Radio signal of axion-photon conversion in neutron stars: A ray tracing analysis

Open Access

Radio signal of axion-photon conversion in neutron stars: A ray tracing analysis

Mikaël Leroy, Marco Chianese, Thomas D. P. Edwards, and Christoph Weniger

Phys. Rev. D 101, 123003 – Published 2 June 2020

Abstract

Axion dark matter can resonantly convert into photons in the magnetospheres of neutron stars (NSs). It has recently been shown that radio observations of nearby NSs can therefore provide a highly sensitive probe of the axion parameter space. Here we extend existing calculations by performing the first three-dimensional computation of the photon flux, taking into account the isotropic phase-space distribution of axions and the structure of the NS magnetosphere. In particular, we study the overall magnitude of the flux and its possible time variation. We find that overall signal strength is robust to our more realistic analysis. In addition, we find that the variance of the signal with respect to the NS rotation is washed out by the additional trajectories in our treatment. Nevertheless, we show that SKA observations toward J0806.4-4123 are sensitive to $g_{a γ γ} \sim 3 \times 10^{- 13} {GeV}^{- 1}$ at $m_{a} \sim 7 \times 10^{- 6} eV$ , even when accounting for Doppler broadening. Finally, we provide the necessary code to calculate the photon flux for any given NS system https://github.com/mikaelLEROY/AxionNS_RayTracing.

Received 7 January 2020
Accepted 13 May 2020

DOI:https://doi.org/10.1103/PhysRevD.101.123003

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP³.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Dark matter Radio, microwave, & sub-mm astronomy

Axions Neutron stars & pulsars

Gravitation, Cosmology & AstrophysicsParticles & Fields

Authors & Affiliations

Mikaël Leroy^1,2,*, Marco Chianese ^1,†, Thomas D. P. Edwards^1,3,‡, and Christoph Weniger^1,§

¹Gravitation Astroparticle Physics Amsterdam (GRAPPA), Institute for Theoretical Physics Amsterdam and Delta Institute for Theoretical Physics, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, Netherlands
²École Normale Supérieure Paris-Saclay, 61 Avenue du Président Wilson, 94230 Cachan, France
³The Oskar Klein Centre, Department of Physics, Stockholm University, AlbaNova, SE-10691 Stockholm, Sweden

^*mikael.leroy@ens-paris-saclay.fr
^†m.chianese@uva.nl
^‡t.d.p.edwards@uva.nl
^§c.weniger@uva.nl

Article Text

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Issue

Vol. 101, Iss. 12 — 15 June 2020

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Images

Figure 1
Diagram of the system. Here we show a diagram of the NS being observed from Earth with all the relevant quantities labeled for clarity.
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Figure 2
Radiated power from individual trajectories. Snapshot of the radiated power from individual trajectories identified by the pixel position $(x, y)$ of the ROI. We consider the benchmark scenario with $θ_{m} = 15 °$ , $θ = 58.31 °$ , $g_{a γ γ} = 1.0 \times 10^{- 12} {GeV}^{- 1}$ , and $m_{a} = 0.5 μ eV$ . The video of the evolution of the radiated power during a NS rotation period can be found at: https://youtu.be/VyA1-qbIqB4.
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Figure 3
Total radiated power during a NS rotation period. Total radiated power as a function of the instantaneous phase ( $ω t = 2 π t / P$ ) from the isolated NS J0806.4-4123, $B_{0} = 2.5 \times 10^{13} G$ and $P = 11.37 s$ . The light blue lines show the ray-tracing results of the present work with an isotropic phase-space distribution of axion particles. The black lines refer to the analytic computation based on radial trajectories [48]. The dashed, solid and dotted lines correspond to polar angles $θ = 36 °$ , 54° and 72°, respectively. The misalignment angle is fixed to $θ_{m} = 18 °$ for all lines. The plots from left to right display the radiated power for an axion masses of $0.5 μ eV$ , $7.0 μ eV$ , and $12.5 μ eV$ , respectively.
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Figure 4
Averaged radiated power. Total radiated power averaged over a NS period as a function of the polar viewing angle $θ$ . The three plots from left to right correspond to misalignment angles of 18°, 54°, and 90°, respectively. The colors of the lines refer to different values of the axion mass.
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Figure 5
Relative variance of the radiated power. Relative variance of the total radiated power with respect to a whole NS period as a function of the polar viewing angle $θ$ . The three plots from left to right correspond to misalignment angles of 18°, 54°, and 90°, respectively. The colors of the lines refer to different values of the axion mass.
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Figure 6
Projected sensitivity to the axion-photon coupling from radio observations. We consider the isolated NS J0806.4-412 and assume $τ_{obs} = 100 hrs$ . The two red lines correspond to the sensitivity limit for two line broadening scenarios as described in the text. The red solid line only accounts for the DM velocity distribution far from the NS where as the red dashed line also accounts for Doppler broadening from the rotation of the NS magnetosphere. The red band shows the minimum coupling required to detect the time variation of the signal (here we neglect Doppler broadening).
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