Green Bank and Effelsberg Radio Telescope Searches for Axion Dark Matter Conversion in Neutron Star Magnetospheres (2004.00011v1)

Abstract: Axion dark matter (DM) may convert to radio-frequency electromagnetic radiation in the strong magnetic fields around neutron stars. The radio signature of such a process would be an ultra-narrow spectral peak at a frequency determined by the mass of the axion particle. We analyze data we collected from the Robert C. Byrd Green Bank Telescope in the L-band and the Effelsberg 100-m Telescope in the L-Band and S-band from a number of sources expected to produce bright signals of axion-photon conversion, including the Galactic Center of the Milky Way and the nearby isolated neutron stars RX J0720.4-3125 and RX J0806.4-4123. We find no evidence for axion DM and are able to set some of the strongest constraints to-date on the existence of axion DM in the highly-motivated mass range between ~5-11 $\mu$eV.

• The paper investigates axion dark matter conversion in neutron star magnetospheres, focusing on axion masses around 5–11 µeV.
• It employs high-resolution L-band and S-band observations with the Green Bank and Effelsberg Telescopes to detect potential monochromatic radio signals.
• The study sets competitive constraints on the axion-photon coupling constant, informing future searches with advanced instruments like the Square Kilometer Array.

Search for Axion Dark Matter Using Radio Telescopes

The paper under review investigates the conversion of axion dark matter into detectable radio-frequency photons within neutron star magnetospheres, employing the Green Bank and Effelsberg Radio Telescopes for observational data. Axions are a leading contender in dark matter theories, originating from the Peccei-Quinn mechanism proposed to resolve the neutron's strong CP problem. The authors focus on the potential conversion of axion dark matter into photons in the magnetic fields of neutron stars.

Overview

The researchers utilize the Robert C. Byrd Green Bank Telescope and the Effelsberg 100-m Telescope to search for axion-photon conversion signals from neutron stars, including isolated sources and dense regions such as the Galactic Center. These neutron stars create strong magnetic fields that could facilitate the conversion of axion dark matter into radio-wave emissions detectable by the telescopes. The expelled radio signal would, theoretically, manifest as a monochromatic spectral line at an axion mass-dependent frequency. This exploration covers axion masses ranging from approximately 4.5 to 10.5 $\mu$eV, corresponding to radio emissions between 1.1 and 2.7 GHz.

Key Findings

The analysis explores a highly-motivated axion mass range, between 5 and 11 $\mu$eV, using the data obtained from both telescopes' observations toward several sources, notably the Galactic Center and two isolated neutron stars. The search yielded no affirmative detection of axion dark matter, allowing the researchers to impose substantial constraints on the axion photon coupling constant, $g_{a\gamma\gamma}$, compared to previous terrestrial experiments.

Methodology

Data acquisition was conducted in various frequency bands, focusing primarily on the L-band, but also extending to the S-band for additional coverage. The observational methodology incorporates advanced radio telescopes equipped with high-resolution spectrometry. The data analysis involved testing the antenna temperature against expected excesses at particular frequency bins using Gaussian likelihood models, thus enabling the extraction of spectral lines relevant to axion signals. A rigorous veto mechanism ensures the exclusion of false positives based on spectral features present in off-source data.

Implications

The constraints derived from these observations are competitive with those derived from previous axion haloscope experiments such as UF and RBF, providing leading limits within the studied axion mass range. These results suggest that axion-photon conversion in neutron star environments is an avenue ripe for further exploration with potential for discovering axion dark matter. Enhanced flux sensitivity with future telescopes, such as the Square Kilometer Array, is likely required to reach additional unexplored parameter spaces within the QCD axion mass spectrum.

The research emphasizes the importance of understanding neutron star models and magnetospheric conditions alongside a thorough exploration of dark matter density profiles in the observed regions, which inherently affect the conversion efficiency and potential detectability of axion signals.

Conclusion

This paper showcases the promise of radio astronomical searches for axion dark matter via photon conversion processes induced by neutron stars' powerful magnetic fields. While no axion-related signals have yet been detected, the substantial constraints set forth in this work lend critical insight into axion mass ranges and coupling strengths, driving forward the field of astroparticle physics and the quest for a better understanding of dark matter's elusive nature. Future research should involve more comprehensive and higher-sensitivity radio observations, alongside better simulation models for neutron star magnetospheres, to potentially reveal axion dark matter and contribute to our understanding of cosmological phenomena.