Extraterrestrial Axion Search with the Breakthrough Listen Galactic Center Survey

Astrophysical and Cosmological Probes of Dark Matter

“…The mixing of axions and photons is greatly enhanced in the magnetospheres of neutron stars, owing to the large ambient magnetic fields and the resonant Snowmass2021 Theory Frontier: Astrophysical and Cosmological Probes of Dark Matter Recent constraints derived on the axion-photon coupling using radio observations of the Galactic Center obtained with the Green Bank Telescope [261] (solid red, labeled "GBT"); shown for comparison is the 10σ discovery limit for SKA in the same frequency band (transparent red). These results are compared with the expected QCD axion parameter space (yellow), two benchmark QCD axion models (brown, dot-dashed), existing constraints from haloscopes (blue) [262][263][264][265][266] and CAST [267] (grey), and indirect searches using magnetic white dwarfs (MWDs) [243,260].…”

“…amplification possible due to the ambient plasma [227,261,[268][269][270][271][272][273][274]. The characteristic plasma mass near typical pulsars spans ∼ 0.1 − 100 µeV [275], which roughly corresponds to the range of masses for which axions can simultaneously solve the strong CP problem and constitute dark matter [276], and further to the frequency band of modern radio telescopes.…”

“…The radio signal is expected to appear as a forest of spectral lines centered about the axion mass, with each line arising from a single neutron star in the Galactic population [261,271]. If dark matter is predominantly in miniclusters rather than smoothly distributed, the events will instead appear as transients spanning hours to weeks [277].…”

“…Initial estimates indicate that near-future radio interferometers like the Square Kilometer Array may be capable of discovering the QCD axion [270,277]. Searches using existing infrastructure (including the Effelsberg 100-m telescope, the Green Bank Telescope, and the Very Large Array) are already underway and have leading limits on the axion-photon coupling in the mass range 1 m a 20 GHz [261,278,279] (see Fig. 2 for the most recent analysis).…”

“…Recently, there has been significant theoretical progress in our understanding of the radio signal, including a careful treatment of photon refraction, resonant cyclotron absorption, plasma-induced line broadening, anisotropic response of the medium in the photon production process, and general relativistic effects [261,273,274,280]. Yet many open questions remain, including how axions and photons mix in a highly magnetized inhomogeneous plasma, how do charge distributions in active pulsars and magnetars impact the radio flux, what are the properties and distributions of neutron stars in dense dark matter environments, do we expect strong deviations from dipolar magnetic fields (and if so how does this impact the radio signal), how are axions distributed on astrophysical scales (i.e., do they reside in Snowmass2021 Theory Frontier: Astrophysical and Cosmological Probes of Dark Matter tidally disrupted axion miniclusters, and if so what are the properties of these objects in the Galactic Center), and how can we exploit the spatio-temporal properties of the radio signal to improve analyses.…”