Pulse and Polarization Structures in Axion-Converted X-rays from Pulsars

TL;DR

The paper investigates axion production in neutron star cores and subsequent conversion to X-rays in the magnetosphere, predicting distinctive phase-dependent intensity and polarization signatures. It introduces analytic pulsation-polarization formalisms applicable to a wide range of NS environments and applies the framework to RX J1856.6-3754, examining whether an axion-induced hard X-ray excess can be reconciled with existing data. A joint spectral-pulsation analysis suggests current observations are compatible with axion scenarios but remain in tension with some bounds, and a substantial improvement in pulsation data could yield a >3σ test of the hypothesis. The study underscores the value of phase-resolved polarization measurements from IXPE and future polarimeters as a powerful probe of axions in NSs and outlines broader applicability to XDINS and magnetars.

Abstract

Neutron stars (NS's) with their strong magnetic fields and hot dense cores could be powerful probes of axions, a classic benchmark of feebly-coupled new particles, through abundant production of axions with the axion-nucleon coupling and subsequent conversion into X-rays due to the axion-photon coupling. In this article, we point out that the pulsation structures in both the intensity and polarization of X-rays from NS's could provide us additional information about axions and their couplings. We develop new analytical formalisms of pulsation-polarization structure applicable to a wide range of NS's in the axion scenario and argue that they hold in complicated astrophysical environments. As a case study, we apply our formalism to a representative X-ray Dim Isolated Neutron Star, RX J1856.6-3754, with an unexpected hard X-ray excess which might be axion-induced. We show with an updated fit that the axion explanation is compatible with both the intensity and pulsation data available, and combining the pulsation data does not shift the posterior by more than $1\,σ$. Yet, the preferred parameter space is close to being excluded by other astrophysical constraints. With a 75% reduction of the uncertainties in the pulsation data, we could potentially draw a definite conclusion on the axion-induced X-rays at more than $3\,σ$ level.

Figures (8)

• Figure 1: Probability of axions converting to photons ($P_{a\to \gamma}$) as a function of $r/r_0$. It approaches half of its asymptotic value at $\sim 85\,r_0$, distant from the surface, for the benchmark shown.
• Figure 2: Schematic of pulsar field configuration and definition of the $\chi$, $\xi$ and $\phi$ angles.
• Figure 3: The phase-averaged attenuation factor of the photon flux from axion conversions compared to $I_{\rm max}$, color-coded. Both the magnetic dipole approximation and the central LOS approximations are adopted. The white contours show the pulse fraction.
• Figure 4: Polarization degree for phase-averaged measurements. The white-dashed curve corresponds to the Stokes parameter $Q=0$, above (below) which we have $Q<0$ ($Q>0$), corresponding to the phase-averaged photons being orthogonal (parallel) to the spin axis, respectively. The color code corresponds to contours of constant polarization degree.
• Figure 5: Posterior distributions of HBB temperature $T_{\rm HBB}^\infty$, $T_c^{\infty}$, and $g_{a\gamma} g_{aN}$. We absorb the uncertainty due to J1856's mass profile into a dimensionless parameter $\kappa$ and present its impact as a vertical grey band of existing bounds on $\kappa g_{a\gamma} g_{aN}$. More details could be found in App. \ref{['app:estim-uncert-fit']}. Different contours correspond to the fit with spectrum data only (blue), combined spectrum and pulse data (orange), and combined data with reduced systematic uncertainties in the pulse data (green). The constraint on $g_{a\gamma}g_{aN}$ (vertical-dashed) is from the CAST CAST:2024eil and SN 1987A 10.1093/ptep/ptaa104 with $\kappa=1$. For $m_a \gtrsim 10^{-5}\,\mathrm{eV}$, the stronger polar cap constraints Noordhuis:2022ljwCaputo:2023cpv do not directly apply. Gaussian smoothing was applied to the contours with $0.5\,\sigma$ kernel size.