Extending the Breakthrough Listen nearby star survey to other stellar objects in the field

We extend the source sample recently observed by the Breakthrough Listen (BL) Initiative by including additional stars (with parallaxes measured by Gaia) that also reside within the full width at half-maximum of the Green Bank 100-m Telescope and Parkes radio telescope target fields. These stars have estimated distances as listed in the extensions of the Gaia DR2 catalogue. Enlarging the sample from 1327 to 288 315 stellar objects permits us to achieve substantially better Continuous Waveform Transmitter Rate Figures of Merit (CWTFM) than any previous analysis, and allows us to place the tightest limits yet on the prevalence of nearby high-duty-cycle extraterrestrial transmitters. The results suggest ≲0.0660 $^{+0.0004}_{-0.0003}$ per cent of stellar systems within 50 pc host such transmitters (assuming an EIRP ≳ 10¹³ W) and ≲0.039 $^{+0.004}_{-0.008}$ per cent within 200 pc (assuming an EIRP ≳ 2.5 × 10¹⁴ W). We further extend our analysis to much greater distances, though we caution that the detection of narrow-band signals beyond a few hundred pc may be affected by interstellar scintillation. The extended sample also permits us to place new constraints on the prevalence of extraterrestrial transmitters by stellar type and spectral class. Our results suggest targeted analyses of Search for Extraterrestrial Intelligence radio data can benefit from taking into account the fact that in addition to the target at the field centre, many other cosmic objects reside within the primary beam response of a parabolic radio telescope. These include foreground and background Galactic stars, but also extragalactic systems. With distances measured by Gaia, these additional sources can be used to place improved limits on the prevalence of extraterrestrial transmitters, and extend the analysis to a wide range of cosmic objects.