Sky & Telescope - September 2019 - 23

DOMINIC PESCE

1.3 mm (ground)

0.8 mm (ground)

OJ 287

1.3 mm (ground)

Cen A

0.8 mm (ground)

3C 273
10

Source strength at 1.3 mm (janskys)

super fascinating," she says. "They had to get mass somehow,
they're supermassive! So what gives?"
Initial answers may already lie in the EHT's data. Soon,
the researchers will start putting together maps of how the
magnetic ﬁelds move near the event horizon, information
encoded in the light's polarization. Theorists think magnetic
turbulence helps slow the gas around black holes, making
accretion possible. Magnetic ﬁelds also enable black holes like
M87* to power their light-saber-esque jets by sapping energy
from the objects' spins.
"We think of black holes as an endpoint of matter - you
fall in, you can't get out," Johnson says. "A black hole's spin is
not sacred in that way." Magnetic ﬁelds in the accreting gas
pile up near the event horizon, and as the black hole spins,
it whirls the ﬁeld lines around, which drag on the black hole
and slow it down. This stolen energy then powers the ﬂingout of charged particles as gigantic jets. "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," he says.
What many onlookers await, however, are the results for
our own galaxy's central black hole, Sgr A*. The team is still
analyzing those observations. Because it's about a thousandth
the mass of M87*, Sgr A* is smaller, and gas whips around
its circumference a thousand times faster. That means we see
much faster changes in its light, making teasing apart the
shadow signature more complex. "We knew it was a more
turbulent child," says Özel. "We have to apply special care."
That includes devising new algorithms, says Bouman,
whom fellow EHT members credit with bringing key insights
from the computing community to their imaging methods.
Although the team won't have to start completely from
scratch, the current algorithms will likely have to be modiﬁed
to handle Sgr A*'s variability, she explains.
Next year, the EHT will integrate stations in France and
Arizona, and it has already added a dish in Greenland.
Researchers are also pushing to a slightly shorter wavelength
than the 1.3 mm currently used, which will boost their resolution considerably.
But M87* and Sgr A* are the only two black holes whose
shadows are large enough for us to detect with ground-based
networks. To expand to a larger number of black holes, they'll

Sgr A*

M87*

1
M104
NGC 1052

IC 1459
M81
M84

IC 4296
0.1
=1010 Suns
=108 Suns
=106 Suns
0.1

NGC 4261
NGC 1399

100

Shadow diameter (microarcseconds)
p SHADOW TARGETS With current facilities, the EHT collaboration can
resolve the silhouettes of two black holes: Sgr A* and M87's. Moving to
a shorter wavelength will increase resolution (moving left on the chart).
By adding one or more satellites in either medium-Earth orbits (10,000 to
20,000 km altitude) or geosynchronous orbits (35,786 km), researchers
could bag three, maybe four more shadows.

have to put radio telescopes in space. Adding geosynchronous orbits would lengthen baselines by more than six times
Earth's radius, enabling the EHT to see black hole shadows
roughly one-tenth as wide as M87*'s.
This utterly serious push to space VLBI is remarkable
progress for a collaboration that, back when I ﬁrst met them
nearly 10 years ago, was once a huddle of astronomers working on what they affectionately referred to as "Shep's Event
Horizon Telescope." Over the years, the team has grown,
complexiﬁed, and faced a stream of setbacks and frustrations.
"I didn't realize how hard it would be when we started out,"
Doeleman admitted to me the afternoon of the announcement, thinking back on the journey. But quitting wasn't an
option. "You know, there are just some projects that you have
to keep pushing on, no matter what."
That they've seen a black hole's shadow - not yet another
blob, not something that violates gravity, but a heart of darkness nestled in the glow of gas hurtling into oblivion - is a
humbling testament to the years of everyone's hard work.
Or, as Doeleman puts it: "Sometimes you have to kiss a lot of
frogs before you get the prince."

¢ Science Editor CAMILLE M. CARLISLE has buzzed around
the EHT team's ears since she was a wee master's student.
This article is based on her reporting for S&T's website.
Watch videos explaining how VLBI and image reconstruction
work: https://is.gd/vlbiprimer.
sk yandtele scope.com * S E P TE M B E R 2 019

https://www.is.gd/vlbiprimer http://www.skyandtelescope.com

Sky & Telescope - September 2019

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