The FAST-SETI Milky Way Globular Cluster Survey. I. A Pilot Multibeam On-the-fly Search of Five Globular Clusters at the <i>L</i> Band

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 Five-hundred-meter Aperture Spherical radio Telescope (FAST)’s 19 beam L -band receiver (1.05–1.45 GHz). We adapted MultiBeam Point-source Scanning (MBPS) to extended targets by gating detections to generalized on-target windows (gOTWs), i.e., the time intervals when a beam’s main lobe intersects a buffered cluster mask, and by enforcing the deterministic multibeam illumination sequence as a geometry test. Spectra at Δ ν ≈ 7.5 Hz resolution and Δ t ≈ 10 s cadence were searched with turboSETI over <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mo stretchy="false">∣</mml:mo> <mml:mover accent="true"> <mml:mrow> <mml:mi>ν</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>̇</mml:mo> </mml:mrow> </mml:mover> <mml:mo stretchy="false">∣</mml:mo> <mml:mo>≤</mml:mo> <mml:mn>4</mml:mn> <mml:mspace width="1em"/> <mml:mi mathvariant="normal">Hz</mml:mi> <mml:mspace width="0.25em"/> <mml:msup> <mml:mrow> <mml:mi mathvariant="normal">s</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> </mml:math> at S/N ≥ 10. From 2.75 × 10 5 raw hits across both linear polarizations, none survived the gOTW gating, array-wide simultaneity veto, in-stripe ordering, and single-drift coherence checks, yielding a robust null result. With SEFD ≈ 1.5 Jy and an effective ∼60 s per illuminated crossing, our per-crossing flux density threshold is <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>S</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>min</mml:mi> </mml:mrow> </mml:msub> <mml:mo>≈</mml:mo> <mml:mn>0.50</mml:mn> </mml:math> Jy, corresponding to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">EIRP</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>min</mml:mi> </mml:mrow> </mml:msub> <mml:mo>≈</mml:mo> <mml:mo stretchy="false">(</mml:mo> <mml:mn>0.72</mml:mn> <mml:mspace width="0.1em"/> <mml:mtext>–</mml:mtext> <mml:mspace width="0.1em"/> <mml:mn>1.8</mml:mn> <mml:mo stretchy="false">)</mml:mo> <mml:mo>×</mml:mo> <mml:mn>1</mml:mn> <mml:msup> <mml:mrow> <mml:mn>0</mml:mn> </mml:mrow> <mml:mrow> <mml:mn>16</mml:mn> </mml:mrow> </mml:msup> </mml:math> W for cluster distances of 4–6.5 kpc; when multiple illuminated crossings occur, noncoherent stacking improves sensitivity by up to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msqrt> <mml:mrow> <mml:mi>N</mml:mi> </mml:mrow> </mml:msqrt> </mml:math> . 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 disfavour bright, persistent, isotropic L -band beacons above the stated thresholds during our epochs and establish a scalable blueprint—geometry-aware gating and verification—for multiepoch MBPS campaigns that expand signal morphologies and combine passes to deepen constraints on transmitters in dense stellar systems.