Sky & Telescope - June 2022 - 14

BLACK HOLE BACKGROUND: SUDAKARN VIVATVANICHKUL; GRAVITATIONALWAVE
EVENT: GREGG DINDERMAN / S&T
Black Holes Galore
period sets the waves' frequency, so as the objects inspiral,
the waves' frequency rises, sweeping up until the objects suddenly
plunge toward each other and collide, creating
a " chirp " in the signal. The resulting remnant
rings like a bell as it settles down. This ringdown,
as well as the objects' masses, spins,
and distance from Earth, is encoded in
the gravitational waves.
LIGO and Virgo can detect waves
from roughly 10 hertz to a few kilohertz.
Although two black holes might
spend a billion years inching closer to
each other, the detectors won't " hear "
them until the last few seconds or so.
When something hits the detectors
After the observing run, collaborators do a much deeper
" We've totally baffled
a lot of the theorists,
because our mass
distribution looks
nothing like any mass
distribution drawn
before. "
that looks like a gravitational-wave event,
it triggers an automated public alert. Dozens
of team members get calls - usually in the
middle of the night for the U.S., when noise from
human activity is low and detectors are therefore more
sensitive, says LIGO's Chad Hanna (Penn State). The scientists
jump to vet the signal. Events that make the cut prompt
a more detailed alert to the astronomical community.
The LIGO and Virgo collaborations issued 39 public alerts
- about one per week - during the second half of the latest
observing run, which lasted from November 2019 to March
2020. Speed is key, because if the event involves a neutron
star, then observers must rally to search the skies for a flash
of light (S&T: Dec. 2021, p. 36).
BREAKDOWN OF A GRAVITATIONAL-WAVE EVENT
Time
Period (Frequency)
The time from crest to crest depends on
the size of the black holes' orbit around
each other.
Period
Merger and Ringdown
The black holes plummet toward each other and merge.
The resulting, larger black hole rings like a bell from the
collision before settling down.
-MAYA FISHBACH
analysis, spending months assessing thousands of data
streams that record things like the detectors'
physical environments and laser power. " People
invest a lot of time in really trying to get the
right answers, " Hanna says - especially
early-career scientists, he emphasizes,
who are often the ones " banging their
heads against the supercomputers. "
The final tally can fluctuate a lot:
Only 18 of the 39 alerts survived. Scientists
also found another 17 that the
automated system hadn't spotted. The
less-than-optimal success rate stems in
part from the fact that every observing
run involves new hardware, as researchers
try to squeeze every improvement they
can out of the instruments. As a consequence,
each time they must relearn the system's vagaries.
Down in the Valley
The black holes LIGO and Virgo caught colliding generally
had masses from 7 to 50 times that of the Sun. They were
usually paired up pretty equally, mass-wise, which is unsurprising
because most ways to build a black hole binary tend
to join objects of near-equal masses. Independent teams have
also trawled the gravitational-wave data and found events
beyond the 90 in the collaborations' catalog, and these follow
the same trends.
Amplitude
The height of the waves' crests
encodes information about the black
holes' masses, spins, and distance
from Earth:
* Gravitational waves fade as they
travel, so smaller amplitudes
(a.k.a. quieter signals) come from
more distant sources.
* More massive black holes create
stronger waves.
* If the black holes spin and lean at
an angle as they orbit each other,
then the orbit will wobble around
and the amplitude will vary.
14 JUNE 2022 * SKY & TELESCOPE
Inspiral
As the black holes spiral in toward each other,
their orbit shrinks. This causes the waves'
frequency to increase.
* Smaller black holes come closer together
before they collide, so their waves reach
a higher frequency.
* The black holes' spins will affect how close
the objects approach before merger.
* The black holes' masses determine how
quickly the inspiral proceeds.
The exact shape and
behavior of the ringdown
depend on the black holes'
spins when they collide, as
well as the remnant's mass
and spin.
Sky & Telescope - June 2022

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