to study gamma-ray bursts —
immensely energetic bursts of
radiation that occur during a
supernova or a neutron star merg-
er. But unlike those bursts, which
last no more than a few minutes,
this radiation just kept going. “It
was unlike anything we had seen
before,” recalls Joshua Bloom, an
astronomer at the University of
California, Berkeley, who was a
lead investigator on the signal.Soon, telescopes spanning the
entire electromagnetic spectrum
were pointed at Swift J1644+57,
as the event came to be known.
Different physical processes emit
different kinds of radiation, and
often the key to unraveling an
astronomical mystery lies in
observing as many wavelengths
as possible.
An incredible picture emerged:
A TDE had occurred around apreviously dormant black hole
and, as part of this process, the
TDE had somehow launched a jet
of material traveling at near light-
speed, so fast that the laws of rela-
tivity must be considered. This
relativistic jet was aimed directly
at Earth — and astronomers were
staring directly into the beam. “It
was one of my only times as an
astronomer where I had an ‘ah-ha!’
moment, and all the little pieces
started coming together in my
head,” recalls Bloom. “We didn’t
have all the details yet, but obser-
vationally and theoretically it
seemed to click.”
In X-ray wavelengths, astrono-
mers noticed a consistent rising
and falling pattern in the TDE’s
brightness. They realized these
were f lare-ups from shredded
material falling into the black hole,
constantly fueling the jet. Another
key piece of information came
from radio wavelengths. By mea-
suring the signal intensity over
multiple frequencies, astronomers
could extract a range of informa-
tion. This included the energy in
the jet, as well as the radius of the
blast wave that created the radio
signals, and even the density ofRelativistic jets
shoot out from the
supermassive black
hole at the heart of
the galaxy
Centaurus A in this
photo-illustration,
plowing into
surrounding
material. ESA/HUBBLE, L.
CALÇADA (ESO)
1
Tidal disruption events begin with a star orbiting a supermassive black hole at
the heart of a galaxy. If it makes a close encounter with another star, it can be
thrown into a death plunge towards the black hole.
2
As the star nears the black hole, tidal forces begin to grow, distorting the star
— the process of spaghettification.Black holeStarStarStar distorted
by tides28 ASTRONOMY • DECEMBER 2021
THE DEATH OF A STAR