Sky & Telescope - October 2023 - 8

NEWS NOTES
STARS
How Soon Will Betelgeuse
Blow?
MANY ASTRONOMERS THINK
Betelgeuse, the red supergiant star
marking Orion's shoulder, will go
supernova soon - that is, within
the next 10,000 to 100,000 years.
In a new study posted June 1st on
the arXiv astronomy preprint server,
Hideyuki Saio (Tohoku University,
Japan) and colleagues claim that star
might be closer to exploding than we
thought. But others disagree.
Betelgeuse " breathes " in and out,
BLACK HOLES
Signal from OJ 287's Second Black Hole
ASTRONOMERS HAVE LONG suspected
that the distant galaxy OJ 287 harbors
not one but two supermassive black
holes in its core - one with the mass of
a whopping 18 billion Suns, the other a
comparatively diminutive, 150-millionsolar-mass
companion. At the 242nd
meeting of the American Astronomical
Society, Mauri Valtonen (University of
Turku, Finland) announced that his
team detected a new set of flares from
the second black hole.
Astronomers have been recording
OJ 287's light, which travels some 3½
billion light-years to Earth, since 1888.
In the 1980s, Aimo Sillanpää (Tuorla
Observatory, Finland) and colleagues
used the long archive to show a roughly
12-year cycle in outbursts from OJ 287,
which the astronomers attributed to a
pair of orbiting black holes.
Further investigation revealed that
the outbursts seen every 12 years are
actually sets of twin, weeks-long flares.
Sillanpää, Valtonen, and colleagues
interpreted these flares as arising when
the secondary black hole plunges into
and exits the gaseous accretion disk surrounding
the primary black hole.
Using models of the black hole pair's
interactions, astronomers were able to
predict when the next set of outbursts
should occur. In 2021 and 2022, the
team unleashed a campaign across
8 OCTOBER 2023 * SKY & TELESCOPE
pThis illustration shows the putative black hole
binary in OJ 287. See the animation at https://
is.gd/OJ287video.
multiple wavelengths to observe the
system. The astronomers caught the
activity from the secondary's plunge
into the accretion disk within days of
the predicted time. But the campaign
also caught two unexpected features.
To the team's surprise, Staszek Zola
(Jagiellonian University, Poland) caught
a brief (day-long), highly energetic spurt
of light in an R-band filter. Analysis
indicates that this brief flare would have
occurred when the secondary black hole
swallowed a massive dose of new gas
during its plunge through the primary's
accretion disk.
The second surprise came when
the Fermi Gamma-ray Space Telescope
detected high-energy radiation. The
team attributes this to the secondary
black hole's jet interacting with gas in
the primary's disk. A trawl through
archival data turned up a similar
gamma-ray flare coinciding with a predicted
outburst in 2013.
Valtonen argues that the newest set
of flares points at multiple stages of
interaction as the secondary black hole
passes through the primary's disk -
including, for the first time, emission
that's coming from the secondary itself.
¢DIANA HANNIKAINEN
with overlapping overtones, resulting
in regular brightness changes over
periods of 2,200 days, 420 days, 230
days, and 185 days. Usually, astronomers
treat the 420-day up-and-down
as the primary in-and-out pulsation,
with the shorter cycles as overtones.
The 2,200-day (or 6-year) period
is dubbed a long secondary period, a
feature of unknown origin found in
SUPERNOVAE
We Could Soon " Hear "
Gravitational Waves from
Dying Stars
A NEW POSSIBLE SOURCE of
gravitational waves is the cocoon of
hot, dense gas that forms within a
massive, dying star. This source is
within reach of the recently begun
observing run of the Laser Interferometer
Gravitational-Wave Observatory
(LIGO).
Hefty stars with 20 to 40 times
the Sun's mass die in particularly
spectacular fashion. First the center
collapses into a black hole, which in
turn powers two opposite jets that
drill their way out of the star. As the
jets ram their way through the outer
stellar layers, they amass a sheath of
hot, dense star-stuff.
Ore Gottlieb (Northwestern
University) and colleagues only
recently realized this accelerating
cocoon of gas could emit gravitaNASA
/ JPL-CALTECH
https://www.is.gd/OJ287video https://www.is.gd/OJ287video
Sky & Telescope - October 2023

Table of Contents for the Digital Edition of Sky & Telescope - October 2023
