Spectropolarimetry of AT 2021biy. The top panel shows the total-flux spectra in flux arbitrary units. The second, third, and fourth panels display the Stokes q, u, and polarization p spectra, respectively. The bottom panel gives the polarization position angle, PA. The vertical cyan bands show the regions affected by telluric bands. With the exception of the total-flux spectra, data are binned to 200 Å and 300 Å for the first and second epochs (respectively), for clarity of presentation.