and the black hole
spins quickly, then there should be a jet, if only a stubby one,
Tchekhovskoy and Narayan agree.
Perhaps the jet has been stifled. A jet will stream out along
the black hole's spin axis while it's still close to the black hole,
but if enough gas flows in along that axis, then the flow will
re-route the jet in a completely different direction. " Through
that process [the jet] loses a lot of power, and so it fizzles out
much more easily, " Ressler says. Stellar winds could potentially
quash the jet this way, because they can feed the black
hole from any direction.
The Quest Continues
The picture drawn so far might be confusing, and it should
be. Although this work involved collecting 3½ petabytes of
data - equivalent to one-seventh of the entire digital collec18
DECEMBER 2022 * SKY & TELESCOPE
tion of the Library of Congress - it's only a first look.
" We've done the best job we can, " Narayan says. But the
inferences are self-contradictory: For example, how can we
have a rapid spin and MAD flow with no jet? " I'm a little
nervous, " he admits.
The EHT team has plenty of data to keep exploring these
questions: Observations from 2018, 2021, and 2022 all
remain to be studied, as well as Sgr A*'s 2017 polarization
data. The light's polarization will reveal the magnetic fields'
orientation and, potentially, a more precise spin estimate
if the black hole drags everything around with it enough to
affect the pattern. The team is still discussing which data set
to attack next.
But what several members of the collaboration are turning
their eyes to is the next-generation EHT (ngEHT). No matter
how good the algorithms, the EHT's true limits are the number
of radio dishes and their locations. M87*'s image used
data from seven stations at five sites; Sgr A*'s image involved
eight at six sites (the eighth being the South Pole Telescope,
for which M87* lies below the horizon). The team has continued
adding telescopes to the array, and in 2022 eleven
stations at nine sites participated in the campaign.
But it's not enough. " Right now we're on the edge, " says
EHT founding director Shep Doeleman (Center for Astrophysics,
Harvard & Smithsonian). " We lose one of these
antennas, it's very difficult to recover the image. . . . If we
have a weather pattern that takes two of our antennas out
in key locations, we're done for the night. Might as well not
even observe. "
The ngEHT could change that. Phase I aims to rope in
five additional dishes, three grabbed and refurbished from a
former National Science Foundation project and shipped to
new locations. Proposed to be up and running in 2026, Phase
I will increase the number of baselines by a factor of eight
compared with 2017. It will also observe in both the existing
230 GHz band and a new 345 GHz one, which will not only
SANE Torus
MAD Torus
MAD Wind
Distance from black hole
(in black hole radii)
Less Visible at 230 GHz More
ACCRETION SCENARIOS: L. MURCHIKOVA ET AL. / ASTROPHYSICAL JOURNAL
LETTERS 2022 / CC BY 4.0; FORECAST: EHT COLLABORATION (2)

Sky & Telescope - December 2022

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