NEWS NOTES
A.^TONITA,^L.^
REZZ
OLLA
&^F.^ PANNARALEONLY TWO MONTHS INTO a new
observing run, gravitational-wave
observatories have announced 13 new
candidate signals — one of which could
turn out to be a black hole swallowing
a neutron star.
Major improvements to both the
Laser Interferometer Gravitational-
wave Observatory (LIGO) in the U.S.
and the Virgo instrument near Pisa,
Italy, have made all three detectors far
more sensitive to ripples in spacetime.
And beginning with the third observ-
ing run, which goes from April 2019
to April 2020, LIGO and Virgo are
announcing gravitational-wave signals
as they happen — that is, before the
sources themselves are fully vetted and
confi rmed as real. Finding electromag-
netic radiation from these candidate
events is crucial to understanding
them, and immediate announcements
allow astronomers to observe the sky
near candidate sources at once.
Of the 13 candidates announced
as of May 24th, 10 appear to be black
hole mergers, while two others seem to
be neutron star crashes. The most tan-
talizing, though, is a possible mashup
between a black hole and a neutron
star. (These categories are preliminary
and await further analysis.)
While black hole pairs aren’t
expected to produce light, astrono-
mers do expect inspiraling neutron
star pairs — or a neutron star spiraling
into a black hole — to fl ash across the
electromagnetic spectrum. No coun-
terparts have been announced so far,
but investigations are still under way.Unfortunately, one of the detectors
— LIGO Hanford — was briefl y offl ine
when the signal from the fi rst neutron
star merger arrived on April 25th. All
three detectors working together can
narrow the fi eld to hundreds of square
degrees, but with Hanford out of com-
mission the sky area to comb through
expanded to 10,000 square degrees.
All detectors were online when the
second such merger was detected.
Meanwhile, the signal from the
black hole–neutron star collision,
which came on April 26th, was weak.
“It’s like listening to somebody whis-
per a word in a busy café,” says LIGO
spokesperson Patrick Brady (University
of Wisconsin, Milwaukee). “It can be
diffi cult to make out the word or even
to be sure that the person whispered
at all. It will take some time to reach a
conclusion about this candidate.”
Despite the challenges involved in
following up on gravitational-wave
signals, early efforts are promising.
“We do have one tantalizing candidate
remaining for which the jury is still
out,” says Mansi Kasliwal (Caltech).
“We are collecting more information,
and I hope to be able to say more soon.”
Regardless, 13 candidates in two
months is an auspicious start. The
team had expected to fi nd a few black
hole mergers per month; however,
with only a single neutron star colli-
sion observed in 2017, predictions for
the rate of these less-massive merg-
ers had ranged from one per month
to one per year. As of the end of May,
we’ve already seen two neutron star
mergers; if things keep going this way,
we can expect to see many more.
■ MONICA YOUNG
Find out how amateurs can participate in
LIGO research by visiting https://is.gd/
proamLIGO.GRAVITATIONAL WAVESLIGO and Virgo Find
Possible Black Hole–
Neutron Star Crasht This simulation
frame shows a
black hole (gray
sphere) devouring
a neutron star.8 AUGUST 2019 • SKY & TELESCOPE
MILKY WAY
Omega Centauri Is
Losing Its Stars
ASTRONOMERS HAVE DISCOV-
ERED a stream of stars pulled
from Omega Centauri, the largest
and most brilliant globular clus-
ter around the Milky Way — and
perhaps the remnant of a one-time
dwarf galaxy.
Omega Centauri is unusually
luminous and massive. What most
puzzles astronomers, though, is
that its stars separate into mul-
tiple populations, suggesting that
the cluster came together over
billions of years instead of all at
once. These peculiarities have led
some astronomers to suggest that
this globular might actually be the
remains of a galaxy that came too
close to the Milky Way. As it wasTHE MOON
Apollo-era Data Reveal
the Moon’s Tectonic
Activity
A NEW LOOK AT OLD SEISMIC
DATA gathered during the Apollo
missions reveals that young active
faults might be the source of shallow
moonquakes.
When the Apollo astronauts
deployed seismometers on the lunar
surface, they revealed 28 shallow,
but sometimes surprisingly power-
ful, quakes between 1969 and 1977.
A new study appearing May 13th in
the journal Nature Geoscience links
these quakes to current tectonic
activity on the Moon.
As the Moon loses heat from its
interior, it shrinks and its surface
wrinkles. Thrust faults form where
the brittle crust breaks: One side of
the break slips downward while the
other side goes upward, a process
that creates steep slopes, or scarps,
typically tens of meters high.
Even though these faults cover
most of the lunar surface, they had
largely gone undetected until 2010,