December 2020, ScientificAmerican.com 33commonly observed in the types of supernovae associ-
ated with gamma-ray bursts. Our discovery implies that
more stars shed gas at the end of their lives than we
thought. We know the gas was lost in the final moments
of the star’s life because it was so close to the star at the
time of the explosion; if it had been cast off earlier, it
would have had time to get farther away. That means
the star lost a significant chunk of its outer atmosphere
in the final days to weeks of its life, after shining for
millions to tens of millions of years. It seems, then, that
this shedding heralds the death of the star.
Once again, we were left with questions. How prev-
alent are these death omens in different types of stars?
What is the physical mechanism that produces them? I
realized that I had a new direction to my research now—
not just gamma-ray bursts and jets but also the warning
signs of soon-to-explode massive stars. And perhaps
these different phenomena were even connected.
It was not until the final six months of my Ph.D. pro-
gram that I finally found a gamma-ray burst afterglow.
On January 28, 2020, I did my usual candidate review
when I saw something that looked promising. I knew
better than to get excited—there had been many, many
false starts over the years. I immediately requested
additional observations with a telescope in La Palma
in the Canary Islands, and they confirmed that this
source was fading away quickly, as would be expected
for an afterglow. That night I requested urgent obser-
vations on the 200-inch Hale Telescope at the Palomar
Observatory that showed the source was still fading.
The next night I obtained observations with the Swift
X-ray space telescope and detected x-rays from the
event, all but confirming this was truly a GRB after-
glow. The night after that I got a brief window of time
on the Keck Telescope on Mauna Kea in Hawaii, with
the hope of measuring how far away the explosion was.
I slept in a sleeping bag in the remote observing
room at my university, the California Institute of Tech-
nology, and set an alarm for 4 a.m. When the time came,
I felt panicked—I was squeezing in this observation
right at the end of the night, the sky was getting bright-
er quickly, the source was very faint, and I was terrified
of being too late. I did the best that I could. When it was
too bright to observe any longer, I called my colleague
Dan Perley of Liverpool John Moores University in Eng-
land on Skype, and we looked at the data together. I was
lucky. The source was faint, but there was a big, boom-
ing, obvious feature in the light from the event that
enabled us to measure the distance, which was vast: a
redshift of 2.9, which means its light had significantly
reddened during its journey through the cosmos. When
this star exploded, the universe was only 2.3 billion
years old. The photons from the blast took 11.4 billion
years to reach Earth. Today the physical location of the
burst is 21 billion light-years away—the explosion hap-
pened so long ago that the universe has expanded sig-
nificantly since then. This was the real deal.
A few months after finding our first afterglow, we
found a second. To put that in perspective, prior to theZwicky Transient Facility, only three afterglows had
ever been found without a gamma-ray burst first occur-
ring and telling astronomers where to look, and we
found two in just a few months. Now that we have our
search strategy ironed out and working, I hope we can
find these routinely. Still, even with two afterglows in
hand, I cannot definitively answer the questions I orig-
inally set out to answer. It is difficult to tell whether
any given afterglow is something new or just a normal
gamma-ray burst that high-energy satellites happened
to miss. We will need to find more events before we can
tell if we are witnessing truly different phenomena.EXPANDING THE CATALOG
since The discovery of an unexpected new type of en gine-^
driven explosion in AT2018cow, my search has uncov-
ered a variety of unusual stellar displays. There was the
weird Ic-BL supernova (the kind associated with GRBs)
crashing into a cocoon of material but showing no evi-
dence for a powerful jet (the hallmark of a GRB). Then
there was another event similar to AT2018cow. There
were also two Ic-BL supernova that probably had jets,
but these were less energetic and wider than those in
traditional gamma-ray bursts. And finally, right at the
end of graduate school, two actual cosmological after-
glows, one of which turned out to have an associated
gamma-ray burst.
So far we astronomers have been like zoologists,
going out into relatively uncharted territory and char-
acterizing all the different creatures (in this case, explo-
sions) that we see. The next stage will be to look for pat-
terns. What are the relative rates of each type of blast?
Do they seem to occur in one type of galaxy but not
another? Are these different categories actually differ-
ent “species” or just different manifestations of the
same phenomenon?
To answer these questions, we will need a much larg-
er catalog. Beginning in a few years, the Vera C. Rubin
Observatory, currently under construction in Chile, will
use the largest digital camera ever constructed (three
billion pixels) to spot 10 million potential transients
every night—10 times more than the Zwicky Transient
Facility does now. With more data, I would like to inves-
tigate which stars lose some of their mass right before
they die and how often. I want to study how we can tell
if there was a jet that got choked inside a star and how
to recognize the kind of faint emission emitted during
a star’s death throes to predict where and when a star
will explode. Ultimately I would like to probe questions
about the factors that lead to these unusual deaths—
perhaps it is something about a star’s rate of spin or its
history of interactions with other stars that causes it to
die in such a spectacular and rare way.FROM OUR ARCHIVES
Stellar Fireworks. Daniel Kasen; June 2016.
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