Sky & Telescope - September 2020 - 11

STARS

Two New Beasts for an
Explosive Menagerie

BILL SA X TON / NR AO / AUI / NSF

ASTRONOMERS HAVE ADDED two

explosive events to a class dubbed Fast
Blue Optical Transients (FBOTs), named
for its members' sudden appearance and
blue-hot colors.
One of the ﬁrst real-time discoveries occurred in 2018. Ofﬁcially designated AT2018cow, this explosive source
earned the nickname "the Cow" (S&T:
May 2020, p. 10). Its proximity enabled
detailed follow-up observations, which
made it the prototype FBOT.
Before then, in 2016, the automated
Catalina Sky Survey and the All Sky
Automated Supernova Survey had independently discovered another source,
designated CSS161010. But according to
Raffaella Margutti (Northwestern), "It
took almost two years to ﬁgure out what
we were looking at, just because it was
so unusual."
Visible light from CSS161010 peaked
in a matter of days, glowing from a
dwarf galaxy some 500 million lightyears away. In the years that followed,
Deanne Coppejans (also at Northwestern), Margutti, and colleagues followed
up with radio and X-ray observations,
presented in the May 20th Astrophysical
Journal Letters. The researchers interpret
the data as a blast that ejected twin jets
of plasma shooting out at half the speed

of light and carrying up to 10% of the
Sun's mass.
Meanwhile, the Zwicky Transient
Facility recorded a similarly luminous
burst in 2018, whose light traveled for
3 billion years to Earth. Ofﬁcially, the
event is ZTF18abvkwla but astronomers
couldn't resist dubbing it "the Koala."
(Soon there may be a whole zoo!) In the
May 20th Astrophysical Journal, Anna
Ho (Caltech) and her colleagues followed up on the Koala with radio observations, ﬁnding evidence of fast-moving
material that had blasted into thicker
surroundings. The radio waves rivaled
gamma-ray bursts in their luminosity.
Most astronomers believe that these
new beasts are massive stellar explosions. Ralph Wijers (University of

Ryugu's reddening reaches too deep to
be caused by normal space weathering.
Red ejecta dug up by impacts streak
the surface, and some small craters are
actually redder than their surroundings, suggesting the red materials are at
least a few meters deep. Furthermore,
the team found that craters are either
red or blue, with few in between. The
dichotomy suggests that the reddening
was an event, not a continuous process,
and that it happened before the blue
craters had formed.
Putting these data together, Morota
and colleagues propose that Ryugu
baked when it ventured too close to the

Sun, between 300,000 and 8 million
years ago.
They also estimate that the heating
happened some 8.5 million years after
the asteroid's formation, making Ryugu
at most about 17 million years old.
This age is surprisingly young. Based
on Ryugu's orbit, scientists thought the
little world was built from the debris of
a main-belt asteroid smashup that happened a few hundred million years ago.
Maybe multiple generations of asteroids
formed and broke up before Ryugu
coalesced, the authors suggest.
But the age estimates are far from
certain, cautions OSIRIS-REX scientist

Normal
supernova

Fast blue optical
transients

Gamma-ray
burst

p Astronomers think FBOTs (center) may be a type of explosion in between regular core-collapse
supernovae (left) and more extreme gamma-ray bursts (right).

Amsterdam), who was not involved in
these studies, thinks FBOTs could help
ﬁll the gap between regular supernova
explosions and the most energetic
gamma-ray bursts. In the case of FBOTs,
thick, dense clouds of material may
surround the supernovae, absorbing and
re-radiating the explosive power.
But other ideas remain in play.
Maybe FBOTs are not supernovae but
tidal disruption events, where intermediate-mass black holes rip apart and
consume unlucky stars.
Whatever produces these hot, brief,
and energetic outbursts, one thing's for
sure: Now that astronomers know what
to look for, these events are likely just
the tip of an explosive iceberg.
■ GOVERT SCHILLING

Daniella DellaGiustina (University of
Arizona). It's possible that space weathering, pulverization, and even carbon
could redden the rocks, she explains.
Researchers will be able to conﬁrm
Ryugu's surface age once they test Hayabusa 2's samples, which are scheduled
to drop in Australia this December.
Interestingly, the scientists say the
proposed heating event couldn't explain
why Ryugu's minerals show so little
evidence of interacting with water (S&T:
July 2019, p. 8). The bluish regions are
just as dry as the reddish ones.
■ CAMILLE M. CARLISLE

* Read more at https://is.gd/ryugu.
sk yandtelescope.org * SEP TEMBER 2 02 0

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