Event Horizon Telescope
22 SEPTEMBER 2019 • SKY & TELESCOPE
puter scientist Katie Bouman (now Caltech), if there were an
elephant in the center of M87, they wanted to see it.
After testing the countless alternatives, they all met and
unveiled their four images — and all looked remarkably alike:
four dark circles surrounded by ringlike structures.
Then they started over. The team members reorganized
themselves, using what they’d learned to systematically attack
each day’s data three different ways. Each method produced
a slightly different image, but once again, the images were
strikingly consistent with one another.
Combining these images into a single one took a long,
long time, Johnson says. The researchers wrestled with how to
convey what was sure versus what might be the byproduct of a
single algorithm’s favorite bits. They fi nally decided to take the
three images from a single day, blur them to match the instru-
ments’ resolution, and then average the blurred results into a
single image. By doing so, they only showed the structure that
appears using all analysis methods. “We stand behind basically
every element” of this conservative image, Johnson says.
And what an image it is. The width of the silhouette is
about 40 microarcseconds — the size of a hydrogen atom seen
at arm’s length. “This is the fi rst time that I saw this image,”
said NSF Director and astrophysicist France Córdova during
the press conference, “and it did bring tears to my eyes.”Around the Rabbit Hole
The team’s primary focus thus far was creating the image. But
they have ascertained some of the underlying physics, too.
Based on the size of the shadow, the researchers calculated
the black hole contains 6.5 billion solar masses, a fi gure close
to the larger of two contested values.
The black hole is spinning clockwise from our perspec-
tive; the bright crescent to the south is the boosted beam of
gas moving toward us, while the dimmer north is where gas
recedes from us. The data do not, however, reveal how quicklythe black hole spins, because the shadow’s shape and size are
independent of the spin except for the most extreme rota-
tions. The observations also don’t yet connect the black hole
with its jet, because the jet is on the wrong scale and too faint
to be resolved with the current data.
Nevertheless, the image boosts astrophysicists’ confi dence
in their theories about what happens near an accreting black
hole. “I have to admit I was a little stunned that it matched
so closely the predictions we had made,” says Broderick.
“Just the fact that our [simulations] came so close to
images like the one we ended up getting for M87* already
tells us that we’re on the right track for understanding accre-
tion physics,” says Özel. “We could have been completely off.”
But there’s little detail yet about how that accretion physics
works. Although many immediately hailed the shadow image
as a verifi cation of Einstein’s theory of gravity, astrophysicists
aren’t content with that. “It’s not like I’m sad we confi rmed GR
again,” says X-ray astronomer Daryl Haggard (McGill Uni-
versity, Canada). But what she cares about is the magnetically
tangled gas wreathing the black hole and how the black hole
eats it. “These are supposed to be the most effi cient engines in
the universe,” she says. And yet when astronomers look closely,
they see stuff fl owing out — winds off the accretion disks, pow-
erful jets. Of all the matter trying to make it across the event
horizon, it’s unclear how much successfully dives in. “That’s“It might look like we’re just taking a picture of a sleeping giant or something,
but this system is alive like the surface of the Sun.”—MICHAEL JOHNSON
pIT’S ALIVE Images of the shadow from four days of the observing run reveal small changes, indicating that the black hole’s environment is evolving
from day to day. Each image is the average of the three reconstruction methods’ results for that day.uTHEY’RE MULTIPLYING Left: EHT members (plus the author, back
row) gathered in January 2010 to strategize. Right: A November 2018
photo from a workshop in the Netherlands shows how much the collabo-
ration has grown — and only includes half the team.VA
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