IMAGE: NASA/BALL AEROSPACE1310 10 DECEMBER 2021 • VOL 374 ISSUE 6573 science.org SCIENCET
his week, NASA will launch a small
mission with an ambitious task: to
peer into violent cosmic objects for
clues to how they work. By detecting
how intense magnetic fields in col-
lapsed stars and black holes align, or
polarize, the x-rays they emit, the Imaging
X-ray Polarimetry Explorer (IXPE) could re-
veal how those objects spew out radiation in
the first place. “We’re just trying to find out
how do you produce these x-rays anyway,”
says principal investigator Martin Weisskopf
of NASA’s Marshall Space Flight Center.
“The community has been waiting for this
for a long time,” says x-ray astronomer Feryal
Özel of the University of Arizona, who is not
part of the project. NASA’s Orbiting Solar
Observatory 8, launched in 1975, detected a
smattering of polarized x-rays from a single
source, the Crab nebula, Weisskopf says. Since
then, the questions such x-rays might answer
have piled up, but no mission has set out
to measure them—in part because the 1975
mission suggested there might be too few to
be worth the effort. Weisskopf says he pro-
posed dedicated x-ray polarimetry missions
several times before succeeding with IXPE, a
$190 million mission that is set to launch on
9 December from the Kennedy Space Center
in Florida on a SpaceX Falcon 9 rocket.
X-rays are emitted when gas is heated into
a 100-million-degree plasma, a roiling soup
of electrons and ions. Photons have mag-
netic and electric fields that oscillate perpen-
dicularly to their path, typically in randomdirections. However, magnetic conditions at
their birth or interactions on their journey
can polarize the photons by forcing those os-
cillations into the same plane.
IXPE will spend at least 2 years scruti-
nizing x-ray sources with three identical
telescopes—cheaper than one big one and
a hedge against failures. Each telescope is
a cylinder containing two dozen concentric
shells that focus x-rays through grazing re-
flections. (X-rays penetrate standard tele-
scope mirrors.) Sensors built by the Italian
Space Agency detect the x-rays and their po-
larization in a layer of dimethyl ether gas.
An x-ray hitting a gas atom knocks out an
electron that tends to shoot off in the direc-
tion of polarization, leaving a visible trail.
Imaging many trails and their spread tells
observers how polarized the light is, and in
which direction.
Pulsars are a prime target for IXPE.
These spinning, city-size remnants of dead
stars emit beams of radio waves, x-rays, and
other radiation that sweep past Earth like a
lighthouse beacon. Weisskopf says rival the-
ories of beam generation suggest the x-rays
originate in different locations on the pul-
sar: all across its surface, at its poles, or in
its atmosphere. Each theory predicts the x-
ray polarization signal should vary with the
timing of the pulses in a way IXPE should
be able to distinguish. “We will get a result,
as long as we launch,” he predicts.
Also in the mission’s sights are magnetars,
stellar remnants like pulsars, but with even
more powerful magnetic fields, 100 million
times stronger than any magnet made onEarth. The magnetic field lines force fast-
moving electrons into helical paths, causing
them to spray out polarized x-rays known
as synchrotron radiation. By measuring
how the polarization of the x-rays changes
as the magnetar spins, observers will be
able to map the geometry of the field across
the entire globe—and watch for tangles that
lead to eruptive outbursts. “With IXPE we
may really see the magnetic field direction,
the twisted magnetosphere,” says theorist
Matthew Baring of Rice University.
Polarization could also reveal how hungry
the supermassive black hole at the center of
our Milky Way has been in recent history. Gas
and dust swirling around an active, accreting
black hole emit x-rays as they are heated by
gravitational forces close to the event hori-
zon. The x-rays now arriving directly from
the Milky Way’s black hole are dim, suggest-
ing it is quiescent. But x-rays emitted longer
ago could also be arriving, having followed
dogleg paths after being scattered by distant
gas clouds. That scattering should imprint a
polarization on the x-rays, a sign that they
came from the galactic center and not the
cloud itself. “If we look at clouds and see po-
larization, that would be a smoking gun,” says
astrophysicist Philip Kaaret of the University
of Iowa, who worked on a rival polarimeter
proposal that lost out to IXPE. Their bright-
ness might show whether the black hole was
more active tens of thousands of years ago.
IXPE’s biggest coup might be in helping
understand the mechanics of powerful jets
launched by supermassive black holes in dis-
tant galaxies. These jets blast material up to
10 million light-years out into space—that’s
100 times the diameter of the Milky Way. Re-
searchers believe the spin of the black hole
funnels material and field lines from churn-
ing charged particles near the accretion disk
into the jets, which spew from both poles.
Other telescopes have seen x-rays emit-
ted close to the base of the jet, and re-
searchers believe synchrotron radiation is
responsible. But what powers the electrons
to near–light-speeds as they spiral around
the field lines? One possibility is shock
waves created by fast-moving plasma col-
liding with slower moving material. An-
other is magnetic reconnection, when
stressed field lines snap and reconnect.
Both processes release energy that can ac-
celerate electrons.
Polarization measurements by IXPE could
offer insight into how big and chaotic this
emission region is and which process accel-
erates the electrons. Both might be at work,
Baring says. “That makes nature richer,” he
says, “and keeps theorists busy.” jIXPE’s three telescopes sit 4 meters from their
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