Sky & Telescope - August 2019 - 8

NEWS NOTES
THE MOON

GRAVITATIONAL WAVES

LIGO and Virgo Find
Possible Black Hole-
Neutron Star Crash
ONLY 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 Gravitationalwave 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 observing 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
conﬁrmed as real. Finding electromagnetic 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 tantalizing, 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, astronomers do expect inspiraling neutron
star pairs - or a neutron star spiraling
into a black hole - to ﬂash across the
electromagnetic spectrum. No counterparts have been announced so far,
but investigations are still under way.
8

AUGU S T 2 019 * SK Y & TELESCOPE

Unfortunately, one of the detectors
- LIGO Hanford - was brieﬂy ofﬂine
when the signal from the ﬁrst neutron
star merger arrived on April 25th. All
three detectors working together can
narrow the ﬁeld to hundreds of square
degrees, but with Hanford out of commission 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 whisper a word in a busy café," says LIGO
spokesperson Patrick Brady (University
of Wisconsin, Milwaukee). "It can be
difﬁcult 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 ﬁnd a few black
hole mergers per month; however,
with only a single neutron star collision observed in 2017, predictions for
the rate of these less-massive mergers 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.

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 powerful, 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,

MILKY WAY

Omega Centauri Is
Losing Its Stars
ASTRONOMERS HAVE DISCOVERED a stream of stars pulled

from Omega Centauri, the largest
and most brilliant globular cluster 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 multiple 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 was

A . TONITA , L. RE Z ZOLL A & F. PA NN A R A LE

t This simulation
frame shows a
black hole (gray
sphere) devouring
a neutron star.
https://www.is.gd/proamLIGO https://www.is.gd/proamLIGO
Sky & Telescope - August 2019

Table of Contents for the Digital Edition of Sky & Telescope - August 2019
