Sky & Telescope - February 2023 - 9

FRB: JINGCHUAN YU; CUBESAT VIEW OF DART IMPACT: ASI / NASA / APL
STARS
New Observations Add Fuel
to Fast Radio Burst Debate
UNTIL RECENTLY, there were dozens of
ideas for what powers fast radio bursts
(FRBs), ultra-bright flashes of radio
waves visible from billions of light-years
away. But when astronomers found
multiple FRB-like bursts coming from
a nearby highly magnetic neutron star,
or magnetar, the question of FRB origins
seemed to be largely settled (S&T: Sept.
2020, p. 10).
However, studies published September
21st in Nature and Nature Communications
on FRB 20201124A disagree on
its origin.
Over several months in 2020, Heng
Xu (Peking University, China) and
colleagues detected 1,863 bursts from
the source using the Five-hundredmeter
Aperture Spherical radio Telescope
(FAST) in China. The FAST data
included polarization, which, to the
team's surprise, revealed that the magnetic
field and/or the plasma density
around the source sometimes changed
SOLAR SYSTEM
Asteroid Deflection Worked Even Better Than Expected
The mission confirmed the measureTHE
DOUBLE ASTEROID REDIRECT
TEST'S (DART's) attempt to alter the
path of an asteroid moon succeeded
beyond expectations. Before the DART
spacecraft slammed head-on into
Dimorphos, the moon of asteroid 65803
Didymos, the moon orbited its primary
in about 11 hours, 55 minutes. Postimpact,
it now takes only 11 hours, 23
minutes (plus or minus 2 minutes) to
complete its orbit. The large change
- 32 minutes - was within the range
predicted before the impact but at the
top end of what was possible.
The plumes of ejecta streaming out
from the head-on impact likely contributed
to the moon's recoil. For every
action, there is an equal and opposite
reaction: By launching material toward
Dimorphos' direction of motion,
Dimorphos' momentum changed just as
much in the opposite direction.
ment using visible light and radar. With
both ground- and space-based optical
telescopes, the team recorded light
curves of the Didymos system as the
two components alternately eclipsed
each other. The cadence of eclipses
changed as the orbital period of Dimorphos
shortened.
In addition, radio telescopes like
those at Goldstone in southern California
and Green Bank Observatory in
West Virginia broadcast radar waves
at the Dimorphos system and watched
for the reflected signal. The new orbital
speed derived from the radar data
closely matched the ones measured
from visible-light observations.
Many questions remain: How much
momentum did the impact transfer?
How much mass was ejected? How big
were the particles? Was the impact
dramatically over only a few days.
Team member Fayin Wang (Nanjing
University, China) led a team in proposing
that there is a magnetar at the heart
of this FRB. But this magnetar has a
companion: an extremely luminous
blue Be star surrounded by a gas disk.
As the two stars dance around each
other, the magnetar's radio bursts pass
through different parts of the Be star's
disk to reach the observer, resulting in
apparently quick changes.
" I think this is an excellent model
that neatly explains the observed
properties of the burst, " says Adam
Lanman (McGill University, Canada),
who wasn't involved in the study. " But
it's certainly not the end of the story! "
For one, the repeating flashes from FRB
20201124A aren't as regular as would be
expected from a binary system.
Another major wrench in the Be/
magnetar scenario is where the FRB
resides: The Nature study presents
observations with the twin 10-meter
Keck telescopes on Mauna Kea in
Hawai'i, showing that the FRB source
lies between the arms of a barred spiral
pA stylized depiction shows a fast radio
burst's circularly polarized emissions traveling
toward the FAST and Keck telescopes. Keck's
observations pinpointed the source's location:
in between the arms of a barred spiral galaxy.
galaxy. " That position is not particularly
attractive for young stars, " says
team member Bing Zhang (University
of Nevada). And if there are few young
stars there, then it's unlikely the source
comes from a fledgling magnetar.
Zhang says he for one is back to the
drawing board, at least for this particular
source. " We need to really rethink
everything, " he says. " Start over. "
¢BENJAMIN SKUSE
pAfter the impact, plumes of ejecta streamed
away from Dimorphos. Each rectangle here
represents a different level of contrast in order
to resolve fine structure in the plumes.
large enough to introduce a wobble
into Dimorphos' spin pole? Scientists
are still analyzing the dramatic ejecta
plumes and long, comet-like dust tail
that the impact caused. It will take years
to reap the full harvest of science!
¢EMILY LAKDAWALLA
See more post-impact images at https://
is.gd/DARTimpact.
sk yand tele scope .o r g * FEBRUARY 2023
9
http://www.is.gd/DARTimpact http://www.is.gd/DARTimpact http://skyandtelescope.org
Sky & Telescope - February 2023

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