Sky & Telescope - December 2021 - 39

in the United States and the Virgo detector
in Italy. At once, algorithms automatically
send us an alert through the Gamma-ray
Coordinates Network (GCN). The network,
originally envisioned to facilitate the search
for GRB afterglows, does more than just
inform us of a new detection; it also serves
as a platform where astronomers promptly
communicate their observations and results
to the community.
The alert can catch you anywhere and
Patiently, we proceed with
our everyday lives until
the sudden ringing of our
phones sets the hunt in
motion.
anytime. Whether you are in a pub, on a
leisurely Sunday morning walk, or fast asleep, you have to
run to a computer and start compiling information. Is the
gravitational wave signal strong and trustworthy? What was
its likely progenitor? A binary black hole merger? There is a
very small chance that such a merger produces a counterpart,
perhaps we should just let it go. Is a neutron star involved?
Now that is interesting! How well is its location known, and
can it be promptly observed with any of the telescopes in our
network? We infer all that from the initial alert, but soon
additional information starts trickling in.
Position in the sky determines which telescope we trigger
first. Smaller telescopes with large fields of view are useful to
scan the localization area as quickly as possible, searching for
bright transients. Large telescopes see much smaller portions
of the sky, but they can find fainter objects. Instead of pointing
them blindly at random patches, we use a shrewd scheme
by targeting luminous and red galaxies, which are more likely
to host mergers because their stellar populations are older.
We scrape through data in the search for transients. By
comparing the positions of any we find to cataloged transients,
we automatically identify, and discard, most of them.
But many are new, and there is little else we can do but
conduct additional observations to learn more about them.
A few hours after the alert, tens of unidentified transients
keep thousands of astronomers busy. Ideally,
things calm down once somebody identifies
the real counterpart, and then all the eyes
and resources focus on that one source.
How Lucky Were We with GW170817?
The LIGO and Virgo observatories carried out their third and
most recent observational campaign between April 2019 and
March 2020. As of late August 2021, scientists have published
results on 41 new gravitational wave events, adding to the
previous sample of 11. From the point of view of the gravitational
wave community, the campaign was a phenomenal
success. However, despite the formidable numbers, we haven't
detected a single convincing electromagnetic counterpart to
any of these mergers.
Two mergers seemed promising. GW190425 was very likely
a binary neutron star merger, only the second one detected
so far. But even if an electromagnetic counterpart existed,
the bad localization and its considerable distance made for a
challenging and ultimately unsuccessful search. GW190814
appeared to be more auspicious. The two compact objects
involved in the merger were a 23-solar-mass black hole and
a mysterious object of 2.6 solar masses that could have been
either the most massive neutron star or the lightest black hole
ever observed (S&T: Oct. 2020, p. 8).
The excellent localization made GW190814 a perfect
target for ENGRAVE. With the Very Large Telescope and a
fleet of smaller survey telescopes at our disposal, we launched
Swift
HST
GROWTH
ENGRAVE
WORLDWIDE ENDEAVOR GROWTH and ENGRAVE are two of many collaborations searching for flashes and afterglows from gravitational-wave
events. Each network utilizes telescopes on the ground and in space.
sk yand tele scope .o r g * DECEMBER 2021 39
ILLUSTRATION: LEAH TISCIONE / S&T; MAP: GREGG DINDERMAN / S&T

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Sky & Telescope - December 2021

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