arXiv subjects

Michael A. Garrett

Publications and source records attributed to Michael A. Garrett.

At least 19 recordsLinked to original sources

arXiv2026

Archival Diagnostics for Potential Background Contaminantsof Project Hephaistos Dyson Sphere Candidates

We report on the diagnostic investigation of nine Project Hephaistos Dyson Sphere candidate M-dwarfs based on archival data. By comparing the Gaia DR3 positions with epoch 2016.0, propagated to the AllWISE epoch, with the mid-infrared centroids measured from the AllWISE images, together with deep archival optical/near-infrared imaging, we identified significant background contamination in candidates B and C--candidate B coincident with a radio counterpart showing a power-law-like spectrum with a radio spectral index alpha = 0.63 +/- 0.11, while candidate C has a near-infrared companion at an offset of 3.75 arcsec. Candidate A provides suggestive evidence through a radio counterpart with spectral index alpha = 0.40 +/- 0.35, while candidates E, F, H and J show marginal evidence. These systems exhibit either significant astrometric offsets or visible interlopers, indicating that the mid-infrared excess likely arises from line-of-sight contamination by hot, dust-obscured galaxies. However, candidates D and I still lack obvious signs of contamination. Dedicated observations are therefore essential to characterise these potential interlopers, eliminate false positives, and ensure that technosignature searches focus on the most robust Dyson Sphere candidates.

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arXiv2026

Project Hephaistos -- IV. James Webb Space Telescope Observations of Two Dyson Sphere Candidates

We report on JWST/MIRI imaging and spectroscopy of two M-dwarf stars previously singled out by project Hephaistos as potential Dyson-sphere candidates (their candidates D and E) due to the presence of excess flux at mid-infrared wavelengths. We find that the infrared excess does not originate from Dysonian megastructures, or other radiation mechanisms close to these stars, but from background galaxies projected within $\sim 1$ arcsec of the M dwarfs, thereby confusing previous mid-infrared photometry obtained with the WISE telescope. The candidate D background galaxy lies at redshift $z\approx 0.9$, appears point-source dominated in imaging and has a mid-infrared spectrum consistent with being a Hot Dust Obscured Galaxy (Hot DOG). The candidate E background galaxy lies at $z\approx 0.4$, displays an extended morphology with bright knots and a spectrum consistent with a dusty starburst.

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arXiv2026

SETI's blind spot: Technological Acceleration and fleeting technosignatures

The search for extraterrestrial intelligence (SETI) has traditionally framed the detection challenge with a focus on the parameter $L$ in the Drake equation - the communicative lifetime of a civilisation. I argue that the more pertinent quantity is $τ_d$, the duration during which a civilisation produces technosignatures that are \emph{actually detectable by us, now}. Modelling technological progress as an exponential process, we show that $τ_d = α^{-1}\ln(K_{\rm max}/K_{\rm min})$, where $α$ is the rate of technological acceleration and $[K_{\rm min},K_{\rm max}]$ brackets the technology levels accessible to our instruments. As $α$ increases, $τ_d$ can shrink to mere decades, dramatically narrowing the window in which civilisations overlap technically. This ``technology mismatch'' has implications for future search strategies, emphasising broadband and technology-agnostic approaches, as well as anomaly detection across multi-wavelength/messenger survey data.

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arXiv2026

Searching for Extraterrestrial Intelligence with the SKA

The search for technosignatures (also known as the Search for Extraterrestrial Intelligence or SETI) depends critically on our ability to distinguish artificial signals from the rich complexity of natural astrophysical phenomena and radio frequency interference from anthropogenic emissions. As the search for technosignatures increasingly aligns with mainstream astrophysics, complementing the search for biosignatures, it demands not only sophisticated statistical and computational approaches, but also deep domain knowledge across the electromagnetic spectrum. The SKA will play a pivotal role in the next-generation of technosignature searches, providing an unprecedented combination of sensitivity, field of view, and spatial resolution over its wavelength range. Integrating wide-field, high-resolution observations with machine learning and multi-wavelength diagnostics will represent key steps forward. The SKA's singular capabilities will render it an indispensable instrument for the rapid identification and follow-up characterisation of promising technosignature candidates. In this chapter, we discuss the request for high temporal and spectral resolution data products with a main focus on frequency-domain SETI. Nevertheless, multiple SKA observing modes have the potential to substantially advance technosignature research.

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arXiv2026

Isolating Broadband Radio Technosignatures (BRaTs): A Framework for Detecting Planetary-Scale Leakage

The search for extraterrestrial intelligence (SETI) has traditionally focused on the detection of narrowband electromagnetic beacons. However, terrestrial technology is increasingly evolving toward distributed, low-power, wideband digital infrastructure. The strict adherence to narrowband filtering that characterises most SETI surveys, therefore, risks discarding the aggregate leakage signatures of advanced civilisations by systematically misclassifying them as unstructured noise. We investigate the feasibility of detecting such planetary-scale broadband radio technosignatures (BRaTs) using a hierarchical observational framework. In this tiered approach, wide-field radio surveys conducted by next-generation arrays (such as the SKA and its precursors) perform the initial deep-field observations, with targeted Very Long Baseline Interferometry (VLBI) providing the definitive, high-resolution follow-up. Because broadband continuum emission is largely insensitive to Doppler drift, long-duration "SETI Deep Fields" are observationally viable, extending the accessible detection volume for Kardashev Type I leakage to 100 pc. To distinguish these signals from other astrophysical confounders, a multi-parameter diagnostic framework is proposed. Candidate technosignatures are identified through a convergence of high brightness temperatures, negligible circular polarisation, spectral non-uniformity, interstellar scintillation, and sub-milliarcsecond astrometric co-motion with nearby Galactic stars/exoplanets.

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arXiv2026

Upper Limits on Radio Emission from the K2-18 System

Stellar and planetary magnetic fields play a crucial role in the habitability of a planet and the integrity of its atmosphere. The recently claimed detection of biosignatures, methane, carbon dioxide and dimethyl sulfide/disulfide, in the atmosphere of K2-18 b, a sub-Neptune orbiting an M dwarf star present an intriguing question regarding the stellar magnetic environment and the resistance of the planet's magnetosphere (if it exists) to erosion by magnetic activity from the host. To probe for radio emission from the system, we have conducted observations using the Karl G. Jansky Very Large Array (VLA) at S, C and X-bands (2-4, 4.5-7.5 and 8-10 GHz respectively) to search for coherent and incoherent radio emission. We detect no radio emission associated with incoherent emission mechanisms. We report $3σ$ Stokes I upper limits of $49.8\ μ\rm{Jybeam}^{-1}$ at S-band, $17.7\ μ\rm{Jybeam}^{-1}$at C-band and $18.0\ μ\rm{Jybeam}^{-1}$ at X-band and an upper limit of the ratio of the radio to the total bolometric luminosity of $\log L_\text{R}/\log L_\text{bol}<-8.8$. We have also searched for short duration bursts associated with coherent emission mechanisms at C and X-bands . No signals above a $3σ$ significance threshold are detected. Although no signals are detected our radio observations offer constraints, albeit limited, on the stellar magnetic environment supporting recent X-ray observations indicating K2-18 is a very faint emitter. Our results also contextualise any planetary transmission spectra by providing constraints on the activity level of the host.

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arXiv2026

Radio Activity from the Rapidly Rotating T dwarf 2MASS 2228-4310

We present the detection of 2MASS J22282889-4310262 (2M2228), a T6/T6.5 brown dwarf, using the Karl G. Jansky Very Large Array (VLA) archival data observed at C band (4-8 GHz) over two observing epochs ($2\times96$ minutes). 2M2228 is detected at time and frequency averaged Stokes I and V peak flux densities of $67.3\pm4.9\ μ \rm{Jy beam}^{-1}$ and $14.4\pm3.0\ μ\text{Jy beam}^{-1}$ in the first epoch and $107.2\pm5.2\ μ\rm{Jy\ beam}^{-1}$ and $-20.7\pm1.2\ μ\text{Jy beam}^{-1}$ in the second epoch. This discovery constitutes the eighth and, notably, the most rapidly rotating T dwarf detected to date at radio wavelengths. Our observations reveal highly polarised bursts at fractional polarisation ratios $f_\text{c}>50$%. Using Stokes I light curves, we measure occurrence intervals of $\sim47$ and $\sim58$ minutes in the two observing epochs respectively with the first burst aligning within a half period timescale of the the previously measured mid infrared photometric period of $85.8\pm0.32$ minutes. We attribute the emission to the electron cyclotron maser emission (ECME) and constrain the magnetic field strength to $B\gtrsim1.4$ kG. We emphasise that the periods inferred are provisional considering the short observing durations. The combination of previously demonstrated atmospheric stability and newly detected radio emission in 2M2228 makes it a promising laboratory for testing magnetospheric currents-driven auroral models and for guiding future coordinated James Webb Space Telescope (JWST) and radio observations to probe the link between auroral activity and atmospheric dynamics in T-type brown dwarfs.

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arXiv2025

Simulating the Stellar Bycatch: Constraining the Prevalence of Extraterrestrial Transmitters within Radio SETI Surveys

Searches for radio technosignatures place constraints on the prevalence of extraterrestrial transmitters in our Galaxy and beyond. It is important to account for the complete stellar population captured within a radio telescope's field of view, or stellar 'bycatch'. In recent years, catalogues from ESA's Gaia mission have enabled SETI surveys to place tighter limits on extraterrestrial transmitter statistics. However, Gaia remains restricted by magnitude limits, astrometric uncertainty at large distances, and confusion in crowded regions. To address these limitations, we investigate the use of the Besançon Galactic Model to simulate the statistical underlying stellar population to derive more realistic constraints on the occurrence of extraterrestrial transmitters. We apply this method to Breakthrough Listen's Enriquez/Price survey, modelling 6,182,364 stellar objects within 1229 individual pointings and extending the search out to distances $\leq 25$kpc. We place limits on the prevalence of high duty cycle transmitters within 2.5kpc, suggesting $\leq (0.000995 \pm 0.000002)\%$ of stellar systems contain such a transmitter (for near-zero drift rates and EIRP$_{\mathrm{min}} \gtrsim 5 \times 10^{16}$W). In support of broader adoption, we provide a simple calculator tool that enables other researchers to incorporate this approach into their own SETI analyses. Our results enable a more complete statistical estimation of the number and stellar type of systems probed, thereby strengthening constraints on technosignature prevalence and guiding the analysis of future SETI efforts. We also conclude that SETI surveys are, in fact, much less biased by anthropocentric assumptions than is often suggested.

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arXiv2025

Radio Emission from a Nearby M dwarf Binary

We present the detection of the binary system 2MASS J02132062+3648506 AB using the Karl G. Jansky Very Large Array (VLA) archive data observed at 4-8 GHz. The system is a triple consisting of a tight binary ($\sim0.2"$) of two M dwarfs of spectral class M4.5 and M6.5 and a wide T3 brown dwarf companion ($\sim$16.4"). The binary displays coronal and chromospheric activity as traced by previously measured X-ray flux and H$α$ emission. We detect the unresolved binary at a peak flux density of $\sim356\ μ\mathrm{Jybeam}^{-1}$ at a signal-to-noise ratio (SNR) of $\sim36$ and determine a radio luminosity of $\mathrm{log}L_R/\mathrm{log}L_\mathrm{bol}\approx-7.76$. The radio emission is quiescent, polarised at a mean circular polarisation fraction $f_\mathrm{c}=45.20 \pm 1.58$ % and exhibits a spectral index $α=-0.44\pm0.07$ . We probe the binary using the Enhanced Multi-Element Remotely Linked Interferometer Network (e-MERLIN) with an angular resolution of $\sim40$ mas at 5 GHz and detect a component at a peak flux density of $\sim90\ μ$Jy $\mathrm{beam}^{-1}$ at a SNR $\sim5$ . We propose a gyrosynchrotron origin for the radio emission and estimate a magnetic field strength $B<174.86$ G, an emitting region of size $L<1.54$ times the radius of the M4.5 primary and a plasma number density $n_\mathrm{e}<2.91\times10^5\ \mathrm{cm}^{-3}$. The brown dwarf companion is not detected. Additionally, we have analysed observations of 2MASS J04183483+213127, a chromospherically active L5 brown dwarf which is also not detected. Accordingly, we place $3σ$ flux density upper limits at $36.9\ μ$Jy $\mathrm{beam}^{-1}$ and $42.3\ μ$Jy $\mathrm{beam}^{-1}$ for Stokes I and V respectively.

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arXiv2025

SETI Post-Detection Protocols: Progress Towards a New Version

The International Academy of Astronautics (IAA) SETI Committee has long provided guiding principles for responding to a potential detection of a SETI signal. The foundational Declaration of Principles Concerning Activities Following the Detection of Extraterrestrial Intelligence, first formulated in 1989, has been widely recognised by the international scientific community. A supplemental set of draft protocols addressing the possibility of a reply to an extraterrestrial signal was prepared in 1995 by the IAA SETI Permanent Committee, with both documents presented in a position paper to the UN Committee on the Peaceful Uses of Outer Space in 2000. In keeping with the evolving landscape of SETI research, the IAA Declaration of Principles was streamlined and updated in 2010. Recognising the need for continued adaptation, the IAA SETI Committee established a Task Group in 2022 to re-examine the protocols in light of recent advances in search methodologies, the expansion of international participation in SETI, and the increasing complexity of the global information environment. The Group recognises the living document nature of the protocols, which will require ongoing refinement to remain relevant and effective in a rapidly changing world. A draft revised Declaration of Principles was presented at the IAC 2024 in Milan, and initial feedback was received from the community, particularly members of the IAA SETI Committee. Since then, we have continued to seek broader community input in a structured process, refining the proposed updates based on further discussions and consultations. A Revised Declaration of Principles, is presented here.

astro-ph.IM↗

arXiv2025

High-resolution imaging of the radio source associated with Project Hephaistos Dyson Sphere Candidate G

We present high-resolution e-MERLIN and EVN (e-VLBI) observations of a radio source associated with Dyson Sphere candidate G, identified as part of Project Hephaistos. The radio source, VLASS J233532.86-000424.9, is resolved into three compact components and shows the typical characteristics of a radio-loud active galactic nucleus (AGN). In particular, the European VLBI Network (EVN) observations show that it has a brightness temperature in excess of $10^{8}$~K. No radio emission is detected at the position of the M-dwarf star. This result confirms our earlier hypothesis, that at least some of the Dyson Sphere candidates of project Hephaistos are contaminated by obscured, background AGN, lying close to the line of sight of otherwise normal galactic stars. High-resolution radio observations of other Dyson Sphere candidates can be useful in distinguishing truly promising candidates from those contaminated by background sources.

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arXiv2024

Background Contamination of the Project Hephaistos Dyson Spheres Candidates

Project Hephaistos recently identified seven M-dwarfs as possible Dyson Spheres (DS) candidates. We have cross-matched three of these candidates (A, B \& G) with radio sources detected in various all-sky surveys. The radio sources are offset from the Gaia stellar positions by $\sim 4.9$, $\sim 0.4$ and $\sim 5.0$ arcseconds for candidates A, B, and G respectively. We propose that DOGs (Dust obscured galaxies) lying close to the line-of-sight of these M-dwarf stars significantly contribute to the measured WISE mid-IR flux densities in the WISE W3 and W4 wavebands. These three stars have therefore been misidentified as DS candidates. We also note that with an areal sky density of $9 \times 10^{-6}$ per square arcsecond, Hot DOGs can probably account for the contamination of all 7 DS candidates drawn from an original sample of 5 million stars.

astro-ph.SR↗

arXiv2024

The VLBA CANDELS GOODS-North Survey. I -- Survey Design, Processing, Data Products, and Source Counts

The past decade has seen significant advances in wide-field cm-wave very long baseline interferometry (VLBI), which is timely given the wide-area, synoptic survey-driven strategy of major facilities across the electromagnetic spectrum. While wide-field VLBI poses significant post-processing challenges that can severely curtail its potential scientific yield, many developments in the km-scale connected-element interferometer sphere are directly applicable to addressing these. Here we present the design, processing, data products, and source counts from a deep (11 $μ$Jy beam$^{-1}$), quasi-uniform sensitivity, contiguous wide-field (160 arcmin$^2$) 1.6 GHz VLBI survey of the CANDELS GOODS-North field. This is one of the best-studied extragalactic fields at milli-arcsecond resolution and, therefore, is well-suited as a comparative study for our Tera-pixel VLBI image. The derived VLBI source counts show consistency with those measured in the COSMOS field, which broadly traces the AGN population detected in arcsecond-scale radio surveys. However, there is a distinctive flattening in the $ S_{\rm 1.4GHz}\sim$100-500 $μ$Jy flux density range, which suggests a transition in the population of compact faint radio sources, qualitatively consistent with the excess source counts at 15 GHz that is argued to be an unmodelled population of radio cores. This survey approach will assist in deriving robust VLBI source counts and broadening the discovery space for future wide-field VLBI surveys, including VLBI with the Square Kilometre Array, which will include new large field-of-view antennas on the African continent at $\gtrsim$1000~km baselines. In addition, it may be useful in the design of both monitoring and/or rapidly triggered VLBI transient programmes.

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arXiv2023

An Interferometric SETI Observation of Kepler-111 b

The application of Very Long Baseline Interferometry (VLBI) to the Search for Extraterrestrial Intelligence (SETI) has been limited to date, despite the technique offering many advantages over traditional single-dish SETI observations. In order to further develop interferometry for SETI, we used the European VLBI Network (EVN) at $21$~cm to observe potential secondary phase calibrators in the Kepler field. Unfortunately, no secondary calibrators were detected. However, a VLBA primary calibrator in the field, J1926+4441, offset only $\sim1.88'$ from a nearby exoplanet Kepler-111~b, was correlated with high temporal $\left(0.25 \ \rm{s}\right)$ and spectral $\left(16384 \times 488\ \rm{Hz \ channels}\right)$ resolution. During the analysis of the high-resolution data, we identified a spectral feature that was present in both the auto and cross-correlation data with a central frequency of $1420.424\pm0.0002$ MHz and a width of 0.25 MHz. We demonstrate that the feature in the cross-correlations is an artefact in the data, associated with a significant increase in each telescope's noise figure due to the presence of \ion{H}{i} in the beam. This would typically go unnoticed in data correlated with standard spectral resolution. We flag (excluded from the subsequent analysis) these channels and phase rotate the data to the location of Kepler-111~b aided by the GAIA catalogue and search for signals with $\rm{SNR}>7$. At the time of our observations, we detect no transmitters with an Equivalent Isotropically Radiated Power (EIRP) > $\sim4\times10^{15}$ W.

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arXiv2023

Simulation of the Earth's radio leakage from mobile towers as seen from selected nearby stellar systems

Mobile communication towers represent a relatively new but growing contributor to the total radio leakage associated with planet Earth. We investigate the overall power contribution of mobile communication towers to the Earthś radio leakage budget, as seen from a selection of different nearby stellar systems. We created a model of this leakage using publicly available data of mobile tower locations. The model grids the planet's surface into small, computationally manageable regions, assuming a simple integrated transmission pattern for the mobile antennas. In this model, these mobile tower regions rise and set as the Earth rotates. In this way, a dynamic power spectrum of the Earth was determined, summed over all cellular frequency bands. We calculated this dynamic power spectrum from three different viewing points, HD 95735, Barnard star, and Alpha Centauri A. Our preliminary results demonstrate that the peak power leaking into space from mobile towers is $\sim 4$GW. This is associated with LTE mobile tower technology emanating from the East Coast of China as viewed from HD 95735. We demonstrate that the mobile tower leakage is periodic, direction dependent, and could not currently be detected by a nearby civilization located within 10 light years of the Earth, using instrumentation with a sensitivity similar to the Green Bank Telescope. We plan to extend our model to include more powerful 5G mobile systems, radar installations, ground based uplinks (including the Deep Space Network), and various types of satellite services, including low Earth orbit constellations such as Starlink and OneWeb.

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arXiv2023

Minor Flares on Cygnus X-3 -- VLBI Prospects

The cm-wavelength radio flares on Cygnus X-3 have been studied for many years. Our recent paper (Spencer et al., 2022) looked again at the minor flares (flux density S of a few 100 mJy) and compared their properties with those of a sample of major flares (S > 1 Jy). We find that the minor flares have rise times and duration of ~ 1 hour, as opposed to ~ days for the major flares. Minor flares show more rapid expansion of the synchrotron radiation emitting material than in the strong flares. They also appear closer to the binary, whereas the large flares form a more developed jet, i.e. the jets formed in minor flares are short and wide, those in major flares are long and thin. We used the results of Fender & Bright (2019) to calculate the magnetic field and expansion velocity as a fraction B of the speed of light under minimum energy conditions when the source is optically thick for samples of minor and major flares. The minimum power in the source was found using the rise time of the flares. The minor flares have lower minimum power but have larger velocities and energy densities than the major flares. Minor flares can occur while a major flare is in progress, suggesting an indirect coupling between them. The spectral evolution of the minor flares can be explained by either an expanding synchrotron source or a shock model. Further investigation requires high resolution VLBI observations at the 1 mas level if we wish to understand the development of the source. The problem is that Cygnus X-3 is strongly scattered by the interstellar medium so high frequencies in the several 10s of GHz are required for the resolution needed. The minor flares are rapid [...] and hence only short snapshot VLBI observations can capture the structure. Large numbers of telescopes are required which is a problem at the highest frequencies. We discuss the VLBI possibilities and trade-offs for this awkward object.

astro-ph.HE↗

arXiv2022

The science case and challenges of space-borne sub-millimeter interferometry

Ultra-high angular resolution in astronomy has always been an important vehicle for making fundamental discoveries. Recent results in direct imaging of the vicinity of the supermassive black hole in the nucleus of the radio galaxy M87 by the millimeter VLBI system Event Horizon Telescope and various pioneering results of the Space VLBI mission RadioAstron provided new momentum in high angular resolution astrophysics. In both mentioned cases, the angular resolution reached the values of about 10-20 microrcseconds. Further developments toward at least an order of magnitude "sharper" values are dictated by the needs of astrophysical studies and can only be achieved by placing millimeter and submillimeter wavelength interferometric systems in space. A concept of such the system, called Terahertz Exploration and Zooming-in for Astrophysics (THEZA), has been proposed in the framework of the ESA Call for White Papers for the Voayage 2050 long term plan in 2019. In the current paper we discuss several approaches for addressing technological challenges of the THEZA concept. In particular, we consider a novel configuration of a space-borne millimeter/sub-millimeter antenna which might resolve several bottlenecks in creating large precise mechanical structures. The paper also presents an overview of prospective space-qualified technologies of low-noise analogue front-end instrumentation for millimeter/sub-millimeter telescopes, data handling and processing. The paper briefly discusses approaches to the interferometric baseline state vector determination and synchronisation and heterodyning system. In combination with the original ESA Voyage 2050 White Paper, the current work sharpens the case for the next generation microarcsceond-level imaging instruments and provides starting points for further in-depth technology trade-off studies.

astro-ph.IM↗

arXiv2021

TeraHertz Exploration and Zooming-in for Astrophysics (THEZA): ESA Voyage 2050 White Paper

This paper presents the ESA Voyage 2050 White Paper for a concept of TeraHertz Exploration and Zooming-in for Astrophysics (THEZA). It addresses the science case and some implementation issues of a space-borne radio interferometric system for ultra-sharp imaging of celestial radio sources at the level of angular resolution down to (sub-) microarcseconds. THEZA focuses at millimetre and sub-millimetre wavelengths (frequencies above $\sim$300~GHz), but allows for science operations at longer wavelengths too. The THEZA concept science rationale is focused on the physics of spacetime in the vicinity of supermassive black holes as the leading science driver. The main aim of the concept is to facilitate a major leap by providing researchers with orders of magnitude improvements in the resolution and dynamic range in direct imaging studies of the most exotic objects in the Universe, black holes. The concept will open up a sizeable range of hitherto unreachable parameters of observational astrophysics. It unifies two major lines of development of space-borne radio astronomy of the past decades: Space VLBI (Very Long Baseline Interferometry) and mm- and sub-mm astrophysical studies with "single dish" instruments. It also builds upon the recent success of the Earth-based Event Horizon Telescope (EHT) -- the first-ever direct image of a shadow of the super-massive black hole in the centre of the galaxy M87. As an amalgam of these three major areas of modern observational astrophysics, THEZA aims at facilitating a breakthrough in high-resolution high image quality studies in the millimetre and sub-millimetre domain of the electromagnetic spectrum.

astro-ph.IM↗