Simulating the Stellar Bycatch: Constraining the Prevalence of Extraterrestrial Transmitters within Radio SETI Surveys

TL;DR

This work addresses how to constrain extraterrestrial transmitter prevalence by accurately accounting for the full stellar population within radio SETI beams, not just Gaia-detected stars. It replaces Gaia-only bycatch estimates with Besançon Galactic Model simulations, applied to Breakthrough Listen's Enriquez/Price survey to produce a bycatch sample of $N_* = 6{,}182{,}364$ stars out to $d \le 25\,\mathrm{kpc}$. For $d \le 2.5\,\mathrm{kpc}$, they derive a stringent upper bound on high-duty-cycle transmitters of $f_{tx} \le (0.000995 \pm 0.000002)\%$ for $EIRP_{min} \gtrsim 5\times10^{16}$ W, illustrating the gain over Gaia-based approaches. A public web calculator enables other researchers to perform BGM-based bycatch and transmitter-prevalence analyses and extends the method to incoherent beamforming and interferometric SETI.

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 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 6,182,364 stellar objects within 1229 individual pointings and extending the search out to distances $\leq 25$kpc. We place limits on the prevalence of high duty cycle transmitters within 2.5kpc, suggesting $\leq (0.000995 \pm 0.000002)\%$ of stellar systems contain such a transmitter (for near-zero drift rates and 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.

Figures (7)

• Figure 1: The sample of stars within a single pointing (in the direction of l = 276.05 degrees, b = -14.4 degrees) simulated by the BGM (in black) compared to the sample of stars from Gaia with accurate parallax measurements (in red). We compare the distribution in absolute magnitudes (right) and distribution of distances (top). The BGM can be used to expand the stellar sample beyond the observational magnitude limitations of Gaia, as well as considering the full breadth of the Milky Way.
• Figure 2: For GBT L-band and S-band observations, we plot a comparison of the EIRP$_{\mathrm{min}}$ and transmitter rate determined from the sample of stars up to 25 kpc using the BGM and up to 10kpc from Gaiawlodarczyk-sroka_extending_2020, for shells of increasing EIRP$_{\mathrm{min}}$. We plot the dashed vertical lines as reference to a transmitter with EIRP$_{\mathrm{min}}$ equivalent to Arecibo ($10^{13}$ W), and a transmitter with equivalent power to a Kardashev Type I civilisation ($10^{17}$ W). This work to extend the stellar bycatch using the BGM challenges the current limits in survey sensitivity and scope, represented in a grey dashed line, referred to as "terra incognito" garrett_constraints_2023
• Figure 3: Hertzsprung–Russell diagram comparing the BGM-simulated stellar population (background) with the subset of Gaia sources (superimposed in red) for which reliable effective temperatures and absolute magnitudes are available. The Gaia sample is restricted to within 1 kpc due to parallax accuracy, and most white dwarfs lack reliable temperature estimates. Utilising the BGM enables consideration of the complete breadth of stellar types captured within the bycatch population, including bright Main Sequence stars and minimal confusion between faint Main Sequence and white dwarfs.
• Figure 4: The distribution of stellar spectral types as a function of galactic latitude for BGM simulations of the 1229 pointing positions. While the overall diversity peaks near the Galactic Plane as expected, the relative proportions of stellar types remain broadly stable across all latitudes.
• Figure 5: The distribution of stellar spectral types in relation to the Galactic Plane. (Upper) The number of stars per pointing per spectral type for $b < |5^\circ|$. (Lower) The number of stars per pointing per spectral type for $l < |5^\circ|$. Both figures highlight the opportunistic bycatch towards the Galactic Centre for SETI endeavours.