FRB 121102
host galaxyWWW.ASTRONOMY.COM 23the sky, and its diameter is 2 million times
bigger than the light waves it observes, and
its resolution is 0.3" or 1⁄12,000°. On the other
hand, a radio telescope with a 210-foot (64
meters) dish — the diameter of Parkes
Observatory, which has discovered the
majority of FRBs — will yield a resolution
of 900" (¼°), about half the diameter of the
Full Moon in the sky. That area may sound
small, but it’s enough to hold hundreds or
even thousands of distant sources. Because
of this, a definitive identification of an
FRB’s origin with a single-dish telescope
like Parkes or Arecibo is impossible.
Luckily, astronomers have a few tricks
to get around the resolution problem. The
first is to link multiple radio telescopes
together in a technique known as interfer-
ometry. By combining simultaneous
observations from multiple telescopes,
astronomers effectively create a radio tele-
scope with a diameter equal to the distance
between dishes.
Interferometry can be carried out using
telescopes thousands of miles apart, giving
resolutions down to 10-millionths of a
degree. You wouldn’t want to do a survey
for new objects with such a small field of
view on the sky — an FRB could be going
off less than a Moon-width away, and you’d
never know — but it is a wonderful tech-
nique for pinpointing a single signal like
the FRB repeater.
Using such an interferometer — the
Karl G. Jansky Very Large Array (VLA),
consisting of twenty-seven 28-foot (25 m)
dishes in New Mexico — astronomers in
fall 2016 detected several outbursts from
the repeating FRB 121102. This allowed
them to narrow down its area of origin and
make detailed radio images of the region
with other telescopes — as well as image it
in optical light using the Gemini North
Telescope in Hawaii — to identify the
source of the repeater.
What the team found was a surprise.
The repeater appears to originate from a
dwarf galaxy 3 billion light-years away,
which would appear completely uninterest-
ing if something weren’t repeatedlythrowing out insane amounts of radio
energy. The galaxy is about the size of the
Small Magellanic Cloud, a satellite galaxy of
the Milky Way with about 1 percent the
mass of our own. Astronomers used the
Hubble Space Telescope and the Spitzer
Space Telescope for further follow-up, and it
appears the bursts originate from a star-
forming region on the outskirts of its host
galaxy. No one knows the source more spe-
cifically than that, but the bursts keep com-
ing from that location — over 150 of them
at last count.The mystery remains
There’s a lot we don’t know about FRBs.
Most glaringly, we don’t know what causes
them. “There are more theories than
bursts,” observes Lorimer, who organized
the first FRB conference where several
dozen theories were put forth.
One popular theory for the repeater, at
least, suggests the bursts originate from a
magnetar, a neutron star dominated by an
extremely strong magnetic field. A magne-
tar’s field can be so powerful that even at
more than 600 miles (1,000 kilometers)
away, it alone would kill you by compress-
ing the electron clouds in your atoms. They
are also known to give off enormous bursts
of high-energy radiation. In 2004, a mag-
netar called SGR 1806–20 experienced a
starquake, or tiny shift in its crust, that
shook its magnetic field so violently that
the event would have registered as a 23 onThe Karl G. Jansky Very Large Array in New Mexico played a key role in identifying the host galaxy
of the only repeating FRB. Over six months, the VLA observed FRB 121102 for 83 hours and recorded
nine outbursts. By narrowing down the FRB’s area of origin, astronomers could chase down an optical
counterpart — the host galaxy. CGP GREY (WWW.CGPGREY.COM)
FRB 121102 originates
from a tiny dwarf
galaxy 3 billion
light-years distant
in the constellation
Auriga. This image,
taken in optical light
with the Gemini
North Telescope,
doesn’t reveal much about the host, but radio observations have led some to believe the FRB may be
associated with a supermassive black hole or a young neutron star. GEMINI OBSERVATORY/AURA/NRC/NSF/NRAOThe first FRB host