Simulating the stellar bycatch: constraining the prevalence of extraterrestrial transmitters within radio SETI surveys

ABSTRACT Searches for radio technosignatures place constraints on the prevalence of extraterrestrial transmitters in our Galaxy and beyond. It is important to account for the complete stellar population captured within a radio telescope’s field of view, or stellar ‘bycatch’. In recent years, catalogues from ESA’s Gaia mission have enabled the search for extraterrestrial intelligence (SETI) surveys to place tighter limits on extraterrestrial transmitter statistics. However, Gaia remains restricted by magnitude limits, astrometric uncertainty at large distances, and confusion in crowded regions. To address these limitations, we investigate the use of the Besançon Galactic model to simulate the statistical underlying stellar population to derive more realistic constraints on the occurrence of extraterrestrial transmitters. We apply this method to Breakthrough Listen’s Enriquez/Price survey, modelling 6182 364 stellar objects within 1229 individual pointings and extending the search out to distances ${\le} 25$ kpc. We place limits on the prevalence of high-duty-cycle transmitters within 2.5 kpc, suggesting ${\le} (0.000995 \pm 0.000002) \, {\rm per \, cent}$ of stellar systems contain such a transmitter (for near-zero drift rates and ${\rm EIRP}_{\mathrm{min}} \gtrsim 5 \times 10^{16}$ W). In support of broader adoption, we provide a simple calculator tool that enables other researchers to incorporate this approach into their own SETI analyses. Our results enable a more complete statistical estimation of the number and stellar type of systems probed, thereby strengthening constraints on technosignature prevalence and guiding the analysis of future SETI efforts. We also conclude that SETI surveys are, in fact, much less biased by anthropocentric assumptions than is often suggested.