The search for extraterrestrial intelligence (SETI) aims to detect technologically capable life beyond Earth through technosignatures. Proxima Centauri, the closest star to the Sun, is an astrobiologically interesting target due to its proximity and the presence of Proxima b, an exoplanet within its habitable zone. The Breakthrough Listen (BL) project observed Proxima Centauri using the Parkes 'Murriyang' radio telescope from April 29 to May 4, 2019, searching for narrowband drifting signals. This observation yielded a signal of interest, blc1, exhibiting characteristics initially suggestive of a technosignature: a narrow bandwidth, non-zero drift rate consistent with a non-terrestrial transmitter, and apparent linearity in drift rate across 30-minute observation panels, with smooth changes over time. The signal's absence in off-source observations further strengthened the initial interest. However, given the ubiquity of human-generated radio interference (RFI), a thorough investigation was warranted to determine blc1's true origin.

The paper references existing literature on the NASA astrobiology roadmap, the Hipparcos catalogue for stellar data, the discovery of Proxima b, the Breakthrough Initiatives, the Parkes telescope's receiver, and previous Breakthrough Listen publications detailing their instrumentation and data analysis. It also cites work on revising the Rio scale for SETI detections, the search for extraterrestrial intelligence (SETI), and methods for verifying fast radio bursts and gravitational waves. The review underlines the existing methodologies and the challenges in distinguishing genuine technosignatures from RFI.

The investigation began with verifying the correct operation of the telescope and backend systems. Consultation with the observatory and the Australian Communications and Media Authority confirmed no catalogued RFI at the observed frequency. The non-standard observing cadence (30-minute on-source, 5-minute off-source observations) and the selection of quasars as off-source calibrators (due to the primary goal of stellar flare detection) were considered. The authors accounted for the asymmetric on/off-source observation times and the inherent higher noise floor of quasars. The large spatial separation of the off-source positions from Proxima Centauri and the telescope's sidelobe pattern were carefully considered when evaluating the signal's localization. To assess potential off-source signal attenuation, de-drifting and incoherent summation of off-source panels were performed. A frequency comb, a set of regularly spaced non-drifting signals, was identified in both on-source and off-source data. The analysis also involved considering various potential sources of the drifting signal: ground-based and aerial transmitters, satellites (LEO, MEO, GEO), deep-space probes, and reflections from asteroids. The authors assessed their drift characteristics against the observed blc1 drift rate, ruling them out as potential sources. The analysis continued with an examination of electronic drift rates. The observed drift was compared to the sidereal drift of Proxima Centauri and the orbital motions of planets in its system. A thorough search for similar signals at other days and frequencies was conducted using archival data from the Parkes telescope. The analysis included both visual inspection and automated search algorithms (turboSETI) to detect potential signals with similar morphology and drift rates, applying harmonic and intermodulation analyses to uncover patterns within the population of identified signals. The investigation culminated in the development of a technosignature verification framework.

The detailed analysis ultimately revealed that blc1 was not of extraterrestrial origin. Instead, it was identified as an intermodulation product of local, time-varying interferers, aligned with the observing cadence. The researchers found dozens of instances of radio interference with morphologies similar to blc1 at frequencies harmonically related to common clock oscillators. Specifically, the analysis uncovered evidence that blc1 was an intermodulation product generated by a ~2 MHz clock oscillator mixed with other zero-drift RFI. The blc1 signal's apparent localization on the sky resulted from its duty cycle or variability, which happened to match the observing cadence of Proxima Centauri exceptionally well. The study identified several other signals with similar characteristics to blc1. A significant number of 'lookalike' signals (36) and 'mirrored lookalikes' (27) were found, all traceable to RFI. The analysis of these signals, in conjunction with blc1, revealed frequency patterns consistent with intermodulation products arising from common clock oscillator frequencies used in digital electronics. These findings strongly indicate a terrestrial, rather than extraterrestrial, origin for blc1.

The findings directly address the initial hypothesis that blc1 could represent a technosignature. The thorough investigation, applying multiple analytical techniques and considering a wide array of potential sources, strongly refutes this hypothesis. The identification of blc1 as RFI underscores the challenges in distinguishing between genuine technosignatures and terrestrial interference, especially in the context of non-standard observation cadences and the complex interplay of RFI sources. The significant number of blc1 lookalikes, all ultimately attributed to RFI, highlights the importance of careful and comprehensive analysis. The study emphasizes the critical need for detailed follow-up procedures, including re-observations and multi-site confirmation, when assessing potential technosignatures. The creation of a technosignature verification framework based on the blc1 analysis is a valuable contribution to the field.

The paper concludes that blc1, initially identified as a potential technosignature, is ultimately attributed to RFI. The study significantly contributes to the SETI field by highlighting the complexities of RFI analysis and introducing a comprehensive technosignature verification framework. Future research should focus on improving RFI mitigation techniques, exploring multi-site observations for independent verification, and further developing real-time data analysis capabilities to enhance the efficiency of SETI searches.

One limitation is the non-standard observing cadence used in the initial Proxima Centauri observation, which contributed to the initial misidentification of blc1. The complex RFI environment around astronomical radio facilities, and the lack of full characterization at the high frequency resolution used, also presents a limitation. The study primarily focuses on the Parkes telescope data; multi-site observations would provide stronger confirmation of the findings.