Sky & Telescope - January 2021 - 20

Dark Matter

In an ideal world, astronomers would only need look for
an anomalous spike in the spectrum of light emanating from
a pulsar to discover the axion's mass. But in reality, they need
to model the precise dynamics of the pulsar's evolution in
order to predict what the signal would look like - no mean
feat when the nearest pulsars are hundreds of light-years
from Earth. Battye is currently sifting through the last 12
years of the UK's Jodrell Bank Observatory archive of pulsar
measurement data to see if he can constrain axion mass this
way. In a parallel study, Safdi is searching nearby neutron
stars with instruments like the Green Bank Telescope in West
Virginia for similar signals.
The axions that would produce these signals would
originate from outside the pulsar or neutron star, and their
wavelengths would be found somewhere in the radio part of
the electromagnetic spectrum. But Safdi is also pursuing an
idea that the hearts of neutron stars themselves are highenergy axion factories. Like the solar axions CAST is hunting, when these axions exit the neutron star and enter the

surrounding magnetosphere, some would transform to X-ray
photons. Recently, Safdi and colleagues searched seven nearby
pulsars for these characteristic photons. In the reams of data
from XMM-Newton and Chandra space telescopes, they
found hints of the elusive particle. Ongoing efforts center on
confirming the discovery.

Black Hole Clues
Though observing pulsars and neutron stars for hints of
axions seems far removed from the aforementioned groundbased lab experiments, almost all of these studies rely on
magnetic fields spontaneously converting axions to normal
photons. Are there other ways we can observe these mysterious hypothetical particles? Masha Baryakhtar (New York
University) is one of a number of researchers exploring a
completely different idea: black hole superradiance.
First proposed by Soviet physicist Yakov Zel'dovich in 1971,
black hole superradiance is an effect in which a particle or
wave grazing a spinning black hole can extract energy and

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WHERE AXIONS MIGHT HIDE
Scientists search a wide range of
masses and interaction strengths
when looking for axions. Particles that
would match QCD axion predictions
would fall in the blue strip. But solar
axions or axion-like particles could
have different properties. Regions
ruled out are in gray; planned experiments cover the possibilities in yellow.

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10-22

10-24
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10-12

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Axion mass (electron volts)

20

JANUARY 2 021 * SK Y & TELESCOPE

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Excluded by black
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Strength of interaction with normal matter (GeV-1)

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Sky & Telescope - January 2021

Table of Contents for the Digital Edition of Sky & Telescope - January 2021

Contents
Sky & Telescope - January 2021 - Cover1
Sky & Telescope - January 2021 - Cover2
Sky & Telescope - January 2021 - 1
Sky & Telescope - January 2021 - Contents
Sky & Telescope - January 2021 - 3
Sky & Telescope - January 2021 - 4
Sky & Telescope - January 2021 - 5
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Sky & Telescope - January 2021 - Cover3
Sky & Telescope - January 2021 - Cover4
Sky & Telescope - January 2021 - SA1
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Sky & Telescope - January 2021 - SA8
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