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GW200208_222617 as an eccentric black-hole binary merger: properties and astrophysical implications

Detecting orbital eccentricity in a stellar-mass black-hole merger would point to a non-isolated formation channel. Eccentric binaries can form in dense stellar environments such as globular clusters or active galactic nuclei, or from triple stellar systems in the Galactic field. However, confidently measuring eccentricity is challenging -- short signals from high-mass eccentric mergers can mimic spin-induced precession, making the two effects hard to disentangle. This degeneracy weakens considerably for longer-duration signals. Here, GW200208_222617 provides a rare opportunity. Originating from a relatively low-mass binary with source-frame chirp mass $\sim20$ M$_\odot$, its gravitational-wave signal spanned $\sim14$ orbital cycles in band, with no indication of data quality issues. Previous analyses for quasi-circular binaries found no evidence for spin precession, and multiple subsequent studies found the data to favour an eccentric merger despite notable technical differences. All in all, we believe GW200208_222617 is the black-hole merger event from GWTC-3 with the least ambiguous detection of eccentricity. We present a critical discussion of properties and astrophysical interpretation of GW200208_222617 as an eccentric black-hole merger using models of field triples, globular clusters, and active galactic nuclei. We find that if GW200208_222617 was indeed eccentric, its origin is consistent with a field triple or globular cluster. Formation in the inner regions of an active galactic nucleus is disfavoured. The outer regions of such a disk remain a viable origin for GW200208_222617; we demonstrate how future detections of eccentric mergers formed in such environments could be powerful tools for constraining the disk geometry.