Indirect detection of the QCD axion

TL;DR

This paper explores a novel radio observational method using the Green Bank Telescope to detect axion dark matter via transient signals from axion-photon conversion near neutron stars, setting new sensitivity limits in the 33-42 μeV mass range.

Contribution

It introduces an observational approach targeting axion-photon conversion signals from neutron stars and dense DM clumps, with detailed strategies and sensitivity analysis.

Findings

No candidate signals detected above 5σ threshold.

Achieved sensitivity of 2 mJy per spectral channel.

Set constraints on axion masses in the 33-42 μeV range.

Abstract

The QCD axion, originally proposed to solve the strong CP problem in QCD, is a prominent candidate for dark matter (DM). In the presence of strong magnetic fields, such as those around neutron stars, axions can theoretically convert into photons, producing detectable electromagnetic signals. This axion-photon coupling provides a unique experimental pathway to probe axions within a specific mass range. We investigate a novel observational approach using the Green Bank Telescope (GBT) to search for radio transients that could arise from interactions between neutron stars and dense DM clumps known as axion miniclusters. By observing the core of Andromeda with the VErsatile GBT Astronomical Spectrometer (VEGAS) and the X-band receiver (8 to 10 GHz), we achieve sensitivity to axions with masses in the range of (33 - 42)$\,\mu$eV, with a mass resolution of $3.8 \times 10^{-4}\,\mu$eV. We detail our observational and analytical strategies developed to capture transient signals from axion-photon conversion, achieving an instrumental sensitivity of $2\,$mJy per spectral channel. Despite our sensitivity threshold, no candidate signals exceeding the 5$\sigma$ level were identified. Future implementations will extend this search across additional spectral bands and refine the modeling used for the processes involved, strengthening the constraints on axion DM models.