ASTRONEWS
FOR HUNDREDS OF
MILLIONS OF YEARS,
two city-sized stars in a galaxy
not so far away circled each
other in a fatal dance. Even
though the dimensions of these
two neutron stars — the col-
lapsed cores left behind after
giant stars explode into superno-
vae — were diminutive, each
still slightly outweighed the Sun.
Closer and closer they spun,
constantly shedding gravita-
tional energy, until the stars
traveled at nearly the speed of
light, completing 100 orbits
every second. By then, dinosaurs
reigned on Earth, and the first
f lowers were just blooming.
That’s when, 130 million years
ago, the dance ended.
The collision was fast and vio-
lent, likely spawning a black hole.
A shudder — a gravitational
wave — was sent out across the
fabric of space-time. And as the
stars’ outer layers launched into
space, the force formed a vast
cloud of subatomic particles that
would eventually cool into many
Earths’ worth of gold, platinum,
and uranium. Seconds later, a
blast of high-energy gamma rays
(the most energetic kind of radia-
tion) punched through the erupt-
ing cloud.
For eons, the invisible space-
time ripple and the intense light
beam from this collision dashed
across the cosmos together,
finally reaching Earth at
8:41 A.M. EDT on August 17. The
gravitational waves first raced
through Italy’s freshly finished
Advanced Virgo detector before
reaching the United States,
stretching and squeezing the
laser beams at the two sites
of the Laser Interferometer
Gravitational-wave Observatory
(LIGO) as the fifth-ever detec-
tion of gravitational waves. Thegamma-ray burst, following just
1.7 seconds behind the gravita-
tional waves, was picked up by
both NASA’s Fermi Gamma-ray
Space Telescope and the
European INTEGRAL satellite.
Immediately, both the LIGO
and Fermi teams implored
astronomers around the world to
start searching for the collision’s
optical afterglow, a phenomenon
never before witnessed from a
gravitational wave source. One
of the search parties was a group
of astronomers from the
Carnegie Institution for Science,
working closely with colleagues
from the University of California,
Santa Cruz.Astronomer Josh Simon, part
of the Carnegie search team, told
Astronomy, “Dave Coulter at UC
Santa Cruz assembled the galaxy
list using public catalogs, and
identified about 100 galaxies
that were the most likely hosts
for the gravitational wave source.
We used the 1-meter Swope
Telescope, as well as the [twin]
6.5-meter Magellan Telescopes,
to image galaxies on this list,
with each telescope observing at
a different wavelength.”
By shortly after sunset at Las
Campanas Observatory in Chile,the Carnegie astronomers were
in full-on search mode. At
9:02 P. M. local time, Ryan Foley,
an astronomer from UC Santa
Cruz, emailed the Carnegie
team in Chile with the simple
subject line: “candidate!”
“I remember being shocked
when we got the email. We had
only been searching for 10 or 15
minutes,” Maria Drout, a NASA
Hubble postdoctoral fellow at
Carnegie who was also part of
the discovery team, said to
Astronomy. “I remember getting
chills when I saw the image. It
was a very clear, bright, new
source. And it was offset from
the center of an elliptical galaxy.“We didn’t know for sure yet
that this was the right source,
but I let a little part of me
believe. It was just too perfect,”
she added.
Drout’s instincts were correct.
About 11 hours after the fifth-
ever gravitational wave detection,
the Carnegie team had managed
to capture the first-ever optical
glimpse of two neutron stars
colliding, bestowing on it the
name Swope Supernova Survey
2017a (SSS17a), in honor of t he
nearly half-century-old telescope
that initially spotted its light.The team took the first-ever
images of the afterglow left by
colliding neutron stars, as well
as the first-ever spectra, which is
essential for distinguishing vari-
ous types of cosmic explosions
from one another. “We were the
only group to get spectra during
the first night, and we even got
multiple. With two spectra just
an hour apart, we saw signifi-
cant evolution, which told us
this is evolving really fast — like
no other astrophysical explosion
we had seen before,” said
Carnegie astronomer Tony Piro.
The discovery of SSS17a —
130 million light-years away in
the galaxy NGC 4993 — marks
the birth of multi-messenger
gravitational wave astronomy, a
whole new approach to studying
the universe. “There are things
that you can discover with gravi-
tational waves that you could
never see with electromagnetic
light, and vice versa,” Simon
said. “Having that combination
should provide us with insights
into these extreme objects.”
Gravitational wave astronomy
is just getting truly started.
When LIGO comes back online
next year after another round of
upgrades, scientists expect to see
one of these mergers every
month or so. But in the years to
come, that number could grow
to once a week, although astron-
omers don’t expect many more
neutron stars to merge this close
to home. “We’ve created a new
field of astronomy,” Foley said.
“We’ve been walking around for
all of humanity being able to see
the universe but not being able
to hear it. Now we get both.
“It’s even hard to predict
where this field will go,” Foley
added, “but I can tell you now
it’s going to be exceptional.”
— Eric Betz, Robert Naeye, Jake ParksThe first observation of a
gravitational wave source
Detectors pick up gravitational
waves — and more — after
neutron stars merge.THREEFOLD. On August 14, 2017, the first three-detector observation of gravitational waves took place
as two black holes merged 1.8 million light-years away.10 ASTRONOMY • FEBRUARY 2018
SWOPE! (THERE IT IS). Carnegie Observatories’ Swope Telescope — a small,
decades-old instrument at Chile’s Las Campanas Observatory — was the first
to image the neutron star merger in optical light. This image includes data from
the Magellan Telescopes as well. Images and spectra of the afterglow allowed
astronomers to learn more about this never-before-seen event. RYAN FOLEYSSS17aAu g u s t 17, 2 017 Au g u s t 21, 2 017