Extraterrestrial Axion Search with the Breakthrough Listen Galactic Center Survey

Axion dark matter (DM) may efficiently convert to photons in the magnetospheres of neutron stars (NSs), producing nearly monochromatic radio emission. This process is resonantly triggered when the plasma frequency induced by the underlying charge distribution approximately matches the axion mass. We search for evidence of this process using archival Green Bank Telescope data collected in a survey of the Galactic Center in the $C$ band by the Breakthrough Listen project. While Breakthrough Listen aims to find signatures of extraterrestrial life in the radio band, we show that their high-frequency resolution spectral data of the Galactic Center region is ideal for searching for axion-photon transitions generated by the population of NSs in the inner pc of the Galaxy. We use data-driven models to capture the distributions and properties of NSs in the inner Galaxy and compute the expected radio flux from each NS using state-of-the-art ray tracing simulations. We find no evidence for axion DM and set leading constraints on the axion-photon coupling, excluding values down to the level $g_{a\gamma \gamma }\sim {10}^{-11}{\mathrm{GeV}}^{-1}$ for DM axions for masses between 15 and $35\mathrm{\mu }\mathrm{eV}$.

Figure 1

Upper limit (95% confidence level) on the axion-photon coupling derived in this Letter using the BL radio observations of the GC. We illustrate the median limit over 100 MC realizations of the NS population (red line), and the corresponding 68% and 95% statistical uncertainties on the population model (see text for details). We compare our upper limit to those from the CAST [37], HAYSTAC [34, 35], and ADMX [31, 45] experiments, the prediction from ALP cogenesis [44], and radio observations of the GC magnetar [15, 16], which we recast using our modeling (see text).

Figure 2

Top: an illustration of the data and the fitted components of our model for a single coarse channel. Middle: the 95% CL upper limits on a flux density excess with bandwidth $\delta f=91.6\mathrm{kHz}$. Bottom: the corresponding significances of the flux density excesses and deficits.

Figure 3

A comparison of the derived flux density sensitivities (with and without the inclusion of the spurious signal nuisance parameter correction) compared with the radiometer equation expected sensitivity. For presentation, the limits have been smoothed with a median filter.