Axion-Like Particles, Magnetars,
and X-ray Astronomy
Jean-François Fortin and Kuver Sinha
Abstract If they exist, axion-like particles (ALPs) can be produced in the core of
magnetars. As they propagate, they then convert to photons in the magnetosphere,
leading to possible signatures in the X-ray photon spectrum. Moreover, since ALPs
only mix with the parallel mode of the photon, interesting modifications to the
photon polarization are also possible. We determine these effects by computing
the Stokes parameters in the presence of ALPs and show that the relevant quantity
factorizes into a product of the ALP-to-photon conversion probability and a factor
that depends on the initial mixture of ALPs and photons at the surface of the
magnetar.
Keywords Astroparticle · Axions · Magnetars · X-rays
1 Introduction
Although the Standard Model (SM) of particle physics explains a wealth of
phenomena, several questions remain unanswered. One of the outstanding problems
of the SM concerns dark matter (DM). DM—non-baryonic matter not included
in the SM—represents approximatively 85% of the total matter content of the
Universe. It was introduced to explain discrepancies between theoretical predictions
and direct observations of galaxy clusters and galactic rotation curves. The strong
CP problem—another issue of the SM—relates to the smallness of the neutron
electric dipole moment and can be explained by the introduction of axions [1, 2].
Axions, and more generically ALPs, could play the role of DM, thus simultaneously
solving two important questions left unanswered by the SM.
J.-F. Fortin ()
Université Laval, Québec, QC, Canada
e-mail: Jean-Francois.Fortin@phy.ulaval.ca
K. Sinha
University of Oklahoma, Norman, OK, USA
e-mail: Kuver.Sinha@ou.edu
© Springer Nature Switzerland AG 2021
M. B. Paranjape et al. (eds.), Quantum Theory and Symmetries, CRM Series in
Mathematical Physics, https://doi.org/10.1007/978-3-030-55777-5_53
569
and X-ray Astronomy
Jean-François Fortin and Kuver Sinha
Abstract If they exist, axion-like particles (ALPs) can be produced in the core of
magnetars. As they propagate, they then convert to photons in the magnetosphere,
leading to possible signatures in the X-ray photon spectrum. Moreover, since ALPs
only mix with the parallel mode of the photon, interesting modifications to the
photon polarization are also possible. We determine these effects by computing
the Stokes parameters in the presence of ALPs and show that the relevant quantity
factorizes into a product of the ALP-to-photon conversion probability and a factor
that depends on the initial mixture of ALPs and photons at the surface of the
magnetar.
Keywords Astroparticle · Axions · Magnetars · X-rays
1 Introduction
Although the Standard Model (SM) of particle physics explains a wealth of
phenomena, several questions remain unanswered. One of the outstanding problems
of the SM concerns dark matter (DM). DM—non-baryonic matter not included
in the SM—represents approximatively 85% of the total matter content of the
Universe. It was introduced to explain discrepancies between theoretical predictions
and direct observations of galaxy clusters and galactic rotation curves. The strong
CP problem—another issue of the SM—relates to the smallness of the neutron
electric dipole moment and can be explained by the introduction of axions [1, 2].
Axions, and more generically ALPs, could play the role of DM, thus simultaneously
solving two important questions left unanswered by the SM.
J.-F. Fortin ()
Université Laval, Québec, QC, Canada
e-mail: Jean-Francois.Fortin@phy.ulaval.ca
K. Sinha
University of Oklahoma, Norman, OK, USA
e-mail: Kuver.Sinha@ou.edu
© Springer Nature Switzerland AG 2021
M. B. Paranjape et al. (eds.), Quantum Theory and Symmetries, CRM Series in
Mathematical Physics, https://doi.org/10.1007/978-3-030-55777-5_53
569