Sky & Telescope - January 2021 - 19

and searching nature's sources of magnetic fields in space for
telltale signs of the chameleon-like phenomenon.

Earthbound Investigations
A leading experiment offering hope for finding axion dark
matter on Earth is the Axion Dark Matter Experiment
(ADMX) at the University of Washington. Started in 1995
based on a detection scheme proposed by Sikivie, ADMX
hunts for axions that could be showering Earth every second
from the wider universe. The experiment aims to switch these
axions to microwave photons via a supercooled magnet whose
magnetic field is about 150,000 times stronger than Earth's.
If axions do exist and the instrument is tuned to precisely
the right wavelength, its cavity will resonate, amplifying the
signal so that ultra-sensitive quantum electronic detectors
can pick it up.
Another promising line of investigation for axion hunters is to search for axions from the Sun. These axions could
be produced within the Sun and other stars through highenergy X-ray photons switching identity to axions when they
interact with charged particles in the stellar plasma. Solar
axions would not explain dark matter - only primordial
axions produced in the early universe and now permeating
space could do that. But finding the solar version would prove
axions exist.
Most of these experiments are based on a design, again
first proposed by Sikivie, that uses specially built telescopes

ESO / L. CA LÇA DA

Many researchers have turned to scouring the vast natural laboratory of outer
space for hints of this tiny hypothetical
elementary particle.
called helioscopes. The CERN Axion Solar Telescope (CAST),
for example, is essentially an extremely good X-ray telescope
that points at the Sun, but whose lens cap never comes off.
It consists of two 10-meter-long tubes and a strong magnetic field generated by an upcycled LHC prototype magnet,
intended to reconvert solar axions back to photons. Each
magnetic pipe leads to an X-ray telescope and photon detector
that records faint flashes of X-ray light. If any photons appear,
they are likely solar axion chameleons.
CAST has been searching for axion signals by following
the Sun for 1.5 hours at both sunrise and sunset since 2003.
Though the team has found no signal, "some of the strongest limits on the properties of axions arise from CAST," says
Safdi. Promising even tighter limits and possibly even detections, CAST's proposed successor - the International Axion
Observatory (IAXO) - may be a game-changer, he adds.
Scientists are also intrigued by recent results from
another kind of experiment. XENON1T is a giant, sensorlined tank, buried beneath the mountains of central Italy.
Inside its relatively modest 3.2 tons of xenon, dark matter

PULSAR LAB Axions
encountering pulsars' powerful magnetic fields could
switch into photons, creating a signal that telescopes
might detect.

hunters captured an unexpected excess of particle interactions. WIMPs were immediately ruled out as the culprit,
leaving three possibilities: contamination inside the experiment, neutrinos exhibiting a new property, or, most excitingly, solar axions. Confirmation will have to wait until first
results from XENON1T's successor experiment, XENONnT,
which at the time this article went to press was scheduled to
go live late in 2020.

Searching Outer Space
Though new experimental ideas and upgrades to existing
ground-based searches promise to explore even more potential masses and interaction strengths, Earth-based hunts
may simply not be up to finding the elusive axion without
more information on its makeup. This is one reason many
researchers have turned to scouring the vast natural laboratory of outer space for hints of this tiny hypothetical elementary particle.
"I think the largest [stable and continuous] magnetic field
that we can create on Earth is about 45 teslas," explains Richard Battye (University of Manchester, UK), referring to the
35-ton device at the National High Magnetic Field Laboratory
in Florida. "But, for example, you can get 1011 T in a pulsar
- even though it's a long way away and you have less control
over it, it's a massive signal."
Pulsars are the highly energetic leftovers from massive
stars' explosive deaths: rapidly spinning, magnetized neutron
stars sending narrow beams of light in opposite directions
like cosmic lighthouses. Battye is interested in these pulsars'
magnetospheres, a region of space encircling the pulsar where
charged particles are trapped by the gargantuan electromagnetic field - the perfect place for axions to flip to photons.
sk yandtelescope.org * JANUARY 2 021

19


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Sky & Telescope - January 2021

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