The FAST-SETI Milky Way Globular Cluster Survey I: A Pilot Multibeam On-the-Fly Search of Five Globular Clusters at L-Band

TL;DR

This work presents the first FAST technosignature survey targeting globular clusters using a multibeam on-the-fly raster (MBOTF) and a geometry-aware MBPS analysis with generalized on-target windows. By gating detections to gOTWs and enforcing deterministic beam-order and cross-pass coherence, the authors robustly suppress RFI and derive tight L-band EIRP limits of approximately ${ m EIRP}_{ m min} oughly (0.72{--}1.8) \times 10^{16}$ W for distances of 4–6.5 kpc, with stacking able to improve sensitivity by up to a factor of ${\sqrt{N}}$ when crossings align. Across five metal-rich globular clusters, the survey records ${\sim}2.75\times10^{5}$ raw narrowband hits but finds no candidate meeting all MBPS criteria, yielding a robust null result and placing FAST in the low-left region of the CW rate–sensitivity plane. The study demonstrates a scalable, geometry-driven observing and verification blueprint for extended-field technosignature searches, enabling future multi-epoch campaigns to deepen constraints on transmitters in dense stellar systems. These results significantly constrain bright, persistent L-band beacons in globular clusters and establish a practical methodology for expanding searches to more clusters and signal morphologies.

Abstract

We report a narrowband technosignature search toward five Milky Way globular clusters (NGC 6171, NGC 6218, NGC 6254, NGC 6838, and IC 1276) using the Five-hundred-meter Aperture Spherical radio Telescope (FAST) 19-beam L-band receiver (1.05-1.45 GHz). We adapt the MultiBeam Point-source Scanning (MBPS) strategy to extended targets by gating detections to generalized on-target windows (gOTWs), i.e. the time intervals when a beam main lobe intersects a buffered cluster mask, and by enforcing the deterministic multibeam illumination sequence as a geometry test. Dynamic spectra with frequency resolution about 7.5 Hz and time resolution about 10 s are searched with turboSETI over drift rates |nu_dot| <= 4 Hz s^-1 at signal-to-noise ratio S/N >= 10. From about 2.75e5 raw hits across both linear polarizations, none survive the gOTW gating, array-wide simultaneity veto, in-stripe ordering, and single-drift coherence checks, yielding a robust null result. With system equivalent flux density SEFD about 1.5 Jy and an effective 60 s per illuminated crossing, our per-crossing flux density threshold is S_min about 0.50 Jy, corresponding to minimum isotropic-equivalent radiated power EIRP_min in the range (0.72-1.8)e16 W for cluster distances 4-6.5 kpc; when multiple illuminated crossings occur, non-coherent stacking improves sensitivity by up to sqrt(N). To our knowledge this is the first FAST technosignature survey dedicated to globular clusters and the first to use MBPS as the primary observing strategy. These limits disfavor bright, persistent, isotropic L-band beacons above the stated thresholds during our epochs and establish a scalable blueprint, based on geometry-aware gating and verification, for multi-epoch MBPS campaigns that expand signal morphologies and combine passes to deepen constraints on transmitters in dense stellar systems.

Figures (6)

• Figure 1: Schematic illustration of the OTF raster scan strategy, an adaptation of the MBPS technique for extended targets like the globular cluster NGC 6254 (central yellow circle). The observation follows the precise path A→B→C→D to ensure complete coverage. The first scan (Pass 1) commences at point A, with the 19-beam array (cyan) slewing in the negative right ascension direction to point B. A single, precise step in declination ($\Delta\text{Dec} \simeq 2.49'$) is then executed to move the array to point C. From here, the second scan (Pass 2) begins, slewing in the reverse right ascension direction (coral) and concluding the observation at point D. The shaded cyan and coral regions illustrate the sky coverage traced by each of the 19 beams. This two-pass methodology provides complete and overlapping coverage of the target's full angular extent. Crucially, this strategy preserves the core principle of MBPS: a genuine extraterrestrial signal must appear in the strict temporal and spatial sequence of on-target beam crossings within each pass, a pattern that terrestrial RFI is highly unlikely to replicate.
• Figure 2: The on-sky layout of the 19-beam L-band receiver of the Five-hundred-meter Aperture Spherical radio Telescope (FAST). For the observation presented in this work, the beams were organized into four distinct Beam Groups (BGs), labeled A through D. This grouping corresponds to the sequence in which different sets of beams scanned across the target globular cluster. Each group is highlighted by a unique, semi-transparent shaded region, with the color coding specified in the legend.
• Figure 3: Simulated appearance of a geometrically consistent narrowband signal under MBPS scanning. Each subpanel (M08, M02, M01, M05, M14) represents one beam of a single beam group (BG) during a raster scan, showing normalized power versus time and relative frequency from 1300 MHz. The injected tone drifts at $\dot \nu=-0.41$ Hz s$^{-1}$ and is visible sequentially as the main lobe of each beam sweeps over the source location. The recurrence pattern, $\sim115$ s timing separations, and consistent drift rate satisfy the MBPS geometric criteria (C1–C4): the signal appears only inside gOTWs, does not illuminate all beams simultaneously, recurs with the correct in-stripe order, and maintains a single drift-rate solution with amplitudes following the beam-response envelope. This example illustrates what a genuine sky-localized technosignature would look like under the FAST MBPS observation strategy.
• Figure 4: Distribution of signal hits detected across the five GCs. The hit density is extremely high in two primary regions. A saturated block of hits from 1220--1300 MHz corresponds to the navigation satellite band (green), rendering it unusable. A dense, but resolved, cluster of hits aligns with the civil aviation allocation (blue, 1030--1140 MHz). Notably, a strong, isolated hit peak at 1381.05 MHz is also explained by a navigation satellite downlink (GPS L3),
• Figure 5: Representative RFI morphologies in FAST GC observations. All four examples fail the MBPS verification sequence (C1–C5): (a,b) multibeam simultaneity with flat gain envelopes (C1–C3); (c) single-feed, irregular track (C4); (d) array-wide coherence inconsistent with in-stripe sequencing (C2–C3).