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New constraints on axion with gamma-ray observations of the Crab Nebula

Kazunori Kohri, Haruki Takahashi

TL;DR

This work targets axion-photon mixing in gamma rays from the Crab Nebula by incorporating a realistic, variable magnetic-field profile into the emission and conversion modeling. By combining a two-component SSC Crab Nebula model with seed photons from synchrotron, dust, and the CMB, and by using transfer-matrix/density-matrix methods to compute the photon survival probability $P_{\gamma\gamma}$, the authors produce ALP-sensitive gamma-ray spectra. A Monte Carlo-based statistical analysis of Fermi-LAT and LHAASO data yields 95% CL exclusions, with the bound $g_{a\gamma\gamma} \lesssim 1 \times 10^{-11}\,\mathrm{GeV}^{-1}$ at $m_a \simeq 10^{-8}\,\mathrm{eV}$, surpassing CAST in the range $10^{-10}\,\mathrm{eV} \lesssim m_a \lesssim 10^{-6}\,\mathrm{eV}$. The results demonstrate the critical influence of astrophysical magnetic-field modeling on ALP constraints and offer a framework applicable to other galactic sources.

Abstract

In this paper, we derive the upper bounds on the coupling of axion-like particles (ALPs) with photon as a function of the mass by considering axion-photon conversion in the Crab Nebula. Previous studies have not considered the influence of the magnetic field within the Crab Nebula. The magnetic field plays a crucial role through the Synchrotron Self-Compton (SSC) process, in which high-energy electrons produce synchrotron radiation that is subsequently up-scattered by the same electrons via inverse Compton scattering to generate gamma rays. Therefore, neglecting the magnetic field in modeling leads to theoretical inconsistencies. In this work, we investigate the significance of the magnetic field effect and demonstrate that even differences in magnetic field modeling can substantially alter the conversion probability. We thus, for the first time, point out that proper consideration of the magnetic field is essential in ALP searches using gamma rays from the Crab Nebula. The resulting constraints reach up to a coupling of $g_{aγγ} \lesssim 1 \times 10^{-11} {\rm GeV}^{-1}$ for ALP masses in the range $10^{-10} {\rm eV} \lesssim m_a \lesssim 10^{-6} {\rm eV}$.

New constraints on axion with gamma-ray observations of the Crab Nebula

TL;DR

This work targets axion-photon mixing in gamma rays from the Crab Nebula by incorporating a realistic, variable magnetic-field profile into the emission and conversion modeling. By combining a two-component SSC Crab Nebula model with seed photons from synchrotron, dust, and the CMB, and by using transfer-matrix/density-matrix methods to compute the photon survival probability , the authors produce ALP-sensitive gamma-ray spectra. A Monte Carlo-based statistical analysis of Fermi-LAT and LHAASO data yields 95% CL exclusions, with the bound at , surpassing CAST in the range . The results demonstrate the critical influence of astrophysical magnetic-field modeling on ALP constraints and offer a framework applicable to other galactic sources.

Abstract

In this paper, we derive the upper bounds on the coupling of axion-like particles (ALPs) with photon as a function of the mass by considering axion-photon conversion in the Crab Nebula. Previous studies have not considered the influence of the magnetic field within the Crab Nebula. The magnetic field plays a crucial role through the Synchrotron Self-Compton (SSC) process, in which high-energy electrons produce synchrotron radiation that is subsequently up-scattered by the same electrons via inverse Compton scattering to generate gamma rays. Therefore, neglecting the magnetic field in modeling leads to theoretical inconsistencies. In this work, we investigate the significance of the magnetic field effect and demonstrate that even differences in magnetic field modeling can substantially alter the conversion probability. We thus, for the first time, point out that proper consideration of the magnetic field is essential in ALP searches using gamma rays from the Crab Nebula. The resulting constraints reach up to a coupling of for ALP masses in the range .

Paper Structure

This paper contains 8 sections, 30 equations, 4 figures, 1 table.

Table of Contents

  1. Introduction
  2. Axion-Photon Conversion
  3. Modeling of the Crab Nebula
  4. Electrons and magnetic fields in the Crab Nebula
  5. Emission mechanism of the Crab Nebula
  6. Axion-photon conversion in the Crab Nebula
  7. Analysis and Results
  8. Conclusion

Figures (4)

  • Figure 1: Spectral energy distribution without and with ALPs in some parameter sets.
  • Figure 2: Same as \ref{['fig:SED 1']} but in different parameter sets.
  • Figure 3: Same as \ref{['fig:SED 1']} but in different parameter sets.
  • Figure 4: Excluded region at 95$\%$ C.L. is shown as green based on the observations of gamma-rays from the Crab Nebula. Red region is excluded by CAST experiments CAST:2024eil. This figure is produced with AxionLimitsAxionLimits.