Sky & Telescope - April 2024 - 40

Neutron-Star Saga
COLLISION Two neutron stars begin to merge in this artist's concept, blasting jets of high-speed particles and producing a disk of debris. Astronomers
detect the former as a short gamma-ray burst; the latter glows as a kilonova.
out into space. Blobs of material in the jet, traveling at ultrarelativistic
speeds, interact and produce a burst of gammarays.
The jets then continue propagating away from the
explosion, eventually colliding with surrounding interstellar
gas. This collision creates shocks that accelerate electrons,
which then cool rapidly, producing a type of emission called
synchrotron radiation.
A perceptive reader may note here the similarity in the
different scenarios encountered in this story: Most involve
material moving at a whisper under the speed of light, which
violently collides with other material, creating shocks and, in
turn, light. Such scenarios are thought to produce all types of
GRBs and explain how magnetar central engines help blow
stars apart, how bow shocks are created around pulsar wind
nebulae, and perhaps, too, how FRBs are generated.
The FRB scenario presented here is just one way astronomers
think these radio bursts could be produced. Presently,
there is no consensus on how they actually form (S&T: Sept.
2022, p. 26). Another complication is that some FRBs occur
only once per source, while others repeat.
Some theories suggest an FRB is emitted toward the very
end of a neutron star's life in a binary system: In the decades
to centuries prior to coalescence, complex interactions
between the stars' magnetic fields might lead to the emission
of coherent radio waves. A similar scenario includes an FRB
being emitted when a lone, rapidly spinning neutron star collapses
into a black hole.
On the other hand, other models suggest an FRB is emitted
soon after a neutron star has been created, powered by
40 APRIL 2024 * SKY & TELESCOPE
a plasma of electrons and positrons escaping from the star's
extremely strong magnetic field. Interestingly, astronomers
recently caught the galactic magnetar SGR 1935+2154
emitting luminous, millisecond-duration bursts, providing
at least one observational connection between an FRB and
a magnetar (S&T: Sept. 2020, p. 10). The current state of
the research field of FRBs is not dissimilar to that of GRBs
30 years ago: There are, theoretically, many ways to produce
an FRB, but not enough observations of well-localized events
that allow astronomers to determine which of the scenarios
is valid. For now, we await more FRB detections for this
puzzle to be solved.
In closing, each of the evolutionary stages our fiducial
MM experienced has been observationally verified - in many
cases, by literally thousands of events (i.e., GRBs, pulsars, and
supernovae). Neutron stars abound in the universe, being
key components of supernovae, GRBs, and systems like X-ray
binaries. They ripple the very fabric of space and time with
bursts of gravitational waves, and they influence the chemical
makeup of later generations of stars and planets.
With present and upcoming ground-based surveys likely to
detect many new and exotic stellar systems in our galaxy and
others, the list of astrophysical events that contain at least
one neutron star will undoubtedly grow larger. A decade from
now, a sequel to our MM's saga might contain even more
astonishing escapades than what it encountered here.
¢ Astronomer and writer ZACH CANO currently meditates on
the mysteries of the universe in rural Hungary.
A. SIMONNET (SONOMA STATE UNIVERSITY) AND NASA'S
GODDARD SPACE FLIGHT CENTER

Sky & Telescope - April 2024

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