Sky & Telescope - February 2019 - 11

IN BRIEF

BLACK HOLES

Gas Seen Moving near the Event Horizon
AN INTERNATIONAL TEAM of astron-

omers has caught the motion of hot,
magnetized gas right near the event
horizon of our galaxy's central supermassive black hole. This gas is likely
part of a puffy disk that the black
hole feeds from. The disk provides
a ﬂickering light source, its steady
glow sometimes spiking in ﬂares that
observers see in wavelengths from
X-ray to radio.
While watching the star S2 whiz
by the black hole last summer (S&T:
Sept. 2018, p. 22), astronomers with
the Gravity Collaboration at the Very
Large Telescope Interferometer in
Chile saw three bright infrared ﬂares
from near the black hole. Each ﬂare
lasted between 30 and 90 minutes
and didn't stay in place. Instead, each
appeared to race around the black hole
at 30% of the speed of light, tracing
out about two-thirds of a clockwise
loop that has the black hole at its center. Changing patterns in the light's
polarization conﬁrmed the twirling.
As the team reports in the October

Astronomy & Astrophysics, the data
point to hotspots orbiting very close to
the black hole's event horizon, completing a pass every 45 minutes or so.
"This is an incredible measurement," says Avery Broderick (University of Waterloo, Canada). If
astronomers see more ﬂares behaving
the same way, then "this presents an
extraordinary opportunity to make
precise tests of gravity in its most
extreme environments."
Attendees at a galactic center
workshop in Germany were excited
but cautious. "Everyone believes there
is some kind of motion in a ﬂare near
the black hole," says Sera Markoff
(University of Amsterdam, The Netherlands). Hotspots are the simplest
solution, which is why the team went
with them. But it's not yet clear if an
alternative solution might be better.
"Most of us believe that more complicated things are possible, such as
motions associated with magnetic
ﬂares or jets."
■ CAMILLE M. CARLISLE

Weird Stripes on Dione
Saturn's moons are a hodgepodge of misfits.
Iapetus looks like the Death Star from Star
Wars. Tiny Pan resembles a cosmic empanada. And now something appears to be
drawing straight lines across the surface of
Dione. The bright lines stretch for up to hundreds of kilometers, are less than 5 kilometers
wide, run parallel to the equator, and reside
only at lower latitudes. They appear to lie atop
extant features such as ridges and craters,
which means they've been emplaced fairly
recently. The most likely culprit is something
in the same plane as Dione's orbit that gently
rained down on the surface, researchers
argue October 15th in Geophysical Research
Letters. A passing comet, Saturn's rings, or
the moons Helene and Polydeuces - which
share Dione's orbit - are all suspects.
■ CHRISTOPHER CROCKETT

Hubble Takes a Nap
The Hubble Space Telescope has fully recovered from a three-week hiatus. The nearly
29-year-old observatory put itself into safe
mode on October 6th after noting anomalous
behavior from one of its gyroscopes, which
are vital for ensuring that the telescope stays
on target. Support crews suspected that the
backup gyro might have developed an air
bubble or stiff power wire after sitting inactive
for years. To clear out the blockage, the team
commanded Hubble to go through a series
of turns, hoping to reset the gyro - and it
worked. The venerable observatory got back
to work on October 27th by turning its gaze
toward a remote star-forming galaxy.
■ CHRISTOPHER CROCKETT

ESO / G R AVIT Y CONSORTIU M / L. CA LÇA DA

Colossal Supercluster Lurks
in Early Universe

This visualization of gas orbiting at about 30% of the speed of light around the Milky
Way's supermassive black hole relies on data from simulations.

Researchers have identified what may be a
gargantuan predecessor to modern superclusters, complex webs of galaxies spanning
hundreds of millions of light-years. The light
from this proto-supercluster has taken about
11 billion years to reach Earth. Seven galaxy
clusters appear linked together by filaments
of galaxies across roughly 20 million billion
billion (2 × 1025) cubic light-years of space.
The entire ensemble is about as massive as
4.6 quadrillion Suns. While not the first galaxy
cluster seen in the early universe, none are
as massive or as sprawling as this one. Olga
Cucciati (National Institute for Astrophysics,
Italy) and colleagues reported the discovery in
the November Astronomy & Astrophysics.
■ CHRISTOPHER CROCKETT
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Sky & Telescope - February 2019

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