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Identifying axion conversion in compact star magnetospheres with radio-wave polarization signatures

Z. H. Xue, K. J. Lee, X. D. Gao, and R. X. Xu
Phys. Rev. D 108, 083009 – Published 9 October 2023
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  • INTRODUCTION
  • METHODS
  • RESULTS
  • DISCUSSIONS AND CONCLUSIONS
  • ACKNOWLEDGMENTS
  • APPENDICES
    • References

    Abstract

    The axion is well motivated in physics. It solves the strong charge conjugation-parity reversal problem CP in fundamental physics and the dark matter problem in astronomy. Its interaction with the electromagnetic field has been expected but never detected experimentally. Such particles may convert to radio waves in the environment with a strong magnetic field. Inspired by the idea, various research groups have been working on theoretical modeling and radio data analysis to search for the signature of radio signals generated by the axion conversion in the magnetosphere of compact stars, where the surface magnetic field as strong as 1013–1014  G is expected. In this work, we calculate the observational properties of the axion-induced radio signals (AIRSs) in the neutron star magnetosphere, where both the total intensity and polarization properties of radio emission are derived. Based on the ray tracing method, assuming 100% linear polarization of radio waves generated in each conversion, we compute the polarization emission profile concerning different viewing angles. We note that plasma and general relativistic effects are important for the polarization properties of AIRSs. Our work suggests that AIRSs can be identified by the narrow bandwidth and distinct polarization features.

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    • Received 2 June 2023
    • Accepted 18 September 2023

    DOI:https://doi.org/10.1103/PhysRevD.108.083009

    © 2023 American Physical Society

    Physics Subject Headings (PhySH)

    1. Research Areas
    Particle dark matterRadio, microwave, & sub-mm astronomy
    1. Physical Systems
    AxionsNeutron stars & pulsars
    1. Properties
    Polarization
    Gravitation, Cosmology & Astrophysics

    Authors & Affiliations

    Z. H. Xue1,2, K. J. Lee1,2,3,*, X. D. Gao4,†, and R. X. Xu1,5

    • 1Department of Astronomy, Peking University, Beijing 100871, China
    • 2National Astronomical Observatories, Chinese Academy of Sciences, 20A Datun Road, Chaoyang District, Beijing 100101, China
    • 3Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, China
    • 4Faculty of Science, Beijing University of Technology, Beijing 100124, China
    • 5State Key Laboratory of Nuclear Physics and Technology, School of Physics, Peking University, Beijing 100871, People’s Republic of China

    • *kjlee@pku.edu.cn
    • †gaoxiangdong@bjut.edu.cn

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    Issue

    Vol. 108, Iss. 8 — 15 October 2023

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    Images

    • Figure 1
      Figure 1

      Radiated power in projection image plane: vacuum versus plasma. Snapshots of the radio photons at frequencies ω=ma produced by axion conversion in the magnetosphere as seen in the projection plane perpendicular to a viewing angle of θ=36°. The benchmark scenario is chosen with an inclination angle of χ=18°, gaγγ=10−12  GeV−1, and ma=0.5  μeV. The left column shows the results of straight line propagation through vacuum, reproducing the results of [23]. The right column results from including plasma effects, reproducing the results of [24]. From top to bottom, we show the results for pulse phases of 0.3, 0.4, and 0.5. All six plots share the same color bar, which is shown next to the right column and they are shown in log scale.

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    • Figure 2
      Figure 2

      Axion-induced radio emission as a function of pulse phases: vacuum versus plasma. A comparison of axion-induced emission pulse profiles and polarizations without and with plasma is shown in the left and right columns, respectively. Each column shows the position angle (P.A.) of polarization (upper), the degree of linear polarization [see Eq. (31)] (middle), and the total power (unit in watts) of axion-induced emission (lower). The viewing angle of this figure is 36°. Other parameters are set the same as the benchmark scenario used in Fig. 1.

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    • Figure 3
      Figure 3

      Axion-induced radio emission as a function of pulse phases: vacuum versus plasma. This figure is the same as Fig. 2, only with a different viewing angle of 54°.

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    • Figure 4
      Figure 4

      Axion-induced radio emission as a function of pulse phases: vacuum versus plasma. This figure is the same as Fig. 2, only with a different viewing angle of 72°.

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    • Figure 5
      Figure 5

      Convergence checks. Left: the comparison of axion-induced emission pulse profiles with different choices of number of pixels (N=7002,10002) and different maximum step sizes and error tolerance in integrating the photon trajectories [h=r/104,10−8;h=r/(3×104),10−9]. Right: histogram of relative error of pulse profile of each pulse phase. Note that the relative error of pulse profiles is different from the error tolerance of photon trajectory. The green bars are for the difference between the case of N=7002 and 10002, whereas the red bars are for the difference between photon trajectory tolerance of h=r/104,10−8 and h=r/(3×104),10−9.

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    • Figure 6
      Figure 6

      Robustness tests. A comparison of axion-induced emission considering a perturbation to the dipole magnetic field configurations where the exponent of r dependence is 3+δ.

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    • Figure 7
      Figure 7

      Robustness tests. A comparison of axion-induced emission considering a perturbation to the dipole magnetic field configuration where a quadrupole component is included to perturb the surface magnetic field by approximately 10%.

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