30 ASTRONOMY • DECEMBER 2021
I began my Ph.D. in astronomy
in 2011, with an aim of specializ-
ing in transient radio astronomy.
(I wanted to be a radio astronomer
ever since I read Contact by Carl
Sagan as a teenager, so this was an
inevitable choice.)
When I heard about Swift
J1644+57, I was impressed by how
much information the radio signal
contained. When I finished my
Ph.D. and the opportunity arose
to become a postdoctoral fellow at
Harvard with the group that had
done the initial Swift J1644+57
analysis, I jumped at it — and got
to analyze the latest data on the
TDE. Sure, I was years after the
main fireworks and it was a ton
of work, but I’d nevertheless stop
every once in a while to marvel
at my luck.The great mysteries
When people hear I’m an astrono-
mer, they’ll often ask me what
the most interesting unanswered
question in the universe is. The
honest answer is that it’s fre-
quently the thing I am working on
at that moment — the more you
research a subject, the more you
come to appreciate its intricacies.
So here is the mystery about
TDEs in general, and Swift
J1644+57 in particular, that keeps
me wondering: While it is the
best-studied TDE on record, Swift
J1644+57 was not predicted and is
not at all like other TDEs we’ve
seen. It was a thousand times
more luminous in radio waves
than other TDEs and over10,000 times more energetic,
thanks to its relativistic jet. On
the other hand, in an “ordinary”
TDE, material tends to flow out
in all directions with energy levels
similar to what we see in a super-
nova explosion — still awesome
and impressive, of course, but a
bit like comparing a conventional
explosion to a nuclear bomb.
Coincidentally, Swift did spot
two more jetted TDEs in 2011, but
at much greater distances, so they
couldn’t be studied in detail.
Otherwise, all the TDEs we’ve
seen have fallen into the ordinarycategory. At this rate, it looks like
just 1 percent of all TDEs launch a
relativistic jet, and we still don’t
understand what makes those
TDEs so special. Perhaps the star
unbinds particularly fast, or the
magnetic fields around the black
hole are extraordinarily high —
but without more events, it’s hard
to know for sure.
Another mystery about TDEs
that keeps me wondering is that
while we know a lot of the mate-
rial falls into the black hole, lots of
it doesn’t. “We know supermassive
black holes are messy eaters,” my
collaborator Kate Alexander, an
astronomer at Northwestern
University, likes to joke. How the
leftover material interacts with thesurrounding environment is still
an open question.
If we are going to understand
TDEs and how these black holes
shape their environments, we need
to follow up on as many new
events — and in as many wave-
lengths — as possible. So far, less
than half of known TDEs were
detected with radio signals.
Alexander hopes to change
that, and oversees a large VLA
observing program (in which I
also participate) to follow up on
all new TDEs discovered in our
local universe. The program hasthe right to “trigger” the VLA
whenever a new TDE is discov-
ered by another facility at another
wavelength — that is, interrupt
the schedule for time-pressing
observations of our own to gather
more data on nearby TDEs.Fresh eyes
However, the VLA can’t do this
work alone. From its location in
New Mexico, it cannot see about a
third of the southern sky. In fact,
until the past few years, all but the
brightest TDEs were unobserv-
able in the southern skies because
there was no facility as sensitive as
the VLA with the right view.
Fortunately, this situation is
changing rapidly, thanks to newBELOW: Every
circled object in
this MeerKAT radio
image is a
supermassive
black hole newly
discovered by the
telescope, firing
relativistic jets into
space. The image
is a wide-field view
spanning 1.78
square degrees.
BELOW RIGHT:
Two bright radio
galaxies launch
relativistic jets in
this MeerKAT radio
image. The top
galaxy is roughly
240 million light-
years away; the
bottom galaxy is
about 130 million
light-years distant.
BOTH: VETTE CENDES/SARAO
The detail and wide field of MeerKAT
images make them unquestionably
the prettiest I’ve seen.