## Main

The discovery of the exoplanet Proxima Centauri b (Prox Cen b) in orbit around Prox Cen1 has sparked excitement over the prospect of a habitable exoplanet in the nearest reaches of the solar neighbourhood. Several studies2,3,4 suggest that Prox Cen b may be able to sustain an atmosphere favourable for life. However, because Prox Cen is an active M-dwarf flare star, doubt has been cast on the ability of Prox Cen b, which is in a much tighter orbit than Earth is to the Sun, to retain an atmosphere amenable to the existence of biological life. A naked-eye visible superflare strong enough to kill any known organisms has been observed from Prox Cen5, although life could still exist on the cold side of a tidally locked planet. Coronal mass ejections from Prox Cen have also been observed6,7, suggesting that Prox Cen b experiences appreciable ionizing radiation. Nevertheless, there remain compelling arguments that M-dwarf stars are viable hosts for life-bearing planets8, and Prox Cen b remains a compelling target for biosignature and technosignature searches.

Prox Cen b is also a candidate for in situ searches for extraterrestrial life by Breakthrough Starshot9. Starshot seeks to launch a gram-sized spacecraft propelled using a laser light sail at a relativistic velocity (0.2c) to the Alpha Centauri system with Prox Cen b as the primary target. If successful, the spacecraft will travel the 4.22 light yr to Prox Cen in 20 yr and then transmit information on its target(s) back to Earth.

Despite being our nearest stellar neighbour, few technosignature searches have been conducted towards Prox Cen. In the 1990s, two search for extraterrestrial intelligence (SETI) programmes were conducted in the southern hemisphere towards nearby stars: the Project Phoenix search of 202 solar-like stars10,11, and a search for technosignatures from 176 of the brightest stars12. Consequently, neither programme selected Prox Cen—a faint M dwarf—as a star target. Until a recent technosignature search of high-resolution optical spectra of Prox Cen13, no technosignature searches had been conducted at optical wavelengths. This search of archival data from the High Accuracy Radial Velocity Planet Searcher spectrometer between 2004 and 2019 would have revealed laser emission from Prox Cen with <120 kW power, and was motivated by the observations detailed here.

In this Article, we present a search for technosignatures from the direction of Prox Cen, using the CSIRO Parkes radio telescope (‘Murriyang’) as part of the Breakthrough Listen (BL) and the Breakthrough Initiatives search for life beyond Earth. BL—a sister initiative of Starshot—is a 10 yr programme to search for signs of intelligent life at radio and optical wavelengths14,15. BL has been conducting observations since 2016 and is undertaking the most rigorous and comprehensive observational SETI campaign to date16,17,18,19,20. Prox Cen is part of the BL survey sample of 60 stars within 5 pc (ref. 15), and was observed as part of a previous data release including 1,327 nearby stars17. These observations were conducted using the Parkes 10 cm receiver, covering 2.60–3.45 GHz. The observations presented here cover a larger bandwidth (0.7–4.0 GHz), with six times longer on-source dwell times (30 min). Our observations allow us to set the lowest equivalent isotropic radiated power (EIRP) detection limit for any stellar target.

We searched our observations towards Prox Cen for signs of technologically advanced life, across the full frequency range of the receiver (0.7–4.0 GHz). To search for narrowband technosignatures we exploit the fact that signals from any body with a non-zero radial acceleration relative to Earth, such as an exoplanet, solar system object or spacecraft, will exhibit a characteristic time-dependent drift in frequency (referred to as a drift rate) when detected by a receiver on Earth. We applied a search algorithm that detects narrowband signals with Doppler drift rates consistent with that expected from a transmitter located on the surface of Prox Cen b (Fig. 1). Our search detected a total of 4,172,702 hits—that is, narrowband signals detected above a signal-to-noise (S/N) threshold—in all on-source observations of Prox Cen and reference off-source observations. Of these, 5,160 hits were present in multiple on-source pointings towards Prox Cen, but were not detected in reference (off-source) pointings towards calibrator sources; we refer to these as ‘events’ (Table 1).

The total hits by frequency are shown in Fig. 2. As expected, our detection pipeline finds the majority of hits (57%) in ranges that have registered transmitters. Distributions of hits and events for our drift rate search range and S/N are shown in Fig. 3. Positive, negative and zero drift rates correspond to 10%, 15% and 75% of the total hits respectively. The slight bias towards a negative drift rate is due to non-geosynchronous satellites21. The majority of events occur below an S/N threshold of 103 because faint signals are less likely to be detected in our shorter reference observations. Stronger signals are also generally associated with nearby ground-based transmitters that will appear in both on-source and off-source observations.

Of the 5,160 events, only one event (Fig. 4) passed all rounds of filtering and visual inspection of dynamic spectra. The event does not lie within the frequency range of any known local radiofrequency interference (RFI), and has many characteristics consistent with a putative transmitter located in another stellar system. This event, which we refer to as a signal of interest, has been previously reported as ‘BLC1’, short for ‘Breakthrough Listen Candidate 1’. We note that ‘signal of interest’ is more appropriate than ‘candidate’22, but for consistency we will adopt BLC1 throughout.

BLC1 was detected at 982.002571 MHz, with a drift rate of 0.038 Hz s−1. The signal of interest was detected over a ~2.5 h period, and is only present in pointings towards Prox Cen. According to the US Federal Communications Commission and the Australian Radiofrequency Spectrum Plan, BLC1 lies within a frequency band reserved for aeronautical radionavigation; however, no transmitters that operate at the detected frequency of BLC1 are registered within 1,000 km of the observatory. Radionavigation stations are ground based, so are less likely to be directionally sensitive. It is unlikely that an aircraft or satellite would be present in the direction of Prox Cen over the course of the signal of interest. BLC1 is analysed in further detail in a companion paper23. As detailed in the companion paper, we ultimately conclude that BLC1 is a complex intermodulation product of multiple human-generated interferers: not a technosignature.

Given our non-detection of technosignatures, we place limits on the detection of narrowband signals from Prox Cen by calculating the minimum detectable EIRP (EIRPmin). The EIRPmin is given by

$${{{{\rm{EIRP}}}}}_{\min }=4\uppi {d}^{2}{F}_{\min }$$
(1)

where d is the distance to the source (1.301 pc for Prox Cen) and Fmin is the minimum detectable flux in watts per square metre. The equation for Fmin depends on the minimum S/N (S/Nmin), the system temperature of the telescope (Tsys), the effective collecting area of the telescope (Aeff), the channel bandwidth (B), the number of polarizations (npol) and the total observation time (tobs)16,17:

$${{{{F}}}}_{\min }={{{\rm{S}}}}/{{{{\rm{N}}}}}_{\min }\frac{2{k}_{\rm{B}}{T}_{{{{\rm{sys}}}}}}{{A}_{{{{\rm{eff}}}}}}\sqrt{\frac{B}{{n}_{{{{\rm{pol}}}}}{t}_{{{{\rm{obs}}}}}}}.$$
(2)

kB is the Boltzmann constant. We calculate Fmin = 9.2 Jy Hz and EIRPmin = 1.9 GW. This EIRPmin is 3.6 times smaller than that previously reported, EIRPmin = 6.2 GW, for observations of Prox Cen17. The improved EIRPmin is due to the lower Tsys of the ultrawide-bandwidth, low-frequency receiver (UWL) (22 K), compared with the Parkes 10 cm receiver (35 K), and our longer tobs, 5 min versus 30 min. Additionally, our EIRPmin is 1.6 times smaller than Green Bank Telescope L-band (1.1–1.9 GHz) and S-band (1.7–2.6 GHz) observations of the second closest star outside the α Cen system, Barnard’s Star (GJ 699). As such, our search of Prox Cen is decisively a very sensitive and comprehensive technosignature search for a stellar target.

On the basis of previous SETI searches and analysis24, it is clear that putative narrowband transmitters are rare. As such, it is statistically probable that any signal of interest is a pathological case of human-generated interference. Extended and rigorous analysis of BLC1 was required to ascertain its progeny; this is presented in the companion paper23, alongside a framework for verification of future signals of interest.

Alone, this search—or more generally any band-limited single-target search—cannot disprove the existence of a technologically advanced society on Prox Cen b. While the UWL receiver has a wide bandwidth, we have still not covered the entire radio spectrum, nor optical, infrared or X-ray bands. In addition to false positives, RFI could also confound detection of technosignatures at coincident frequencies. Prox Cen remains an interesting target for technosignature searches, and we encourage continued observations with other facilities and alternative approaches.

## Methods

For the observations of Prox Cen presented here, we used the Parkes UWL25. The receiver has an effective Tsys of 22 K and system equivalent flux density of 28 Jy across ~60% of the band. The UWL receiver covers a 3.3 GHz wide bandwidth from 0.704 to 4.032 GHz.

Observations were conducted from ut 2019 April 29 to ut 2019 May 4, as part of the P1018 project ‘Wide-band radio monitoring of space weather on Prox Cen’. In this project, Parkes observations were part of a multiwavelength campaign in which Prox Cen was monitored for stellar flare activity6. Observations were conducted using the BL Parkes digital recorder26,27 (BLPDR) in parallel with the primary UWL digital signal processor, ‘Medusa’. For the purposes of a narrowband technosignature search, we deal solely with Stokes I data from BLPDR. For the purposes of detecting flare activity, data from Medusa—which was configured to produce a full-Stokes data product at high time resolution (128 μs) but low frequency resolution (1 MHz)—were also recorded; however, these data are not included in our analysis given their relative insensitivity to narrowband signals.

### Observation strategy

Over the period ut 2019 April 29 to ut 2019 May 4, we observed Prox Cen for a total of 26 h 9 min. The observations were conducted as a series of on-source, off-source pointings (a ‘cadence’) to enable rejection of RFI, similar to previous BL observations16,17. The on-source pointings (A) were towards Prox Cen (14 h 29 min 42.95 s, −61° 59′ 53.84″) while the off-source pointings (B) were primarily the calibrator source PKS 1421-490 (14 h 24 min 32.24 s, −49° 26′ 50.26″) with the exception of two of the pointings, which used PKS 1934-638 (19 h 39 min 25.03 s, −62° 57′ 54.34″), a well characterized flux calibrator for Parkes.

Our observations differed from standard BL searches in two ways. First, our observation lengths were ~30 min on source and ~5 min off source; previous BL searches16,17 employed a 5 min on-source, 5 min off-source observation style. A longer observation time was chosen to maximize time on Prox Cen to search for flare emission: a key element, which may determine the habitability of Prox Cen b. Longer observations mean we are insensitive to signals lasting less than 30 min because we do not have intervening off-source pointings needed to discern if a hit is caused by RFI. However, we are sensitive to signals that are broadcast over a long period of time (an hour or longer) because we can see how that signal changes over time (for example, drift rate changes). We are also more sensitive to weaker signals because we can integrate over the whole 30 min observation to look for a persistent signal.

Second, we used longer cadences—sets of pointings towards the target source and reference sources—than the six-pointing default for BL. The total number of pointings in a single observation ranged from 12 to 17. This gives us the flexibility to choose a subset of cadences from a larger number of pointings per day. In our initial search, we considered four-pointing cadences, meaning, for example, that if an observation consisted of 12 pointings we looked at nine subset cadences in that observation.

### Data format

Data are processed using a pipeline run on the BLPDR. BLPDR provides data in 26 separate files, each containing a 128 MHz subband from the 0.7–4.0 GHz band. The high-spectral-resolution products, as used here for detection of artificial narrowband signals, have a frequency resolution of ~3.81 Hz (that is, 225 channels across each 128 MHz band) and time integrations of ~16.78 s. The final data product is stored in filterbank format, which can then be opened by the Blimpy Python package28. The final data volume for the 6 d observation period is 19.5 TB: about 118 times more data than were obtained for any single source in previous BL searches17.

The Python/Cython package turboSETI29 is used to search over a range of drift rates in the data. Reference 16 describes how the program works in greater detail. Two important parameters required by turboSETI are a minimum S/N, and a maximum possible drift rate. The minimum S/N was set to 10, following previous work17. However, we tailored the drift rate range to the specific characteristics of Prox Cen b’s rotation and orbit.

### Expected drift rate

The most dominant factors affecting the drift rate of a signal are the rotations and orbits of the Earth and the source body. The following equation30 gives us the maximum expected Doppler drift rate ($${\dot{\nu }}_{\max }$$) by accounting for planet rotation $$\left(\frac{4{\uppi }^{2}R}{{P}^{2}}\right)$$ and orbit $$\left(\frac{GM}{r}\right)$$:

$${\dot{\nu }}_{\max }=\frac{{\nu }_{0}}{c}\left(\frac{4{\uppi }^{2}{R}_{\oplus }}{{P}_{\oplus }^{2}}+\frac{4{\uppi }^{2}{R}_{{{{\rm{Pb}}}}}}{{P}_{{{{\rm{Pb}}}}}^{2}}+\frac{G{M}_{\odot }}{{r}_{\oplus }^{2}}+\frac{G{M}_{{{{\rm{PC}}}}}}{{r}_{{{{\rm{Pb}}}}}^{2}}\right).$$
(3)

The term v0 is the emitted frequency from the transmitter; R, P, M and r are the planetary radii, rotational periods, solar masses and orbital radii for Earth, Prox Cen b (subscript Pb) the Sun and Prox Cen (subscript PC), respectively. Other contributions to the drift rate, such as the bodies’ movement through the Milky Way, are negligible.

To limit computation time, an initial search of ±3 Hz s−1 was performed across all frequencies. However, we expect $${\dot{\nu }}_{\rm{max}}=4.191$$ Hz s−1 at 4,000 MHz using the parameters from Table 2 and equation (3). Therefore, a supplementary search over ±4 Hz s−1 from 2,752 to 3,648 MHz and ±5 Hz s−1 from 3,648 to 4,032 MHz was necessary to search the complete range of possible drift rates expected. Nevertheless, putative transmitters orbiting Prox Cen b could exhibit drift rates orders of magnitude higher30; extending to such high drift rates is computationally challenging, and we do not consider these here.

### Finding events

To find candidate events, we run the hits (signals above the S/N threshold) found by turboSETI through a secondary pipeline, which compares on-source and off-source pointings. We classify an event as any narrowband hit that exists in an on-source pointing, but not any of the off-source observations. Typical BL SETI searches with single-dish telescopes use a cadence length of six (ABABAB, three on-source and three off-source observations); however, we use a cadence length of four (ABAB) due to our longer observation times. A shorter cadence relaxes the requirement that events are detected in all on-source observations; that is, we allow events with ~1 h duration. Once an event is found in a cadence of four, we search additional pointings to see if it occurs over a longer time period. Note that cadences are primarily used as a discriminant for RFI; as the narrowband search is run separately on each pointing, longer cadences do not increase sensitivity.

### Filtering events

After events that occur in a four-pointing cadence (ABAB) are found, we generate plots that have an additional two pointings to make a six-pointing cadence (ABABAB). A longer cadence allows visual inspection of the additional pointings for low-S/N hits. For example, an intermittent signal may be present in only the on-source pointings for the four pointings that were searched, but then be present in a successive off-source pointing (Supplementary Fig. 1). We discard events that are clearly present in the off-source observations but are not detected by the search pipeline (that is, the event is present in a successive off-source pointing, but did not meet the S/N threshold of 10, or the drift rate threshold17).

After an initial list of promising events with a six-pointing cadence is found, we plot cadences of length 12. This longer cadence allows us to see the entire duration of the event and if it occurs in any off-source observations. If an event is present in any off-source observations, it is discarded as local RFI (Supplementary Fig. 2). During this step we also discard events that share similar characteristics (drift rate, frequency or profile) with other hits that are found in off-source observations. Finally, any candidate event that lies in the frequency range of nearby registered ground or satellite transmitters is marked as suspicious (Supplementary Fig. 3).

Every event that passes the two rounds of visual inspection and lies within no registered transmitters is scrutinized. Extensive research is done on the frequency bands within which the event lies. We use allocation charts such as the ARSP (Australian Radiofrequency Spectrum Plan), which contains an extensive list of the types of transmitter allocated to specific frequency bands, and the Australian Communications and Media Authority Register of Radiocommunications Licences (https://web.acma.gov.au/rrl/).