Sky and Telescope - January 2018 - 10

NEWS NOTES

This artist's concept shows the neutron star collision at the heart of the gravitational-waveand light-wave-emitting event known as GW170817.

Two Massive Collisions and a Nobel Prize
ASTRONOMERS HAVE ﬁnally wit-

nessed the collision of two neutron
stars. On Thursday, August 17th, at
12:41:04 Universal Time, the two Laser
Interferometer Gravitational-wave
Observatory (LIGO) detectors bagged
roughly 100 seconds' worth of spacetime ripples from the coalescing binary,
an event now known as GW170817.
Just 1.7 seconds later, NASA's Fermi
Gamma-ray Space Telescope registered a
2-second blast of gamma rays from the
same event.
Since the cosmic smash-up occurred
near one of its blind spots, the Virgo
detector in Italy - LIGO's European
counterpart - didn't register a convincing gravitational-wave signal. Using
LIGO's data and Virgo's weak detection,
scientists triangulated the source of the
ripples to an area of sky measuring just
28 square degrees, as they explained at
an October 16th press conference.
Aug 17,
2017

Aug 21,
2017

What followed was a feverish hunt
for an electromagnetic counterpart.
Some 11 hours after the gravitational
waves arrived, the 1-meter Swope Telescope at the Las Campanas Observatory
in Chile discovered a optical source in
the galaxy NGC 4993, 130 million lightyears away in the constellation Hydra,
the Water Monster. The 17th-magnitude
optical transient changed color from
blue to red within 48 hours, before fading away. By then, dozens of telescopes
on the ground and in space, including
Hubble, had studied the source.
Infrared spectra conﬁrmed that the
merger's rapidly expanding radioactive ﬁreball produced huge amounts
of heavy elements, probably including
many Earth masses' worth of gold. The
kilonova (S&T: Nov. 2013, p. 12) also
generated X-rays observed by NASA's
Chandra space observatory nine days
after the event, as well as radio waves
These are the ﬁrst
optical photons from
a gravitational-wave
source. These images
combine optical and
near-infrared data from
the Swope and Magellan telescopes of the
GW170817 signal in its
host galaxy, NGC 4993.

seen yet another week later by the Very
Large Array in New Mexico.
Some 3,600 scientists from more
than 900 institutions were involved in
collecting and analyzing the wealth of
observations of GW170817, described in
more than 100 papers in many different
journals. Look for an in-depth report on
the effort and the scientiﬁc implications
of the discovery in next month's issue.
The neutron-star merger was actually
the second gravitational-wave event
"seen" by all three detectors since Virgo
joined LIGO's second observing run
on August 1st. In late September, the
LIGO-Virgo Collaboration announced
GW170814, which came from the collision of two black holes some 1.8 billion
light-years away. The spacetime ripples
from that event reached Earth on
August 14th at 10:30:43 UT, just three
days before GW170817.
In this earlier case, the original black
holes weighed in at 31 and 25 solar
masses before merging into a 53-solarmass behemoth, radiating away the
energy equivalent of 3 solar masses in the
form of gravitational waves. Interestingly,
these masses are comparable to those in
two of LIGO's three earlier detections,
GW150914 and GW170104, suggesting
the previous events weren't outliers, says
Gijs Nelemans (Radboud University, The
Netherlands). If future detections conﬁrm that masses tend to cluster in a narrow range of values, that could support
different formation scenarios, he adds.
GW170814's localization was less
precise (60 square degrees), due in part
to the signal's shorter duration - 0.1
second, compared to 1½ minutes for
GW170817. Counterpart searches were
unsuccessful, which is unsurprising:
Astronomers don't expect light from
stellar-mass black hole mergers.
To recognize the gravitational-wave
revolution, the Royal Swedish Academy
of Sciences awarded the 2017 Nobel
Prize in Physics on October 3rd to three
of LIGO's principal contributors: interferometer inventor Rainer Weiss (MIT),
former LIGO director Barry Barish, and
theorist Kip Thorne (both Caltech).
■ GOVERT SCHILLING

JA N UA RY 2 018 * SK Y & TELESCOPE

TOP: ROBIN DIENEL (CA R NEGIE INSTIT U TION FOR SCIENCE ); BOT TO M: 1M2H / UC SA N TA CRUZ A ND CA R NEGIE
OBSERVATORIES / RYA N FOLE Y

GRAVITY

Sky and Telescope - January 2018

Table of Contents for the Digital Edition of Sky and Telescope - January 2018