WWW.ASTRONOMY.COM 29August 17, 2017, began like any other day.
But at 8:41 A.M. EDT, everything changed.
For 100 seconds, ripples in the fabric of
space-time stretched and squished Earth’s
extremely precise gravitational wave
telescopes. A bright f lash of gamma rays
followed shortly afterward. And so began
weeks of intense analysis by 3,500 astrono-
mers and physicists using 70 observatories
scattered across all seven continents and
in space.
Scientists announced the results of that
analysis October 16 during several press
conferences and in more than three dozen
papers. They had spotted gravitational
waves from a source never before detected:
two neutron stars spiraling together and
merging. The collision also let out a
gamma-ray burst (GRB), thus proving a
long-held assumption that neutron star
mergers cause at least one type of GRB. But
this event also emitted other forms of light:
visible light, infrared radiation, radio
waves, ultraviolet light, and X-rays, all of
which astronomers examined for details.
On the morning of August 17, the gravi-
tational waves reached Earth first. The
Laser Interferometer Gravitational-wave
Observatory (LIGO) in Louisiana — com-
posed of fine-tuned lasers, mirrors, and
detectors — saw a strong signal, as did its
twin in Washington state just milliseconds
later. The European gravitational wave
observatory, Virgo, uses similar technology
and had come online just weeks before, but
it couldn’t make out the August 17 event
even though the signal should have been
visible. “The event happened in one of the
very few areas on the sky that they have
low sensitivity to,” says LIGO team mem-
ber Amber Stuver of Villanova University.
“That gave us information that made us
better able to pinpoint where on the sky the
source could have come from.”
While gravitational wave scientists were
alerting their colleagues, gamma-ray
astronomers were processing a signal that
arrived at the Fermi gamma-ray space
observatory. “We were absolutely sure we
had a gamma-ray burst,” says Eric Burns, a
team member with Fermi’s Gamma-ray
Burst Monitor (GBM). The GRB arrived
1.74 seconds after the gravitational wave
signal ended. Once the LIGO/Virgo scien-
tists narrowed the location of their source
on the sky, it lined up with the GBM’s sig-
nal. “That’s when all of us were certain it
was the same thing,” says Burns.
The observatories had seen two neutron
stars spiraling toward one another, circling
some 1,500 times before merging. And they
saw the resulting energetic light. What else
could we learn about this incredible event?
That region of sky hosts some 50 galax-
ies. Pinpointing the signal’s origin required
other forms of light. Eleven hours after
the LIGO and Fermi events, the area
came into view of the Swope Telescope atLas Campanas Observatory in Chile. It
spied a new source of light in the outskirts
of galaxy NGC 4993. Several other tele-
scopes confirmed the find.
By spreading out the received optical
and infrared light by wavelength, astrono-
mers found chemical fingerprints of heavy
elements, including those heavier than iron
— elements like gold. While researchers
know that stellar explosions called super-
novae produce these elements, which then
diffuse throughout the cosmos and provide
material for new stars and planets, super-
nova blasts account for only part of the
universe’s heavy element content. Neutron
star collisions, it turns out, create the rest.
Plenty of questions remain. This round
of analysis was just the first, says Burns.
“Now it’s time for the community to
come in and figure out what really hap-
pened by combining all of this informa-
tion, hopefully into a single coherent
picture,” he says. Perhaps the neutron star
merger marks the beginning of multi-
messenger gravitational-wave astronomy
and the future of the field, in which the
entire astronomical community studies all
forms of light and gravitational waves
together to reveal how the universe’s most
energetic events work.The day multi-messenger
astronomy burst forth
Liz Kruesi, a contributing editor to Astronomy,
writes about our universe from the sunny locale
of Austin, Texas.Astronomers spotted
gravitational waves,
gamma rays, and
several other forms
of light from a pair of
merging neutron stars
August 17. The event,
called a kilonova,
produced heavy
elements such as gold
and platinum. ESO/
L. CALÇADA/M. KORNMESSER