8 AUSTRALIAN SKY & TELESCOPE April 2019
A RE-ANALYSIS OF DATAfrom the
Laser Interferometer Gravitational-
wave Observatory (LIGO) and the Virgo
interferometer in Italy has found four
new events, bringing the total number
of gravitational-wave events detected
sofarto11.Thetallyincludesthemost
distant and most powerful black hole
merger yet discovered.
LIGO and Virgo announced the
new events, designated GW170729,
GW170809, GW170818 and
GW170823,atagravitational-wave
conference on December 1, 2018. The
first of these became the most massive
merger detected to date — and the most
distanttoo,asthesignalhadtravelled
5billionyearstoEarth.Tocreatethe
signal, two black holes weighing in at
34and51solarmasseshadcoalesced
into an 80-solar-mass monster,
unleashingtheenergyequivalentoffive
solarmassesasgravitationalwaves.
Thefindshelpcharacterisetheorigin
of this population of binary black holesand the frequency with which they
merge.Thecollaborationhasusedthe
detections to date to show that between
10 and 100 binary black holes merge
per year in a volume 3 billion light-years
on a side. This number appears to grow
with increasing distance, which would
be expected since stars formed more
rapidly in the past than now.
However, while the black holes
areallofstellarmassandhave
likely supernova origins, GW
mightposeachallengetostellar
evolution models, which have trouble
producing hefty black holes. Then
again, uncertainties in the mass
measurements of the progenitors
mean the signal is still consistent with
current models.
In addition to the four new
statistically significant finds, the LIGO/
Virgo Collaboration also published a
list of 14 “marginal event candidates”.
The collaboration was due to release
all data from the second observingrun in February 2019. Meanwhile, all
threegravitational-wavedetectorsare
being upgraded for a new simultaneous
observing run beginning in April 2019.
Given the instruments’ improved
sensitivity, scientists expect to find at
leastafeweventspermonth.
■ GOVERT SCHILLING
TolearnmoreaboutwhatLIGOand
Virgo are teaching us about black hole
spin, visithttps://is.gd/LIGOspins.NASA’S OSIRIS-REX arrived at
asteroid 101955 Bennu on December
3 and kept pace with the asteroid
for several weeks before entering
into orbit on December 31. Now, the
spacecraft is mapping Bennu from
about 730 metres above its surface.
Even before it entered into orbit,
Osiris-REX’s preliminary surveys
had led to the detection of hydrated
minerals on Bennu’s surface,
suggesting that the asteroid’s larger
parent body once hosted water. Amy
Simon (NASA Goddard) announced
the results December 10 at a meeting
of the American Geophysical Union in
Washington, D.C.
Two spectrometers onboard
Osiris-REX picked up the presence
of hydroxyls, molecules that contain
oxygen and hydrogen atoms bonded
together. The mission team suspects
that these molecules are lockedup in clay minerals, formed in
interactions with liquid water or
water vapour.
Bennu itself is too small to have
ever hosted liquid water. However,
researchers think that the roughly
500-metre-wide space rock is actually
a chunk knocked off a much larger
asteroid long ago. The find suggests
that water existed at some point on
Bennu’s home world.
Most of the next year is dedicated
to mapping and scanning Bennu’s
surface, surveying for the perfect
spot from which to grab some
material. Then, around July 2020,
the real excitement begins, as Osiris-
REX moves in for a series of sampling
manoeuvres. The spacecraft will
return the sample to Earth, with a
planned homecoming in 2023.
■ DAVID DICKINSON &
SBennu, as seen by Osiris-REX CHRISTOPHER CROCKETTOsiris-REX arrives,
finds hints of ancient
waterLIGO and Virgo discover four more black hole collisions
BLACK HOLES: LIGO / CALTECH / MIT / SONOMA STATE (AURORE SIMONNET); BENNU: NASA / GODDARD / UNIV. OF ARIZONAIllustration of a black hole duoNEWS NOTES