Axion Emission Can Explain a New Hard X-Ray Excess from Nearby Isolated Neutron Stars

Area of Science:

• * Astrophysics
• * Particle Physics

Background:

• * Neutron stars (NSs) are expected to emit primarily in UV and soft X-ray bands.
• * An unexplained excess of hard X-ray emission (2-8 keV) has been observed from the Magnificent Seven isolated NSs.
• * Conventional astrophysical models do not account for this observed hard X-ray excess.

Purpose of the Study:

• * To investigate if axionlike particles (ALPs) can explain the observed hard X-ray excess from isolated neutron stars.
• * To constrain the properties of ALPs, specifically their mass and coupling constants, based on X-ray observations.

Main Methods:

• * Theoretical modeling of axion production in neutron star cores.
• * Simulation of axion-to-photon conversion in stellar magnetic fields.
• * Comparison of model predictions with observational data of the Magnificent Seven neutron stars.

Main Results:

• * The hard X-ray excess from the Magnificent Seven can be consistently explained by ALPs.
• * The proposed mechanism involves thermal axion production in NS cores and subsequent conversion to X-rays.
• * Constraints are placed on ALP properties: mass (m_a ≲ 2×10⁻⁵ eV) and couplings (g_aγγ × g_ann ∈ (2×10⁻²¹, 10⁻¹⁸) GeV⁻¹).

Conclusions:

• * Axionlike particles provide a viable explanation for the hard X-ray emission observed from isolated neutron stars.
• * This finding opens new avenues for detecting axions and probing fundamental physics using astrophysical objects.
• * The study highlights the importance of considering exotic particle physics in understanding neutron star emissions.