Smoking gun signature from axion and the constraints with radio telescopes

TL;DR

The paper addresses the challenge of detecting QCD axions via axion-photon conversion in magnetar magnetospheres using radio telescopes, focusing on the 1–100 μeV mass range. It introduces a refined resonance calculation and a time-weighted, matched-filter search framework to overcome confusion limits, validated by end-to-end simulations and TMRT data. A Lorentzian spectral template with a periodic Gaussian temporal profile is developed, enabling a two-stage detection strategy that combines candidate identification with precise parameter inference, achieving forecasted sensitivities near the KSVZ/DFSZ couplings for 10 h of FAST or SKA observations. Applying the method to TMRT data yields no detection but provides stringent upper bounds on $g_{a\gamma\gamma}$ across S- and X-band ranges, demonstrating both feasibility and a clear path toward next-generation axion constraints with upcoming radio facilities.

Abstract

Axions are an elegant solution to the strong CP problem for particle physics and a promising dark matter candidate. They can convert into photons under a strong magnetic field, while magnetars with extreme magnetic fields are natural labs for axion detection. Radio telescopes can detect the radio emission from axion-photon conversion near magnetars. In this study, we have refined the calculation of axion-photon conversion and developed the matched filtering integration method to largely improve the signal-to-noise ratio. We validate our method using end-to-end simulation and real observational data from TMRT. A new constraint is set with only 687 seconds of observations with TMRT. Using 10 hours of observation with the high-frequency receiver in FAST or SKA, we can reach the theoretical coupling constant prediction for the axion mass range from 1$μ$eV to 100$μ$eV. We validate the possibility of axion detection with radio telescopes and avoid spectrum confusion.

Figures (3)

• Figure 1: Schematic diagram of axion-photon resonance conversion in magnetars' magnetosphere. The yellow-shaded zone in the schematic represents the axion-photon resonant conversion region, where magnified views reveal characteristic fringes from the axion-photon oscillations.
• Figure 2: The upper panel shows the simulated signal in the beam of the radio telescope, with the simulated overall signals in the upper left corner, the simulated periodic axion signal in the upper right corner, the simulated confusion signal in the lower left corner and the simulated telescope standing wave in the lower right corner. The lower panel is the comparison between the simulated axion signal extracted by time-weighted integration and the total signal. This method can effectively extract the known periodic signal from the interference sources. The bottom panel shows the weight template used.
• Figure 3: Constraints on axion-photon coupling $g_{a\gamma}$ from various observation approaches, including both forecasts and actual observational constraints from this work. The dashed line is the forecast constraints. The green dashed lines show the upper bound of the axion-photon coupling for axion masses of 10 $\mu$eV and 100 $\mu$eV, calculated using FAST's parameters as an example. The blue dashed line shows the upper bound calculated for SKA-mid's band 5b, using J0901-4046 as the observation source. The solid black region represents the coupling upper limits derived from the observational data search of J1809-1943 using TMRT, with the constraints provided by the S-band and X-band, respectively.