http://www.skyandtelescope.com.au 39The 1-metre Henrietta Swope
TelescopeattheLasCampanas
ObservatoryinnorthernChilewas
the first to strike gold. Their success
dependedonacleverstrategy.TheLIGO
data provided them with an indication
ofthesource’sdistance,andwithin
thesearchareatherewereonlyafew
dozengalaxiesatthisdistancerange.
Astronomers with the Swope Supernova
Surveyrapidlycheckedthegalaxiesone
byone,inorderofprobability,toseeif
they could find an optical transient.
Around 23:00 UT, they found a
surprisingly bright (17th-magnitude)
pointoflightatthenorth-eastern
edgeofthegalaxyNGC4993,near
thebinarystarGammaHydrae.The
sourcewasbrightenoughforamateur
astronomerstohavepickedoutwith
large (40-cm) telescopes. The galaxy’s
redshiftputsitatadistanceof130
millionlight-years.Withoutdoubt,herewas the optical counterpart of both the
neutron star collision that produced
the gravitational-wave signal and the
short gamma-ray burst — smack in
the overlap region of LIGO’s banana,
Fermi’s error box and Virgo’s blind spot.
In the subsequent days and weeks,
dozens of ground-based telescopes and
space observatories observed that point,
including the Hubble Space Telescope,
theGeminiSouth,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 (16 days after the crash).
“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 “multi-
messenger paper”) is co-authored by
almost 4,000 astronomers from morethan 900 institutions. “This represents
about one-third of the worldwide
astronomical community,” she says.Striking gold
The fading aftermath of the neutron
star collision has now been observed
at every possible wavelength, from
X-rays and ultraviolet through optical
and infrared, all the way to millimetre
and radio waves. The aftermath
phenomenon is known as a kilonova —
an explosive event less luminous than a
supernova, but about a thousand times
as bright as a normal nova. Only once
before, in June 2013, have astronomers
found a kilonova in conjunction with
a short gamma-ray burst, but that one
was extremely faint, occurring at a
distance of some 4 billion light-years.
The kilonova is basically the
sizzling fireball from the neutron
star smash-up. Chunks of hot, dense
nuclear matter are hurled into space, in
all possible directions, with velocities
easily reaching 20% or 30% the speed
of light. Liberated from the neutron
stars’ extreme gravity, the debris
expands, rapidly losing its ultra-high
density. Neutrons now start to decay
into protons, and in the resulting
thermonuclear cauldron, these two
types of particles combine into heavy
atomic nuclei, many of which are
highly radioactive. What remains is an
incredibly hot expanding shell, loaded
with some of the heaviest elements in
the periodic table.
Spectroscopic observations by theThe VIMOS instrument on ESO’s Very Large Telescope
captured the galaxy NGC 4993, along with the kilonova (arrowed).ESO
RADIO WAVES REVEALED
Astronomers worldwide were keen to see
if they could pick up radio emission from
the neutron star collision event. Australia’s
radio astronomers were ready to go as
soon as word reached them.
“We started planning our observations
immediately: we knew the target area would
rise over Australia at about 11am Sydney
time,” wrote the University of Sydney’s As-
sociate Professor Kate Murphy and Associ-
ate Professor David Kaplan of the University
of Wisconsin-Milwaukee. What followedwas two weeks of collaborative research
that lead to the first confirmation of radio
emission from a gravitational wave event.
The astronomers used the power of the
CSIRO’s Australia Telescope Compact Array
near Narrabri in NSW to pick up the event’s
faint radio signals. “This was really exciting;
our chance to see a gravitational wave event
with conventional telescopes for the first
time. Astronomers had been waiting for this
for more than 20 years, ever since the LIGO
CSIRO project started,” added Murphy and Kaplan.
The CSIRO’s Australia
Telescope Compact
Array.