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The $100^\circ$-long thin stellar stream in the Milky Way halo, GD-1, has anensemble of features that may be due to dynamical interactions. Usinghigh-resolution MMT/Hectochelle spectroscopy we show that a spur of GD-1-likestars outside of the main stream are kinematically and chemically consistentwith the main stream. In the spur, as in the main stream, GD-1 has a lowintrinsic radial velocity dispersion, $\sigma_{V_r}\lesssim1\,\rm km\,s^{-1}$,is metal-poor, $\rm [Fe/H]\approx-2.3$, with little $\rm [Fe/H]$ spread andsome variation in $\rm [\alpha/Fe]$ abundances, which point to a commonglobular cluster progenitor. At a fixed location along the stream, the medianradial velocity offset between the spur and the main stream is smaller than$0.5\,\rm km\,s^{-1}$, comparable to the measurement uncertainty. A flyby of amassive, compact object can change orbits of stars in a stellar stream andproduce features like the spur observed in GD-1. In this scenario, the radialvelocity of the GD-1 spur relative to the stream constrains the orbit of theperturber and its current on-sky position to $\approx5,000\,\rm deg^2$. Thefamily of acceptable perturber orbits overlaps the stellar and dark-matterdebris of the Sagittarius dwarf galaxy in present-day position and velocity.This suggests that GD-1 may have been perturbed by a globular cluster or anextremely compact dark-matter subhalo formerly associated with Sagittarius.

High-resolution spectroscopy of the GD-1 stellar stream localizes the perturber near the orbital plane of Sagittarius