Sky & Telescope - September 2022 - 29

suggests that they're made in a way we've never seen before.
The only other sources we know of that emit radio bursts
of similar duration are pulsars. But we've only found radio
pulsars in the Milky Way and nearby Large and Small Magellanic
Clouds; we can't see them beyond our galactic backyard.
Conversely, FRBs originate in faraway galaxies, at distances
sometimes a million times greater than pulsars, and their
emission must be a trillion times more energetic. This has
sparked a flurry of theoretical speculation about what's causing
these bursts.
According to theorists, some FRBs could arise in catastrophic
scenarios, such as the collapse of a heavyweight
neutron star or the violent and rare collisions of compact
objects, such as neutron stars and white dwarfs. These cases
would only produce one-off events, though. Repeatable bursts
could instead be magnificent flares from young, rapidly spinning
neutron stars, such as pulsars or magnetars (the latter
are neutron stars' highly magnetized cousins).
Alternatively, some astronomers have suggested that both
types of FRBs might arise from interactions within the hot
accreting gas surrounding the supermassive black hole at the
heart of a galaxy - an active galactic nucleus (AGN).
Astronomers still can't pinpoint the positions of most
FRBs, limiting our ability to identify their host galaxies. But
one advantage of the repeaters is that they enable astronomers
to point interferometers at the patch of sky from
whence these bursts emerge, in order to capture the source
in action. The source of the first repeating FRB 121102A
did not disappoint: For the first time, observers pinpointed
an FRB's location - in this case, a tiny star-forming dwarf
galaxy that lies some 3 billion light-years away. The location
overlapped with a compact persistent radio source in the
galaxy, suggesting that the burst's source is probably embedded
in a nebula.
Further to that, FRB 121102A's repeat bursts showed signs
of originating from within a highly magnetic environment.
This information, combined with the properties of the host
galaxy, led to a model for FRBs in which bursts originate
from young magnetars themselves produced in high-powered
supernovae, which fill the magnetar's surrounding environment
with gas and dust.
A few years later, however, astronomers found another
repeating FRB in a massive spiral galaxy that was associated
with neither a radio source nor a highly magnetic environment.
An eagle-eye view of its neighborhood revealed that
the burst was offset from the galaxy's star-forming region.
Star-forming regions are chaotic factories that produce a large
number of stars quickly; these stars will also die frequently,
resulting in many young magnetars. The offset of this FRB
from such a region meant its source is likely older than we'd
expect for a young magnetar. Thus, with only two sources, we
began to notice diversity in repeaters' host environments.
Observations from the world's largest radio telescope,
the Chinese Five-Hundred-Meter Aperture Spherical Radio
Telescope (FAST), recently revealed an FRB 121102A twin:
FINDING ONE-OFF BURSTS This image by the Very Large Telescope
in Chile shows the host galaxy of the non-repeating FRB 181112,
pinpointed thanks to ASKAP's data. The ellipse marks the FRB's approximate
location. Analysis revealed that the signal had passed through
the halo of a massive galaxy (top of image) en route to Earth, giving
astronomers unique information about the halo's gas.
FRB 190520B, also located in a dwarf galaxy and accompanied
by a persistent radio source. Furthermore, these two
FRBs repeat more frequently than others, implying that more
active repeaters may be young sources surrounded by dense,
magnetized plasma.
Another recently discovered repeater was pinpointed to a
globular cluster in the nearby spiral galaxy M81 (S&T: June
2022, p. 11). This was a jaw-dropping discovery! Globular
clusters consist of hundreds of thousands of tightly bound
and very old stars. The massive stars in such a cluster will
have exploded early in its history, and the remnant magnetars
they formed will have lost their magnetic energy after 10,000
years or so. If magnetars are indeed the source of repeating
FRBs, then how can one still exist in an old globular cluster?
This discovery has us scratching our heads, wondering if
there is another way to make a magnetar. The best guesses
include merging neutron stars or white dwarfs, or the collapse
of a white dwarf that siphoned too much material from
a companion star.
But what about FRBs that do not repeat? What does their
home environment look like? Catching one-off bursts well
enough to pinpoint their locations is indeed a difficult task,
but ASKAP has accomplished it. Thanks to a special mode,
ASKAP captures a burst just as it hits the telescope, saves the
rawest form of data from each of the 36 dishes, and replays it
to create an image of the FRB, thereby determining its precise
location on the sky.
When my colleagues and I zoomed in on the host galaxies
of one-off FRBs, we discovered that the sources prefer the
outer, quieter suburbs of the galaxy rather than the downtown
center. As a result, we knew right away that the progenitors
had nothing to do with the supermassive black holes
that reside at the center of these galaxies, hence ruling out
sk yand tele scope .o r g * SEPTEMBER 2022 29
ESO / X. PROCHASKA ET AL.
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Sky & Telescope - September 2022

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