Sky & Telescope - December 2021 - 38
On the Edge of Our Seats
before the merger, tidal
forces disrupt or completely
tear the stars apart. Some of
the resulting debris swirls
around the newly formed
central object, makes an
accretion disk, and launches
two powerful jets moving at
nearly the speed of light. The
jets emit a short GRB and,
upon colliding with the surrounding
medium, give rise
to a longer-lived afterglow
emission. Some of the debris
may be flung away from the
central object, giving rise to
a bright transient called a
kilonova that can stay visible
for a few days to a few weeks
(S&T: Feb. 2018, p. 32).
To everyone's surprise and
Observer
GRB170817A
14°-19°
Jet
core
Slower
jet cocoon
ANATOMY OF A GRB When
two neutron stars collide, a hot
disk of debris forms around them
and a powerful, ultrarelativistic
jet shoots away from each pole.
Sheathing the jet is a slowermoving
cocoon of material. With
GW170817, observers saw both
bluish and red/infrared light, which
they think came from different
parts of the debris.
Polar wind off
accretion disk
(blue kilonova
emission)
vational tactics and write
software to automate and
expedite real-time analysis.
Our research group even
delight, GW170807 produced
everything we could have
imagined: a GRB, an afterglow,
and kilonova emission.
Nevertheless, the breakthrough
left us with many
questions. How different is
one kilonova from another,
and what is their yield of rare earth elements? Does every
merger produce a GRB? We were also anxiously awaiting the
first detection of the NS-BH counterpart. Building on the
experience with GW170817, we started to think of the best
way to find the next counterpart. We had pulled it off once -
how hard could it be?
Needle in a Haystack
Gravitational-wave observatories cannot pinpoint the location
of detected mergers very well. So to find the counterpart,
we need to find a new and short-lived source of light in an
area stretching over hundreds to thousands of square degrees
of sky. But that source of light may be too faint for our
telescopes. Or even worse, the merger might have played out
without producing the counterpart in the first place.
It doesn't help that the universe sparkles and twinkles like
a Christmas tree. A swarm of transients hangs out in that
big patch of the sky. Among all that clutter, we need to find
our counterpart and be smart about it - we wouldn't want to
spend all our resources on a transient that turned out to be
nothing more than a boring supernova.
Preparation is key to success. In truth, planning takes up
most of the work. Research groups spend years and many
resources building their own telescopes dedicated to the transient
search. Others write proposals to get time on existent
state-of-the-art machines. Scientists need to develop obser38
DECEMBER 2021 * SKY & TELESCOPE
Merger debris
and ejecta
(red kilonova
emission)
had a dry run to improve our
reaction times and decision
process. Senior members set
up a scenario with a mock
gravitational-wave event and
a virtual observatory. Once
the simulation started, the
organizers sporadically fed
us new pieces of information
to guide us in our follow-up
process. The tension peaked
when an error caused by
something as mundane as
file mismanagement led
one group of participants to
triumphantly announce the discovery of a kilonova, causing
confusion among others who couldn't find the claimed signal.
Thank goodness it was a dry run and not the real thing.
Scientists are a competitive bunch, but faced with the
challenging task of transient follow-up, we have to close
ranks and work together. Recently, large collaborations
between institutions from all over the world have emerged,
such as the Caltech-led project Global Relay of Observatories
Watching Transients Happen (GROWTH). " The idea behind
GROWTH is to establish a large collaboration distributed
around the world, but also distributed in wavelength, " says
Alessandra Corsi (Texas Tech University).
European researchers combined their resources at the
European Southern Observatory and formed the Electromagnetic
Counterparts of Gravitational Waves at the Very
Large Telescope (ENGRAVE) collaboration. Levan, the chair
of ENGRAVE's executive committee, points out that there is
another angle to large collaborations. " You get more voices,
you're more likely to make a good decision, " he says. " It
means that all our data come into one clearinghouse. When
you're making decisions about follow-up, you have the full
picture, not just part of the picture. "
And then we wait. Patiently, we proceed with our everyday
lives until the sudden ringing of our phones sets the hunt in
motion. It begins with the detection of gravitational waves
by at least one of three observatories: the two LIGO detectors
LEAH TISCIONE / S&T
Sky & Telescope - December 2021
Table of Contents for the Digital Edition of Sky & Telescope - December 2021
Contents
Sky & Telescope - December 2021 - Cover1
Sky & Telescope - December 2021 - Cover2
Sky & Telescope - December 2021 - 1
Sky & Telescope - December 2021 - Contents
Sky & Telescope - December 2021 - 3
Sky & Telescope - December 2021 - 4
Sky & Telescope - December 2021 - 5
Sky & Telescope - December 2021 - 6
Sky & Telescope - December 2021 - 7
Sky & Telescope - December 2021 - 8
Sky & Telescope - December 2021 - 9
Sky & Telescope - December 2021 - 10
Sky & Telescope - December 2021 - 11
Sky & Telescope - December 2021 - 12
Sky & Telescope - December 2021 - 13
Sky & Telescope - December 2021 - 14
Sky & Telescope - December 2021 - 15
Sky & Telescope - December 2021 - 16
Sky & Telescope - December 2021 - 17
Sky & Telescope - December 2021 - 18
Sky & Telescope - December 2021 - 19
Sky & Telescope - December 2021 - 20
Sky & Telescope - December 2021 - 21
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