Astronomy - USA (2020-01)

WWW.ASTRONOMY.COM 29

2

FRB science

starts really

popping off

Since fast radio bursts (FRBs)
f lashed onto the scene in 2007, these
powerful, milliseconds-long blips of radio
energy have intrigued astronomers. Most
FRBs appear and disappear only once, and
astronomers don’t know what they are
— only that they originate outside our gal-
axy. In 2012, astronomers discovered the
first repeating burst, FRB 121102, and in
2017, they tracked the repeater to a dwarf
galaxy 2.5 billion light-years away. In 2019,
astronomers again made FRB history with
the discovery of nine additional repeating
FRBs, as well as, for the first time, track-
ing two non-repeating FRBs to their hosts.
Astronomers announced in work pub-
lished in Nature January 9, 2019, that the
Canadian Hydrogen Intensity Mapping
Experiment (CHIME) had spotted the
second-ever repeater, cataloged as
FRB 180814.J0422+73. In August 2019,
CHIME team members announced the
discovery of eight more repeating FRBs,
boosting in a single year the number of
known repeaters from one to 10.
Current theories suggest that FRBs
come from compact objects with high
magnetic fields, such as neutron stars,
black holes, or mergers between white
dwarfs. But whether the cause is one, all,
or a combination
of those options
remains unclear.
More repeaters
lend credence to
the idea that not
all FRBs come from the same types of
sources. “There are a lot of theories of
what FRBs might be, and some of them
predict that they are cataclysmic — they
are only one-off events, some kind of
explosion that’s created the burst. But
now if you have to explain these repeat-
ing bursts, then you need to come up
with another theory that can generate
bursts again and again,” says CHIME
team member Cherry Ng of the Dunlap
Institute for Astronomy and Astrophysics
at the University of Toronto.
But repeating FRBs were not all 2019
had in store. On June 27, astronomers
announced they’d tracked, for the first

time, a non-repeating FRB to its home

galaxy. The burst, FRB 180924, was

spotted September 24, 2018, with the

Commonwealth Scientific and

Industrial Research Organisation’s

Australian Square Kilometre Array

Pathfinder radio telescope. Using tech-

nology to isolate the moment the array

had received the burst, the team evalu-

ated the billionths-of-a-second differ-

ences in the time it took each dish in the

array to detect the f lash. The technique,

called interferometry, provided a precise

location on the sky where the burst had

come from: the outskirts of a massive

galaxy, DES J214425.25−405400.81,

about 3.6 billion light-years away.

Days later, on July 2, a team led by

Vikram Ravi at Caltech’s Cahill Center for

Astronomy and Astrophysics announced

they’d used the ten 4.5-meter dishes of the

Deep Synoptic Array-10 at Caltech’s

Owens Valley Radio Observatory to track

FRB 190523 to its home galaxy PSO

J207.0643+72.4708, about 7.9 billion light-

years away.

“By looking at galaxies, you can say a

lot about the stellar populations in the gal-

axy,” Ravi says. And learning about the

stars, gas, and dust in the galaxy is crucial,

he says, to determining the cause of FRBs

and whether their progenitors are strange

or commonplace. The host galaxies of the

non-repeating FRBs are remarkably simi-

lar. Both with about the same heft as the

Milky Way, they are older galaxies that

aren’t forming many stars. That’s in sharp

contrast to the home of the repeater FRB

121102, which is about a thousand times

less massive than the Milky Way and

forming stars at a rapid pace. FRB

180924’s location rules out its galaxy’s cen-

tral supermassive black hole — about

13,000 light-years away from the FRB —

as its source. And

both massive, qui-

escent host galax-

ies reduce the

chances that these

FRBs are related to

supernovae, which occur less frequently in

older galaxies.

One thing is for sure: The mystery of

FRBs is only deepening. “I think being

able to leave every option on the table,

however strange it might be, and investi-

gate every possible avenue, is really inter-

esting and allows you to think in more

creative ways,” Ravi says. Finding more

FRBs and tracing them to their homes will

be key to learning what causes them. Both

Ng and Ravi speculate that perhaps all

FRBs repeat over time. “Repetition is also

a question of timeline. If you look longer,

they might pop up again,” Ng says.

Only time will tell.

The Australian Square Kilometre Array Pathfinder radio telescope tracked the non-repeating fast radio burst

FRB 180924 to the outskirts of a distant galaxy, imagined here in this artist’s concept. CSIRO/DR. ANDREW HOWELLS

Finding more FRBs and tracing them to their homes

will be key to learning what causes them.