Breakthrough Listen Candidate 1 (BLC1)

• Breakthrough Listen Candidate 1 (BLC1) is a narrowband radio signal detected near Proxima Centauri, exhibiting a persistent drift rate and narrow spectral width.
• A comprehensive verification framework using statistical modeling and RFI analysis revealed its origin as an instrumental intermodulation artifact rather than an extraterrestrial signal.
• The case study underscores the need for multiwavelength follow-up, real-time observations, and rigorous protocol in SETI searches to reliably exclude terrestrial interference.

Breakthrough Listen Candidate 1 (BLC1) is a narrowband radio signal detected in 2019 from the direction of Proxima Centauri during a search for extraterrestrial technosignatures with the Parkes “Murriyang” 64-m radio telescope. The signal exhibited characteristics broadly consistent with hypothetical artificial emissions, including narrow spectral width, significant persistence, and a time-dependent drift rate. However, subsequent analyses—incorporating rigorous statistical modeling, waveform characterization, verification protocols, null optical counterparts, and astrophysical priors—have converged on an unambiguous terrestrial origin for BLC1, rooted in complex radio-frequency interference (RFI) and intermodulation products within instrumentation.

1. Detection Campaign and Signal Properties

The Breakthrough Listen (BL) campaign observed Proxima Centauri between April 29 and May 4, 2019, using the CSIRO Parkes Ultra-Wideband Low (UWL) receiver covering 0.704–4.032 GHz. The core observing cadence alternated 30-minute on-source pointings with 5-minute off-source calibrator offsets. Approximately 26 hours on-source were accumulated in Stokes I mode, channelized to 3.81 Hz frequency resolution and 16.78 s time integrations (Smith et al., 2021).

A narrowband feature—dubbed BLC1—was identified at ν₀ = 982.002571 MHz, persisting for ~2.5 hours across multiple on-source scans. The signal exhibited a positive frequency drift rate, measured at +0.038 Hz s⁻¹, consistent over all on-source panels. The average SNR was 17.96, and the signal remained unresolved at Δν < 3.81 Hz, the instrumental bin width. BLC1 survived coincidence filtering, appearing in all on-source pointings and absent in the contemporaneous off-source data above SNR thresholds (Smith et al., 2021, Sheikh et al., 2021).

Parameter	Value	Context
Central frequency	982.002571 MHz	All on-source panels
Drift rate	+0.038 Hz s⁻¹	Consistent on-source
SNR (average)	17.96	Dynamic spectrum
Duration	~2.5 h	Multi-scan event
Spectral width	<3.81 Hz	Instrument limited

Instrumental sensitivity enabled detection of transmitted isotropic power down to EIRP_min ≈ 1.9 GW from the distance of Proxima Centauri, representing an unprecedented depth for targeted search campaigns (Smith et al., 2021).

2. Verification Framework and Systematic RFI Exclusion

A ten-step technosignature verification protocol was deployed to scrutinize BLC1 and any comparably compelling candidate (Sheikh et al., 2021). Key elements included:

• Cross-verification of receiver/digital backend logs to exclude hardware anomalies.
• Explicit absence of the candidate in off-source pointings, even at lowered SNR cut thresholds.
• Systematic consultation of local and national RFI registries, including frequency allocation checks for licensed transmitters and satellites at 982.002571 MHz.
• Kinematic modeling of signal drift, compared against expected Doppler contributions from Earth rotation, orbital motion, and hypothetical transmitters at the source (e.g., on Proxima b).
• Deep archival search strategies for “lookalike” signals with matching frequency and drift properties.
• Statistical parametric assessment: kernel-density estimations of (ν, |·f|, SNR) space demonstrating that BLC1 lay within a cluster of local RFI signals.

Further, the occurrence rate of morphologically similar drifting narrowband lines was established by inspecting ∼7000 archival UWL observations. At least 15 distinct features near 982 MHz, with similar drift rates and time profiles, were documented. Intra-campaign analysis identified additional sub-threshold “blc1-like” events with confirmed off-source detection, directly implicating terrestrial or instrumental origins (Sheikh et al., 2021).

3. Instrumental Intermodulation and Electronic Artifact Origin

A comprehensive mathematical model of intermodulation products was central to attributing BLC1's origin. The non-linear mixing of independent clock oscillators (e.g., 2.000004 MHz, 15 MHz, 133.33 MHz, 80.1 Hz), sampled in the analog-to-digital conversion chain, produces a broad array of narrowband intermodulation artifacts. These manifest at frequencies

$$f_{\mathrm{IM}}(n, m) = |n f_1 \pm m f_2|,\quad n, m \in \mathbb{Z}$$

Empirically, BLC1's characteristics—frequency, drift rate and “butterfly” time-frequency morphology— mirrored a population of lookalike features attributed to intermixing of digital local oscillators. The dominant drift rate distribution for the RFI lookalike population was Gaussian (μ ≈ 0.030 Hz s⁻¹, σ ≈ 0.004 Hz s⁻¹), indistinguishable from BLC1’s measured drift (Sheikh et al., 2021). Observing-cadence modulation (duty cycle sync of the RFI brightness with on/off pointing) induced the appearance of sky-localized persistence, which was shown to be a selection effect rather than spatial localization.

4. Multiwavelength Follow-up and Optical Constraints

Contemporaneous and archival high-resolution optical spectroscopy (HARPS, ESO 3.6m) was conducted to search for laser technosignatures from Proxima Centauri, focusing on 107 spectra (29 overlapping with the BLC1 campaign window) (Marcy, 2021). The search targeted both isolated emission lines and “combs” of evenly-spaced frequency teeth, with detection sensitivity for continuous laser emitters of 20–120 kW within a 1.3 au field of view.

No credible non-astrophysical emission was detected. All spectral artifacts, including multiple combs, were traced to terrestrial or instrumental (Fabry–Perot etalon/ghosting) origins. The absence of optical laser emission concurrent with BLC1 places strong upper bounds on multiwavelength technosignature scenarios, further disfavouring an extraterrestrial technology attribution for the radio event (Marcy, 2021).

5. Bayesian Priors: The Copernican Principle Constraint

A quantitative application of the Copernican principle—a non-privileged observer assumption—generates stringent astrophysical priors on the interpretation of BLC1 as a technosignature. Using a Poisson-Gott estimator for the emergence time and observable window of radio-transmitting civilizations across Earth-like planets orbiting Sun-like stars, Siraj and Loeb (2021) derive

$P_{\text{med}} \sim 3 \times 10^{-8}$

as the probability that Alpha Centauri currently hosts a radio transmitter (Siraj et al., 2021). Adopting BLC1 as a genuine extraterrestrial signal thus violates the Copernican framework by approximately eight orders of magnitude. This analysis robustly excludes a Proxima-centric transmitter model for BLC1 given current planetary demographic and technological emergence statistics, even under assumptions maximally optimistic about the prevalence of technosignatures.

6. Lessons Learned and Protocol Recommendations

The BLC1 case underscores several critical lessons for technosignature searches:

• Sky-localized, narrowband, and drifting signals are insufficient for an extraterrestrial attribution; complex, time-variable, and duty-cycled RFI can mimic such features convincingly.
• Rigid multi-step verification protocols—including hardware/instrumentation audit, RFI population analysis, and multiwavelength cross-correlation—are required for candidate vetting.
• Broad bandwidth observations facilitate identification of intermodulation by exposing harmonically related “twin” lines.
• Real-time, multi-telescope corroboration and high-fidelity RFI mapping are necessary to reliably exclude terrestrial or instrumental contaminants.
• Archival population analysis and sub-threshold event extraction are invaluable for establishing RFI morphologies and background rates (Sheikh et al., 2021, Smith et al., 2021).

7. Implications for Future SETI Searches

BLC1 remains a central case paper in the methodology of technosignature searches. While the campaign produced the most sensitive radio technosignature search yet performed toward a stellar target, its outcome emphasizes the necessity of:

• Extensive automation and open-source verification tool development (e.g., blimpy, turboSETI).
• Investing in simultaneous multi-site observations and developing robust RFI characterization campaigns preemptively.
• Prioritizing exhaustive multiwavelength and time-coincident searches for plausible candidates.

Proxima Centauri and the Alpha Centauri system persist as high-priority targets, but future interpretation of candidate events will require reconciliation with Copernican priors and detailed systematic artifact modeling across all available observational domains (Smith et al., 2021, Siraj et al., 2021).