Steve Croft

LHS 1140b, a rocky exoplanet in its host star’s habitable zone, was recently shown to have an escaping helium atmosphere. We present a search for narrowband radio technosignatures in archival Breakthrough Listen observations toward the LHS 1140 system using the 100 m Robert C. Byrd Green Bank Telescope at 1–12 GHz. We report a nondetection of candidate signals above 300 GW.

In 2025 July, the third-ever interstellar object, 3I/ATLAS, was discovered on its ingress into the solar system. Similar to the NASA Voyager missions sent in 1977, science probes by extraterrestrial life (“artifact technosignatures”) could be sent to explore other stellar systems like our own. In this campaign, we used the SETI Institute’s Allen Telescope Array to observe 3I/ATLAS from 1–9 GHz. We detected nearly 74 million narrowband hits in 7.25 hr of data using the newly developed search pipeline bliss . We then blanked hits by frequency and drift rate to mitigate radio frequency interference in our dataset, narrowing the dataset down to ∼2 million hits. These hits were further filtered by the localization code NBeamAnalysis , and the remaining 211 hits were visually inspected in the time-frequency domain. We did not find any signals worthy of additional follow-up. Accounting for the Doppler drift correction and given the nondetection, we are able to set an effective isotropic radiated power upper limit of 10–110 W on radio technosignatures from 3I/ATLAS across the frequency and drift rate ranges covered by our survey.

We report on the observation and measurement of astrometry, photometry, morphology, and activity of the interstellar object 3I/ATLAS, also designated C/2025 N1 (ATLAS) with the NSF-DOE Vera C. Rubin Observatory. Comet 3I/ATLAS, the third known interstellar object, was discovered on UT 2025 July 1. Rubin Observatory had coincidentally collected images of the object’s region of the sky during routine commissioning. Facilitated by Rubin’s high resolution and large aperture, we successfully recovered object detections from Rubin observations spanning UT 2025 June 21 (10 days before discovery, when 3I/ATLAS was 4.5 au from the Sun) through the date of discovery, and we acquired additional images through UT 2025 July 20 as part of commissioning. We measure on-sky locations of 3I/ATLAS in Rubin ugrizy bands, with a typical precision of ∼70 mas, and briefly describe the reason this is coarser than our measured static source astrometric precision of ∼3 mas in Rubin images. We measure grizy magnitudes of 3I/ATLAS photometry at ∼0.01 mag precision, detecting no short-term photometric variability above 0.01 mag. We derive an estimated near-nucleus dust-to-nucleus scattering cross-sectional ratio of η ≳ 13 on UT 2025 July 2 based on Rubin photometry and an upper limit nucleus size computed from Hubble Space Telescope observations. We find Rubin colors of g − r = (0.657 ± 0.013) mag, r − i = (0.235 ± 0.018) mag, i − z = (0.147 ± 0.042) mag, and z − y = (0.047 ± 0.052) mag. These data represent the earliest observations of this object by a large (≳8 m class) telescope and illustrate the type of measurements (and discoveries) Rubin’s Legacy Survey of Space and Time will provide after it begins in early 2026.

Modern time-domain surveys produce alert streams at a scale that makes exhaustive manual inspection infeasible, requiring automated methods to identify unusual transients for follow-up. In this work, we present an unsupervised anomaly detection pipeline applied to the ZTF alert stream using the Lasair broker. We define normal objects as SN Ia, SN II, and SN Ib/c. Anomalous objects include (i) more exotic transients (AGN, TDEs, SLSNe, CVs, and nuclear transients) and (ii) supernova-labeled objects, either spectroscopically or by Lasair, with anomalous properties, such as incorrect or absent host associations, or non-supernova-like light curves. Our pipeline consists of three independently trained simple autoencoders operating on distinct alert stream data products: object features, triplet image cutouts, and light curves. Each model is trained on predominantly normal transients, and performance is assessed using the recall of exotic objects and the purity of all anomalous objects across both a spectroscopically classified held-out test set and the live alert stream. In the test set, performance is evaluated at a fixed rank corresponding to the top ten scoring candidates, while in the alert stream it is evaluated using an anomaly threshold defined from test set behavior. Across both settings, the algorithms consistently recover exotic transients and anomalous supernovae among their top-ranked candidates. Over 25 days of live alert stream application, we identify 87 unusual supernova candidates for follow-up. The overlap between anomalies flagged by different autoencoders in the test set is non-existent, and in the alert stream is small, with maximum overlap between any two algorithms being 11 objects. The framework is data-efficient, requiring only a few thousand training examples, making it well suited for early and ongoing application to the Rubin Observatory alert stream.

We report the results of one of the most sensitive pulsar surveys to date targeting the innermost region of the Galactic center (GC) using the Robert C. Byrd Green Bank Telescope at X band (8–12 GHz) using data from the Breakthrough Listen initiative. In total, we collected 9.5 hr of data covering the wider ∼ 8 ′ diameter of the GC bulge, and 11 hr on the inner 1 . ′ 4 region between 2021 May and 2023 December. We conducted a comprehensive Fourier-domain periodicity search targeting both canonical pulsars (CPs) and millisecond pulsars (MSPs), using constant and linearly changing acceleration searches to improve sensitivity to compact binaries. Assuming weak scattering, our searches reached luminosity limits of L min ≈ 0.14 mJy kpc 2 for CPs and L min ≈ 0.26 mJy kpc 2 for MSPs—sensitive enough to detect the most luminous pulsars expected in the GC. Among 5282 signal candidates, we identify an interesting 8.19 ms MSP candidate (dispersion measure (DM) of 2775 pc cm ⁻³ ), persistent in time and frequency across a 1 hr scan at a flux density of S min ≈ 0.007 mJy . We introduce a novel randomization test for evaluating candidate significance against noise fluctuations, including signal persistence via Kolmogorov–Smirnov tests and flux-versus-DM behavior. We are unable to make a definitive claim about the candidate due to a mixed degree of confidence from these tests and, more broadly, its nondetection in subsequent observations. This deepens the ongoing missing pulsar problem in the GC, reinforcing the idea that strong scattering and/or extreme orbital dynamics may obscure pulsar signals in this region.

3I/ATLAS, an interstellar object, made its closest approach to Earth, at ∼1.8 au, on 2025 December 19. On 2025 December 18, the Breakthrough Listen program conducted a technosignature search toward 3I/ATLAS using the 100 m Robert C. Byrd Green Bank Telescope at 1–12 GHz. We report a nondetection of candidate signals down to the 100 mW level.

3I/ATLAS, an interstellar object, made its closest approach to Earth on 2025 December 19. On 2025 December 18, the Breakthrough Listen program conducted a technosignature search toward 3I/ATLAS using the 100 m Robert C. Byrd Green Bank Telescope at 1-12 GHz. We report a nondetection of candidate signals down to the 100 mW level.

With the discovery of the third confirmed interstellar object (ISO), 3I/ATLAS, we have entered a new phase in the exploration of these long-predicted objects. Though confirmed discovery of ISOs is quite recent, their utility as targets in the search for technosignatures (historically known as the Search for Extraterrestrial Intelligence -- SETI) has been discussed for many decades. With the upcoming NSF-DOE Vera C. Rubin Observatory's Legacy Survey of Space and Time (LSST), the discovery and tracking of such objects is expected to become routine, and thus so must our examination of these objects for possible technosignatures. Here we review the literature surrounding ISOs as targets for technosignatures, which provides a well-developed motivation for such exploration. We outline four broad classes of technosignatures that are well suited for ISO follow-up, including the type of data needed and the best timing for study. Given the limitations in the current understanding of ISOs, we show that care must be taken in identifying technosignatures based primarily on comparison to objects in the Solar System. We therefore provide a roadmap for careful and consistent study of the population of ISOs in the hope of identifying technosignatures.