(1) the assumption that the BCR has zero frequency in the WH frame, and (2) the local sound speed and flow velocity. \[We used spatial windowed Fourier transform of the density and of the GP wave function to identify the background flow and determine its density (for the sound speed) and wavevector (for the velocity).\] This detailed spectral analysis gives us confidence that there is nothing going on beyond the mechanisms we have identified. It also allows us to establish that the Hawking temperature prediction is consistent with the relative amplitude of the Hawking radiation and partner waves, insofar as would be expected. More generally, it reveals the utility of windowed Fourier transform in characterizing the local structure of an inhomogeneous BEC flow.

Finally, we conclude this introduction by mentioning that related studies having some overlap with ours have been reported in Refs. [@Tettamanti; @SR]. We comment on the relation between that work and our conclusions in Sec. \[sec:comparison\].

Methods {#sec:method}
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Reference [@nphys3104] reported a step-sweeping experiment on a quasi-one-dimensional (quasi-1D) condensate. A detailed discussion of our approach to simulating this experiment is given in Appendix \[app:sim\_details\]. We describe a three-dimensional (3D) model of the experimental potential and condensate 