32 Scientific American, December 2020
relying on a trigger from a satellite. My plan was to use
the Zwicky Transient Facility, a robotic telescope at the
Palomar Observatory in California, to patrol the sky for
unusually fleeting, unusually bright points of light—and
then react quickly. When I presented my thesis propos-
al in May 2018, my faculty advisers warned me that I
might not find what I was looking for. They urged me to
keep an open mind because new avenues of inquiry
might arise. One month later that is exactly what hap-
pened. And two years later when I graduated, my thesis
looked very different from what I had expected.HOLY COW
When i began my Work, I wrote a program to find celes-
tial phenomena that were changing in brightness
more rapidly than ordinary supernovae. On a normal
day I examined 10 to 100 different candidates and con-
cluded that none of them were what I was looking for.On some days, though, I encountered something that
gave me pause.
In June 2018 I saw a report from a robotic telescope
facility called ATLAS, reporting a strange event dubbed
AT2018cow. “AT” stood for “astronomical transient,” the
prefix automatically given to all new transients, “2018”
for the year of discovery, and “cow” was a unique string
of letters. In the next couple of days there were reports
of similarities between this event and gamma-ray
bursts, yet there had been no detected show of gamma
rays. “Aha,” I thought, “this is it!” Because AT2018cow
was so bright and so nearby, there was intense world-
wide interest in this object, and astronomers observed
it all across the electromagnetic spectrum. I immedi-
ately made plans to ob serve AT2018cow using a radio
telescope in Hawaii called the Submillimeter Array.
AT2018cow stunned just about everyone. It unfold-
ed completely differently than any cosmic explosion
seen before. We were like the people in a classic para-
ble who are trying to identify an elephant in the dark.
One person feels its trunk and says it is a waterspout,
whereas another feels the ear and thinks it must be a
fan, and a third feels the leg and says it is a tree. Simi-
larly, AT2018cow shared characteristics with several
different classes of phenomena, but it has been diffi-
cult to put a complete picture together.My collaborators and I spent long days and nights
going over our data repeatedly, trying to figure out how
to interpret them. Some of those moments—calculat-
ing the properties of the shock wave together on a
chalkboard, a team member running down the hallway
waving a piece of paper with new results, and meeting
a colleague’s eyes in shock when a beautiful new mea-
surement came in—remain my most treasured memo-
ries from graduate school. In the end, we concluded
that there were two important components to
AT2018cow. The first was a central engine, as in a gam-
ma-ray burst, but lasting for much longer—weeks rath-
er than the typical days; x-rays shining from the heart
of the explosion stayed bright for much longer than
expected. The second was that for some reason, when
the star burst apart, it was surrounded by a cocoon of
gas and dust with about one one-thousandth the mass
of the sun. Our evidence for the cocoon is indirect:
when the star exploded, we saw a
flash of optical light and radio
waves that seemed to indicate
debris hitting a mass surrounding
the star. Such cocoons have been
seen in other types of explosions,
but we do not know how they get
there—it may be that the material
is shed by the star shortly before
exploding.
If this theory is correct, it would
be the first time astronomers have
directly witnessed the birth of a
compact object like a neutron star
or a black hole; most of the time the
corpse is completely shrouded by what remains of the
star. In the case of AT2018cow, we think we could actu-
ally see down to the compact object that produced all
of this amazingly variable and bright x-ray emission.
Still, we are left with many questions. What kind of star
exploded? Was the central engine a neutron star or a
black hole? Why did the star shed mass shortly before
exploding? To make progress, we needed to find simi-
lar events, so my colleagues and I set out to find anoth-
er AT2018cow using the Zwicky Transient Facility.
Three months later I thought we found one—the
bright, fast-rising explosion of September 9, 2018. Ini-
tially it looked very similar to AT2018cow. Within a week,
however, it became clear that this event was a Type Ic-
BL supernova—the kind associated with gamma-ray
bursts. Its name was SN2018gep. I was excited. Sure, it
was not another AT2018cow, but we finally had some-
thing that looked like a gamma-ray burst. Within five
days we had collected detailed observations all across
the electromagnetic spectrum. We searched the data for
evidence of a jet—but we found none. Instead, yet again,
my collaborators and I concluded that we were seeing
bright, fast-evolving optical emission from the collision
of explosion debris with a cocoon of material.
This was a surprise. Although cocoons have been
seen surrounding other types of stars, they are notExtreme deaths appear to be rare,
but the fact that they happen at
all tells us there is much we still
do not understand about the
basics of how stars live and die.
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