38 AUSTRALIAN SKY & TELESCOPE January 2018
FOLLOWING THE FIREBALL
The Australian National University (ANU) plays a leading role in Australia’s
partnership with the US-based Advanced Laser Interferometer Gravitational-Wave
Observatory(LIGO).Anditsowntelescopeshelpedwithfollow-upobservations
aftertheneutronstarcollisiondiscovery.
DrChristianWolf’steamusedtheANU’sSkyMapperand2.3-metretelescopes
at Siding Spring Observatory. “SkyMapper was the first telescope to report the
colourofthefireball,whichindicatesthetemperatureofthefireballwasabout6,000
degreesCelsius—roughlythesurfacetemperatureoftheSun,”saidDrWolf.
Professor David McClelland from the ANU Research School of Physics and
Engineering is leader of a team that is developing new components for LIGO’s
detectors. “Using quantum mechanical techniques, we will make the largest optical
sensorseverbuiltevenmorepowerful,”saidProfessorMcClelland.“Wewillthen
detect many more gravitational waves from cataclysmic events in space, involving
black holes, neutron stars and things not yet known.”box’ measured a few tens of degrees
in diameter (the full Moon is only
half a degree wide), and NASA’s Swift
satellite, which sometimes can catch a
Fermi event with its more precise X-ray
telescope, didn’t see any X-ray emission
immediately after the GRB.
As for the gravitational-wave signal,
the situation looked even worse. The
event had been observed by both the
LIGO detector in Hanford, Washington,
and its twin in Livingston, Louisiana
(although it took a while before the
Livingston signal was retrieved from
the data stream because of a technical
glitch). From the tiny difference in
arrival time (just a few milliseconds),
it was possible to trace the origin of
the gravitational waves back to a long,
thin banana-shaped strip of sky. But
although the banana was extremely
thin in this particular case (thanks to
the long duration of the event), it was
also very long.
The thin LIGO banana did cross theFermi error box, in the constellations
Virgo and Hydra. Alas, the overlap
region was still much too large to start
a focused search for a possible optical
counterpart of the event, which would
probably be extremely faint.
But wait a minute — what about
the third gravitational-wave detector,
in Italy? Virgo had been up and
running in tandem with LIGO since
August 1. Differences in arrival time
for three detectors make it possible to
triangulate the source location much
more precisely. In fact, that was exactly
what had happened three days before,
with the black hole merger GW170814.
So wouldn’t the Virgo observations of
GW170817 provide an answer?
Almost two months after the
events, Vicky Kalogera is still high on
adrenalin when she explains the role of
the European Virgo detector in solving
the case. “In August,” she says, “I was
vacationing with my family in Colorado
and Idaho, where we would observe theAugust 21st total solar eclipse. I had
promised not to be working all the time.
Then came GW170814, and three days
later the neutron star event. I’ve been at
my laptop and in telecons ever since.”
Surprisingly, she recounts, Virgo
did not ‘trigger’ on GW170817. The
90-second Einstein wave signal of
the coalescing neutron stars almost
doesn’t show up in Virgo’s data
stream, even though the European
instrument shouldn’t have had any
problem detecting it. “The great thing,”
says Kalogera, “is that Virgo’s ‘non-
detection’ turned out to be the key to
localising the source.”
Laser interferometers like LIGO and
Virgo can detect gravitational waves
from nearly every direction. But because
of their design, there are four regions of
sky on the instrument’s local horizon
for which the detection sensitivity is
much lower than average. At the very
centre of those regions are blind spots.
Virgo hadn’t registered a strong, passing
gravitational wave because the source
of the waves was located near one of
Virgo’s blind spots.
Lo and behold, this spot coincided
with the overlap region between LIGO’s
thin ‘banana’ and Fermi’s error box.
Given the upper limits on the Virgo
signal, astronomers were able to fence
off a much smaller, elongated part of
the sky, with an area of just some 28
square degrees.Counterpart search
Now the hunt was on. Over the past
years, the LIGO-Virgo Collaboration
had signed a formal agreement with
some 70 teams of astronomers all
over the world to share this kind of
information under strict embargo.
This 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 co-ordinates of the
search area for GW170817 in hand,
everyone trained their instruments at
the suspected crime scene in southern
Virgo and eastern Hydra. ANUCOSMIC COLLISIONANU’s SkyMapper telescope
at Siding Spring.