Sky and Telescope - February 2018 - 33

ILLUSTR ATION: NSF / LIGO / SONOM A STATE UNIV ERSIT Y / A . SIMONNE T; CHIRP: LSC / A LE X NIT Z; SK Y M A P: DATA SOURCE: IAIR A RCAVI E T A L. / NATUR E 2017

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n August 17th, a new age of astronomy began. That
day, the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) registered tiny
ripples in spacetime, produced by a pair of frantically
orbiting neutron stars right before they collided. But the reason that they herald a new age is that they didn't come alone:
Telescopes on the ground and in space detected the cosmic
smash-up and the fading glow of its radioactive ﬁreball all
across the electromagnetic spectrum (S&T: Jan. 2018, p. 10).
Astronomers have known neutron star binaries exist since
1974, when Russell Hulse and Joseph Taylor (then at the
University of Massachusetts, Amherst) discovered the ﬁ rst
one, PSR 1913+16. The two objects have an average separation of less than a million kilometers and an orbital period of
7.75 hours. But that separation and period are shrinking with
time. In fact, the binary's very slow decrease in orbital period,
measured over subsequent years by Taylor, Joel Weisberg (now
at Carleton College), and others, perfectly matches Einstein's
prediction for energy loss due to the emission of gravitational
waves. Some 300 million years from now, the two neutron
stars in the Hulse-Taylor binary will collide and merge.
This slow death dance provided the ﬁrst hard evidence,
even if it was indirect, that gravitational waves were real, and
the discovery of PSR 1913+16 ultimately earned Hulse and
Taylor the 1993 Nobel Prize in Physics. The objects' inexorable inspiral also gave a huge boost of conﬁdence for physicists such as Rainer Weiss (MIT) and Kip Thorne (Caltech),
who were designing the ﬁrst prototypes of LIGO-like laser
interferometers. If one binary neutron star would coalesce in
300 million years, others might do so tomorrow - and the
energetic burst of gravitational waves the collision produced
should be detectable with extremely sensitive instruments
here on Earth.
On August 17th, tomorrow arrived. "We've been waiting for this for 40 years," says Ralph Wijers (University of
Amsterdam, The Netherlands).
"I couldn't believe my eyes," adds LIGO lead astrophysicist Vicky Kalogera (Northwestern University). Back when
LIGO caught its ﬁrst event in September 2015, many team
members didn't believe it was real (S&T: Sept. 2017, p. 24).
But in the neutron stars' case, it was immediately clear that
here was the thing they'd all been waiting for. "It's a lot more
exciting than the ﬁrst gravitational-wave detection."
Astronomers around the world share Kalogera's elation.
Observing both gravitational waves and electromagnetic
radiation from the catastrophic coalescence of two hyperdense neutron stars provides astronomers with a wealth
of new, detailed information. The new buzzword is multimessenger astronomy, the study of objects or phenomena in
the universe using fundamentally different types of emission. The detection of neutrinos from supernova 1987A had
provided a tantalizing glimpse of this future, but as Edo
Berger (Harvard) comments, "2017 August 17 will always be
remembered as the singular moment when multi-messenger
astronomy was born."

Seconds before collision
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p THE CHIRP This spectrogram combines the signals from both LIGO
detectors to show the characteristic sweeping "chirp" signal of a merger.
As the neutron stars came closer to each other, circling faster, they produced higher-frequency gravitational waves, shown by the greenish line
sweeping upwards, until eventually they merged (not shown).

How History Was Made
Rumors about the neutron star event had circulated since
August 18th, when Craig Wheeler (University of Texas, Austin) tweeted: "New LIGO. Source with optical counterpart.
Blow your sox off!" Then, on September 27th, the LIGO and
Virgo collaborations announced the detection of GW170814,
the gravitational-wave signal of a black hole merger. The discovery led some to assume that the earlier rumors had been
just hype: Because these colliding black holes shouldn't give
off any light, you wouldn't expect an optical counterpart.
But in a speech October 3rd after his co-reception of the
2017 Nobel Prize in Physics (shared with Thorne and former
LIGO director Barry Barish of Caltech), Weiss conﬁrmed
another announcement was coming - and wouldn't say
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p FINDING GW170817 Gravitational-wave data homed in on a
banana-shaped region in the Southern Hemisphere (blue lines), more
specific than the region specified by gamma-ray data (red lines). The
black dot marks the location of the kilonova, in the galaxy NGC 4993.
sk yandtele scope.com * FE B RUA RY 2 018

http://www.skyandtelescope.com/

Sky and Telescope - February 2018

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