Sky and Telescope - February 2018 - 36

Gravitational Waves

August 21, 2017

200

Brightness
(in millions of Suns)

August 17, 2017

150
100
50
0
1

2

3

4

Days (since merger)

Counterpart Search
Now the hunt was on. Over recent years, the LIGO-Virgo
Collaboration had signed a formal agreement with about 100
teams of astronomers all over the world to share this kind of
information under strict embargo, meaning they couldn't go
public with it before a specified date. The alert system would
enable the teams to search for electromagnetic counterparts
of any gravitational-wave signals with telescopes on the
ground and in space, preferably right after the detection.
With the latest coordinates of the search area for GW170817
in hand, some 70 teams trained their instruments at the
suspected crime scene.
The 1-meter Henrietta Swope Telescope at the Las Campanas Observatory in northern Chile was the first to strike gold.
The team's success depended on a clever strategy. The LIGO
data provided them with a rough indication of the source's
distance, and within the search area there were only a few
dozen galaxies at this distance range. Astronomers with the
Swope Supernova Survey rapidly checked the galaxies one by
one, in order of probability, to see if they could find an optical
transient in any of them.
Around 23:33 UT, they found a 17th-magnitude point of
light some 10 arcseconds (7,000 light-years) northeast of the
core of the lenticular (S0) galaxy NGC 4993, which lies near

the binary star Gamma Hydrae. The source was surprisingly
bright, enough for experienced amateur astronomers to have
picked it out with large (16-inch) telescopes. The galaxy's redshift puts it at a distance of 130 million light-years, the same
distance as inferred from the gravitational waves.
Without doubt, here was the optical counterpart of both
the neutron star collision that produced the gravitationalwave signal and the short gamma-ray burst.
In the subsequent days and weeks, dozens of groundbased telescopes and space observatories observed that point,
including the Hubble Space Telescope, Gemini South, Keck,
the European Southern Observatory's Very Large Telescope,
ALMA, the Chandra X-ray Observatory (it picked up X-rays
some 9 days after the event), and the Very Large Array (radio
waves 16 days after the crash). Researchers even searched for
high-energy neutrinos in data from the IceCube neutrino
detector in Antarctica and the Pierre Auger Observatory in
Argentina, but they found no matches.
"I would think this is the most intensely observed astronomical event in history," Kalogera says. The paper describing
the follow-up observations (unofficially known as the "multimessenger paper") is coauthored by some 3,600 physicists
and astronomers from more than 900 institutions. According
to some estimates, a whopping 15% of the worldwide astronomical community are on the author list. And it's only one
of many dozens of papers on GW170817 released on October
16th, in journals including Physical Review Letters, The Astrophysical Journal Letters, Science, and Nature.

Striking Gold

p FIRST IMAGES These are the first six observations of the kilonova
(three left-hand columns), all taken within 12 hours of the gravitationalwave signal. On the right are the first detection in X-rays 9 days later
(top) and in radio 16 days later.

36

FE B RUA RY 2 018 * SK Y & TELESCOPE

Astronomers have now observed the fading aftermath of the
neutron star collision at every possible electromagnetic wavelength. The aftermath phenomenon is known as a kilonova
- a bright, transient event less luminous than a supernova,
but about a thousand times as bright as a normal nova and
some 100 million times more luminous than the Sun. Only
once before, in June 2013, have astronomers found a possible
kilonova in conjunction with a short gamma-ray burst, but
that one was extremely faint, due to its distance of some 4
billion light-years (S&T: Nov. 2013, p. 12).

M ULTIWAV ELENGTH THU MBN AILS: B. P. A BBOT T E T A L. / ASTR OPHYSICAL JOUR NAL LE T TERS 2017 (848:L12, DOI 10.3847); IM AG ES: 1M2H / U.C. SA N TA CRUZ A ND CA R NEGIE
OBSERVATORIES / RYA N FOLE Y; G R A PH: LE A H TISCIONE / S&T, SOURCE: SA R A H WILK INSON / LCO

p FIRST LIGHT Left: These composite images of NGC 4993 show the kilonova (marked by yellow arrows) upon discovery on August 17th and four
days later, on August 21st, when it had dramatically reddened. Both images use data from the Swope and Magellan telescopes, taken in different
filters. The left-hand image contains the first optical photons received from the afterglow, called SSS17a. Right: The kilonova reddened and faded by a
factor of more than 20 in just a few days, as shown here in data from Las Cumbres Observatory telescopes.



Sky and Telescope - February 2018

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