First, astronomers discovered the “Cow.”
Now they’ve rounded up a small herd.
A short-lived celestial flare-up with a
bovine nickname has been joined by two similarly unusual outbursts. The
mysterious events were brighter than typical supernovas, explosions of stars
that are a common source of temporary light shows in the sky. And the novel bursts
came and went quickly, with their visible light brightening and dimming over
days instead of the weeks typical of normal supernovas.
As particularly luminous examples of a
poorly understood class known as fast blue optical transients, the three novel
bursts have unknown origins. But they seem to be kin. “It’s like people going
out to find different creatures and find out how they’re related to each other.
We’re in the early stages of the ‘zoology’ of this class,” says astronomer Anna
Ho of Caltech.
Detected in June 2018, the Cow earned
its moniker thanks to the automatically assigned letters within its official astronomical
name, “AT2018cow.” It is joined by the “Koala,” Ho and colleagues report in the May 20 Astrophysical Journal. Named for the
ending letters in its handle, ZTF18abvkwla, the Koala appeared in September
2018. The third event, reported in the May 20 Astrophysical Journal Letters, resisted cute nicknames. Known as CSS161010, it was detected before the other
events, in 2016, but its significance wasn’t understood until now.
A possible explanation for the events is
that they result from an unusual type of supernova, one that explodes into a
dense shell of material. For all three events, telescopes detected radio waves
in addition to a short-lived flare of visible light. Those radio waves could have
been produced by accelerated electrons, kicked up when a blast of debris from
the explosion slammed into that surrounding shell. If an aging star shed its outer layers before it exploded, that could have created the
sheath (SN: 6/21/19).
But the events are still enigmatic. “We
don’t actually know what they are yet,” says astrophysicist Deanne Coppejans of Northwestern University in
Evanston, Ill., a coauthor of the paper on CSS161010. Scientists still can’t
rule out the possibility that the events could be the result of a black hole
ripping apart a star, rather than the aftermath of a supernova, she says.
After the Cow was discovered, scientists
had speculated that whatever created it may have involved some kind of cosmic “engine,”
a dense object like a black hole or spinning neutron star that could add some
oomph to the eruption by launching powerful jets of material. Now, in light of
the two teams’ new results, “I’m driven to think that there really is some kind
of central compact object driving these explosions,” says astrophysicist Brian Metzger of Columbia University.
That’s because the two new events both
spewed matter at blazing speeds. The Koala ejected its detritus at more than 38
percent of the speed of light; CSS161010’s ejecta reached more than 55 percent
light speed. The Cow’s offal reached a relatively measly 10 percent of the
speed of light. In the case of CSS161010, the researchers also observed X-rays,
which could likewise have been stirred up by such an engine.
Scientists have been regularly detecting
fast blue optical transients only for the last several years. Transient blips
of light in the sky are spotted by comparing different images taken of the same
locations at different times. In the past, telescopes surveyed the sky at
intervals short enough to catch normal supernovas but missed briefer events. Faster-paced
surveys have recently made it possible to spot changes that occur on timescales
of days. Those surveys include the Zwicky Transient Facility, which detected
the Koala, and the Catalina Real-time Transient Survey and
the All-Sky Automated Survey for Supernovae, both of
which detected CSS161010.
But watch this space: “There are more
surveys coming online that are going to be capable of detecting these things,”
says astrophysicist Patricia Schady of the
University of Bath in England. That could include the Vera Rubin Observatory, which will start up in 2022 (SN: 1/10/20). To fully pin down the source of the events, Schady
says, “we really do need more of these things.”