All-sky Ultra-narrowband Spectral Imaging with the OVRO–LWA: Technosignature Constraints and Axion-like Particle Prospects

We present an imaging-domain search for technosignatures at decametric wavelengths with the Owens Valley Radio Observatory Long Wavelength Array, targeting ultra-narrowband continuous-wave signals between 50 and 86 MHz. We implement an offline graphics processing unit pipeline that processes raw voltage data with upchannelization to approximately 10 Hz frequency resolution, producing all-sky images for each fine channel and totaling more than 3 × 10⁶ images for a single 30 s epoch. Candidate selection is performed using multikernel matched filtering across frequencies, empirical noise standardization, and false-discovery-rate control. After applying quality cuts that remove extended sources, corrupted images, and obvious radio frequency interference, three narrowband candidates with signal-to-noise ratios above 10σ were selected for detailed analysis. By reimaging these candidates with finer temporal and spectral resolution, we resolved their structure and found them to be inconsistent with compact celestial narrowband emitters. Consequently, we report no detection of extraterrestrial technosignatures. The representative sensitivity of the search is ∼100 Jy per channel across the entire visible hemisphere. For an unresolved emitter, this corresponds to 10σ equivalent isotropic radiated power limits of about 10¹⁴ W at a distance of 10 pc and 10¹⁸ W at 1 kpc. The wide field of view and ultrafine spectral resolution of this approach enable simultaneous probing of technosignature signals from millions of stellar systems. This method further establishes a scalable framework for deeper integrations and stacked searches toward neutron star targets relevant to axion-like particle line conversion.