arXiv subjects

Steve Croft

Publications and source records attributed to Steve Croft.

At least 37 records · Page 2Linked to original sources

arXiv2023

Developing a Drift Rate Distribution for Technosignature Searches of Exoplanets

A stable-frequency transmitter with relative radial acceleration to a receiver will show a change in received frequency over time, known as a "drift rate''. For a transmission from an exoplanet, we must account for multiple components of drift rate: the exoplanet's orbit and rotation, the Earth's orbit and rotation, and other contributions. Understanding the drift rate distribution produced by exoplanets relative to Earth, can a) help us constrain the range of drift rates to check in a Search for Extraterrestrial Intelligence (SETI) project to detect radio technosignatures and b) help us decide validity of signals-of-interest, as we can compare drifting signals with expected drift rates from the target star. In this paper, we modeled the drift rate distribution for $\sim$5300 confirmed exoplanets, using parameters from the NASA Exoplanet Archive (NEA). We find that confirmed exoplanets have drift rates such that 99\% of them fall within the $\pm$53 nHz range. This implies a distribution-informed maximum drift rate $\sim$4 times lower than previous work. To mitigate the observational biases inherent in the NEA, we also simulated an exoplanet population built to reduce these biases. The results suggest that, for a Kepler-like target star without known exoplanets, $\pm$0.44 nHz would be sufficient to account for 99\% of signals. This reduction in recommended maximum drift rate is partially due to inclination effects and bias towards short orbital periods in the NEA. These narrowed drift rate maxima will increase the efficiency of searches and save significant computational effort in future radio technosignature searches.

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arXiv2023

A Simultaneous Dual-site Technosignature Search Using International LOFAR Stations

The Search for Extraterrestrial Intelligence aims to find evidence of technosignatures, which can point toward the possible existence of technologically advanced extraterrestrial life. Radio signals similar to those engineered on Earth may be transmitted by other civilizations, motivating technosignature searches across the entire radio spectrum. In this endeavor, the low-frequency radio band has remained largely unexplored; with prior radio searches primarily above 1 GHz. In this survey at 110-190 MHz, observations of 1,631,198 targets from TESS and Gaia are reported. Observations took place simultaneously with two international stations (noninterferometric) of the Low Frequency Array in Ireland and Sweden. We can reject the presence of any Doppler drifting narrowband transmissions in the barycentric frame of reference, with equivalent isotropic radiated power of 10 17 W, for 0.4 million (or 1.3 million) stellar systems at 110 (or 190) MHz. This work demonstrates the effectiveness of using multisite simultaneous observations for rejecting anthropogenic signals in the search for technosignatures.

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arXiv2023

Signal Synchronization Strategies and Time Domain SETI with Gaia DR3

Spatiotemporal techniques for signal coordination with actively transmitting extraterrestrial civilizations, without the need for prior communication, can constrain technosignature searches to a significantly smaller coordinate space. With the variable star catalog from Gaia Data Release 3, we explore two related signaling strategies: the SETI Ellipsoid, and that proposed by Seto, which are both based on the synchronization of transmissions with a conspicuous astrophysical event. This dataset contains more than 10 million variable star candidates with light curves from the first three years of Gaia's operational phase, between 2014 and 2017. Using four different historical supernovae as source events, we find that less than 0.01% of stars in the sample have crossing times, the times at which we would expect to receive synchronized signals on Earth, within the date range of available Gaia observations. For these stars, we present a framework for technosignature analysis that searches for modulations in the variability parameters by splitting the stellar light curve at the crossing time.

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arXiv2023

Real-Time Technosignature Strategies with SN 2023ixf

Several technosignature techniques focus on historic events such as SN 1987A as the basis to search for coordinated signal broadcasts from extraterrestrial agents. The recently discovered SN 2023ixf in the spiral galaxy M101 is the nearest Type II supernova in over a decade, and will serve as an important benchmark event. Here we review the potential for SN 2023ixf to advance ongoing techonsignature searches, particularly signal-synchronization techniques such as the "SETI Ellipsoid". We find that more than 100 stars within 100 pc are already close to intersecting this SETI Ellipsoid, providing numerous targets for real-time monitoring within ~3$^\circ$ of SN 2023ixf. We are commencing a radio technosignature monitoring campaign of these targets with the Allen Telescope Array and the Green Bank Telescope.

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arXiv2023

A 4-8 GHz Galactic Center Search for Periodic Technosignatures

Radio searches for extraterrestrial intelligence have mainly targeted the discovery of narrowband continuous-wave beacons and artificially dispersed broadband bursts. Periodic pulse trains, in comparison to the above technosignature morphologies, offer an energetically efficient means of interstellar transmission. A rotating beacon at the Galactic Center (GC), in particular, would be highly advantageous for galaxy-wide communications. Here, we present blipss, a CPU-based open-source software that uses a fast folding algorithm (FFA) to uncover channel-wide periodic signals in radio dynamic spectra. Running blipss on 4.5 hours of 4-8 GHz data gathered with the Robert C. Byrd Green Bank Telescope, we searched the central 6' of our Galaxy for kHz-wide signals with periods between 11-100 s and duty cycles ($δ$) between 10-50%. Our searches, to our knowledge, constitute the first FFA exploration for periodic alien technosignatures. We report a non-detection of channel-wide periodic signals in our data. Thus, we constrain the abundance of 4-8 GHz extraterrestrial transmitters of kHz-wide periodic pulsed signals to fewer than one in about 600,000 stars at the GC above a 7$σ$ equivalent isotropic radiated power of $\approx 2 \times 10^{18}$ W at $δ\simeq 10\%$. From an astrophysics standpoint, blipss, with its utilization of a per-channel FFA, can enable the discovery of signals with exotic radio frequency sweeps departing from the standard cold plasma dispersion law.

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arXiv2023

A deep-learning search for technosignatures of 820 nearby stars

The goal of the Search for Extraterrestrial Intelligence (SETI) is to quantify the prevalence of technological life beyond Earth via their "technosignatures". One theorized technosignature is narrowband Doppler drifting radio signals. The principal challenge in conducting SETI in the radio domain is developing a generalized technique to reject human radio frequency interference (RFI). Here, we present the most comprehensive deep-learning based technosignature search to date, returning 8 promising ETI signals of interest for re-observation as part of the Breakthrough Listen initiative. The search comprises 820 unique targets observed with the Robert C. Byrd Green Bank Telescope, totaling over 480, hr of on-sky data. We implement a novel beta-Convolutional Variational Autoencoder to identify technosignature candidates in a semi-unsupervised manner while keeping the false positive rate manageably low. This new approach presents itself as a leading solution in accelerating SETI and other transient research into the age of data-driven astronomy.

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arXiv2023

The Breakthrough Listen Search for Intelligent Life: A Laser Search Pipeline for the Automated Planet Finder

The Search for Extraterrestrial Intelligence (SETI) has traditionally been conducted at radio wavelengths, but optical searches are well-motivated and increasingly feasible due to the growing availability of high-resolution spectroscopy. We present a data analysis pipeline to search Automated Planet Finder (APF) spectroscopic observations from the Levy Spectrometer for intense, persistent, narrow bandwidth optical lasers. We describe the processing of the spectra, the laser search algorithm, and the results of our laser search on 1983 spectra of 388 stars as part of the Breakthrough Listen search for technosignatures. We utilize an empirical spectra-matching algorithm called SpecMatch-Emp to produce residuals between each target spectrum and a set of best-matching catalog spectra, which provides the basis for a more sensitive search than previously possible. We verify that SpecMatch-Emp performs well on APF-Levy spectra by calibrating the stellar properties derived by the algorithm against the SpecMatch-Emp library and against Gaia catalog values. We leverage our unique observing strategy, which produces multiple spectra of each target per night of observing, to increase our detection sensitivity by programmatically rejecting events which do not persist between observations. With our laser search algorithm we achieve a sensitivity equivalent to the ability to detect an 84 kW laser at the median distance of a star in our dataset (78.5 ly). We present the methodology and vetting of our laser search, finding no convincing candidates consistent with potential laser emission in our target sample.

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arXiv2022

A Green Bank Telescope search for narrowband technosignatures between 1.1-1.9 GHz during 12 Kepler planetary transits

A growing avenue for determining the prevalence of life beyond Earth is to search for "technosignatures" from extraterrestrial intelligences/agents. Technosignatures require significant energy to be visible across interstellar space and thus intentional signals might be concentrated in frequency, in time, or in space, to be found in mutually obvious places. Therefore, it could be advantageous to search for technosignatures in parts of parameter space that are mutually-derivable to an observer on Earth and a distant transmitter. In this work, we used the L-band (1.1-1.9 GHz) receiver on the Robert C. Byrd Green Bank Telescope (GBT) to perform the first technosignature search pre-synchronized with exoplanet transits, covering 12 Kepler systems. We used the Breakthrough Listen turboSETI pipeline to flag narrowband hits ($\sim$3 Hz) using a maximum drift rate of $\pm$614.4 Hz/s and a signal-to-noise threshold of 5 - the pipeline returned $\sim 3.4 \times 10^5$ apparently-localized features. Visual inspection by a team of citizen scientists ruled out 99.6% of them. Further analysis found 2 signals-of-interest that warrant follow-up, but no technosignatures. If the signals-of-interest are not re-detected in future work, it will imply that the 12 targets in the search are not producing transit-aligned signals from 1.1-1.9 GHz with transmitter powers $>$60 times that of the former Arecibo radar. This search debuts a range of innovative technosignature techniques: citizen science vetting of potential signals-of-interest, a sensitivity-aware search out to extremely high drift rates, a more flexible method of analyzing on-off cadences, and an extremely low signal-to-noise threshold.

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arXiv2022

A Search for Radio Technosignatures at the Solar Gravitational Lens Targeting Alpha Centauri

Stars provide an enormous gain for interstellar communications at their gravitational focus, perhaps as part of an interstellar network. If the Sun is part of such a network, there should be probes at the gravitational foci of nearby stars. If there are probes within the solar system connected to such a network, we might detect them by intercepting transmissions from relays at these foci. Here, we demonstrate a search across a wide bandwidth for interstellar communication relays beyond the Sun's innermost gravitational focus at 550 AU using the Green Bank Telescope (GBT) and Breakthrough Listen (BL) backend. As a first target, we searched for a relay at the focus of the Alpha Centauri AB system while correcting for the parallax due to Earth's orbit around the Sun. We searched for radio signals directed at the inner solar system from such a source in the L and S bands. Our analysis, utilizing the turboSETI software developed by BL, did not detect any signal indicative of a non-human-made artificial origin. Further analysis excluded false negatives and signals from the nearby target HD 13908. Assuming a conservative gain of 10^3 in L-band and roughly 4 times that in S-band, a ~1 meter directed transmitter would be detectable by our search above 7 W at 550 AU or 23 W at 1000 AU in L-band, and above 2 W at 550 AU or 7 W at 1000 AU in S-band. Finally, we discuss the application of this method to other frequencies and targets.

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arXiv2022

Multiband Detection of Repeating FRB 20180916B

We present a multiband study of FRB 20180916B, a repeating source with a 16.3 day periodicity. We report the detection of 4, 1 and 7 bursts from observations spanning 3 days using upgraded Giant Metrewave Radio Telescope (300-500 MHz), Canadian Hydrogen Intensity Mapping Experiment (400-800 MHz) and Green Bank Telescope (600-1000 MHz), respectively. We report the first-ever detection of the source in the 800-1000 MHz range along with one of the widest instantaneous bandwidth detection (200 MHz) at lower frequencies. We identify 30 $μ$s wide structures in one of the bursts at 800 MHz, making it the lowest frequency detection of such structures for this FRB thus far. There is also a clear indication of high activity of the source at a higher frequency during earlier phases of the activity cycle. We identify a gradual decrease in the rotation measure over two years and no significant variations in the dispersion measure. We derive useful conclusions about progenitor scenarios, energy distribution, emission mechanisms, and variation of downward drift rate of emission with frequency. Our results reinforce that multiband observations are an effective approach to study repeaters and even one-off events to better understand their varying activity and spectral anomalies.

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arXiv2022

Searching the SETI Ellipsoid with Gaia

The SETI Ellipsoid is a geometric method for prioritizing technosignature observations based on the strategy of receiving signals synchronized to conspicuous astronomical events. Precise distances to nearby stars from Gaia makes constraining Ellipsoid crossing times possible. Here we explore the utility of using the Gaia Catalog of Nearby Stars to select targets on the SN 1987A SETI Ellipsoid, as well the Ellipsoids defined by 278 classical novae. Less than 8% of stars within the 100 pc sample are inside the SN 1987A SETI Ellipsoid, meaning the vast majority of nearby stars are still viable targets for monitoring over time. We find an average of 734 stars per year within the 100 pc volume will intersect the Ellipsoid from SN 1987A, with ~10% of those having distance uncertainties from Gaia better than 0.1 lyr.

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arXiv2022

Searching for broadband pulsed beacons from 1883 stars using neural networks

The search for extraterrestrial intelligence at radio frequencies has largely been focused on continuous-wave narrowband signals. We demonstrate that broadband pulsed beacons are energetically efficient compared to narrowband beacons over longer operational timescales. Here, we report the first extensive survey searching for such broadband pulsed beacons towards 1883 stars as a part of the Breakthrough Listen's search for advanced intelligent life. We conducted 233 hours of deep observations across 4 to 8 GHz using the Robert C. Byrd Green Bank Telescope and searched for three different classes of signals with artificial (or negative) dispersion. We report a detailed search -- leveraging a convolutional neural network classifier on high-performance GPUs -- deployed for the very first time in a large-scale search for signals from extraterrestrial intelligence. Due to the absence of any signal-of-interest from our survey, we place a constraint on the existence of broadband pulsed beacons in our solar neighborhood: $\lesssim$1 in 1000 stars have transmitter power-densities $\gtrsim$10$^5$ W/Hz repeating $\leq$500 seconds at these frequencies.

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arXiv2022

Setigen: Simulating Radio Technosignatures for SETI

The goal of the search for extraterrestrial intelligence (SETI) is the detection of non-human technosignatures, such as technology-produced emission in radio observations. While many have speculated about the character of such technosignatures, radio SETI fundamentally involves searching for signals that not only have never been detected, but also have a vast range of potential morphologies. Given that we have not yet detected a radio SETI signal, we must make assumptions about their form to develop search algorithms. The lack of positive detections also makes it difficult to test these algorithms' inherent efficacy. To address these challenges, we present Setigen, a Python-based, open-source library for heuristic-based signal synthesis and injection for both spectrograms (dynamic spectra) and raw voltage data. Setigen facilitates the production of synthetic radio observations, interfaces with standard data products used extensively by the Breakthrough Listen project (BL), and focuses on providing a physically-motivated synthesis framework compatible with real observational data and associated search methods. We discuss the core routines of Setigen and present existing and future use cases in the development and evaluation of SETI search algorithms.

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arXiv2022

The Breakthrough Listen Search for Intelligent Life: Technosignature Search of Transiting TESS Targets of Interest

The Breakthrough Listen Initiative, as part of its larger mission, is performing the most thorough technosignature search of nearby stars. Additionally, Breakthrough Listen is collaborating with scientists working on NASAs Transiting Exoplanet Survey Satellite (TESS), to examine TESS Targets of Interest (TOIs) for technosignatures. Here, we present a $1-11$ $\textrm{GHz}$ radio technosignature search of $61$ TESS TOIs that were in transit during their Breakthrough Listen observation at the Robert C. Byrd Green Bank Telescope. We performed a narrowband Doppler drift search with a minimum S/N threshold of $10$, across a drift rate range of $\pm 4$ $\textrm{Hz}$ $\textrm{s}$ $^{-1}$, with a resolution of $3$ $\textrm{Hz}$. We removed radio frequency interference by comparing signals across cadences of target sources. After interference removal, there are no remaining events in our survey, and therefore no technosignature signals-of-interest detected in this work. This null result implies that at L, S, C, and X bands, fewer than $52\textrm{%}$, $20\textrm{%}$, $16\textrm{%}$, and $15\textrm{%}$, respectively, of TESS TOIs possess a transmitter with an equivalent isotropic radiated power greater than a few times $10^{14}$ $\textrm{W}$.

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arXiv2021

One of Everything: The Breakthrough Listen Exotica Catalog

We present Breakthrough Listen's "Exotica" Catalog as the centerpiece of our efforts to expand the diversity of targets surveyed in the Search for Extraterrestrial Intelligence (SETI). As motivation, we introduce the concept of survey breadth, the diversity of objects observed during a program. Several reasons for pursuing a broad program are given, including increasing the chance of a positive result in SETI, commensal astrophysics, and characterizing systematics. The Exotica Catalog is a 963 entry collection of 816 distinct targets intended to include "one of everything" in astronomy. It contains four samples: the Prototype sample, with an archetype of every known major type of non-transient celestial object; the Superlative sample of objects with the most extreme properties; the Anomaly sample of enigmatic targets that are in some way unexplained; and the Control sample with sources not expected to produce positive results. As far as we are aware, this is the first object list in recent times with the purpose of spanning the breadth of astrophysics. We share it with the community in hopes that it can guide treasury surveys and as a general reference work. Accompanying the catalog is extensive discussion of classification of objects and a new classification system for anomalies. Extensive notes on the objects in the catalog are available online. We discuss how we intend to proceed with observations in the catalog, contrast it with our extant Exotica efforts, and suggest similar tactics may be applied to other programs.

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arXiv2021

A radio technosignature search towards Proxima Centauri resulting in a signal-of-interest

The detection of life beyond Earth is an ongoing scientific endeavour, with profound implications. One approach, known as the search for extraterrestrial intelligence (SETI), seeks to find engineered signals (`technosignatures') that indicate the existence technologically-capable life beyond Earth. Here, we report on the detection of a narrowband signal-of-interest at ~982 MHz, recorded during observations toward Proxima Centauri with the Parkes Murriyang radio telescope. This signal, `BLC1', has characteristics broadly consistent with hypothesized technosignatures and is one of the most compelling candidates to date. Analysis of BLC1 -- which we ultimately attribute to being an unusual but locally-generated form of interference -- is provided in a companion paper (Sheikh et al., 2021). Nevertheless, our observations of Proxima Centauri are the most sensitive search for radio technosignatures ever undertaken on a star target.

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arXiv2021

Analysis of the Breakthrough Listen signal of interest blc1 with a technosignature verification framework

The aim of the search for extraterrestrial intelligence (SETI) is to find technologically-capable life beyond Earth through their technosignatures. On 2019 April 29, the Breakthrough Listen SETI project observed Proxima Centauri with the Parkes 'Murriyang' radio telescope. These data contained a narrowband signal with characteristics broadly consistent with a technosignature near 982 MHz ('blc1'). Here we present a procedure for the analysis of potential technosignatures, in the context of the ubiquity of human-generated radio interference, which we apply to blc1. Using this procedure, we find that blc1 is not an extraterrestrial technosignature, but rather an electronically-drifting intermodulation product of local, time-varying interferers aligned with the observing cadence. We find dozens of instances of radio interference with similar morphologies to blc1 at frequencies harmonically related to common clock oscillators. These complex intermodulation products highlight the necessity for detailed follow-up of any signal-of-interest using a procedure such as the one outlined in this work.

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arXiv2021

The Galactic Faraday rotation sky 2020

This work gives an update to existing reconstructions of the Galactic Faraday rotation sky by processing almost all Faraday rotation data sets available at the end of the year 2020. Observations of extra-Galactic sources in recent years have, among other regions, further illuminated the previously under-constrained southern celestial sky, as well as parts of the inner disc of the Milky Way. This has culminated in an all-sky data set of 55,190 data points, which is a significant expansion on the 41,330 used in previous works, hence making an updated separation of the Galactic component a promising venture. The increased source density allows us to present our results in a resolution of about $1.3\cdot 10^{-2}\, \mathrm{deg}^2$ ($46.8\,\mathrm{arcmin}^2$), which is a twofold increase compared to previous works. As for previous Faraday rotation sky reconstructions, this work is based on information field theory, a Bayesian inference scheme for field-like quantities which handles noisy and incomplete data. In contrast to previous reconstructions, we find a significantly thinner and pronounced Galactic disc with small-scale structures exceeding values of several thousand $\mathrm{rad}\,\mathrm{m}^{-2}$. The improvements can mainly be attributed to the new catalog of Faraday data, but are also supported by advances in correlation structure modeling within numerical information field theory. We furthermore give a detailed discussion on statistical properties of the Faraday rotation sky and investigate correlations to other data sets.

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