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Polarization measurements of gamma-ray burst (GRB) afterglows are a promisingmeans of probing the structure, geometry, and magnetic composition ofrelativistic GRB jets. However, a precise treatment of instrumental calibrationis vital for a robust physical interpretation of polarization data, requiringtests of and validations against potential instrumental systematics. Weillustrate this with ALMA Band 3 (97.5 GHz) observations of GRB 171205A taken$\approx5.19$ days after the burst, where a detection of linear polarizationwas recently claimed. We describe a series of tests for evaluating thestability of polarization measurements with ALMA. Using these tests tore-analyze and evaluate the archival ALMA data, we uncover systematics in thepolarization calibration at the $\approx0.09\%$ level. We derive a 3$\sigma$upper limit on the linearly polarized intensity of $P<97.2~\mu$Jy,corresponding to an upper limit on the linear fractional polarization of$\Pi_{\rm L}<0.30\%$, in contrast to the previously claimed detection. Ourupper limit improves upon existing constraints on the intrinsic polarization ofGRB radio afterglows by a factor of 3. We discuss this measurement in thecontext of constraints on the jet magnetic field geometry. We present acompilation of polarization observations of GRB radio afterglows, anddemonstrate that a significant improvement in sensitivity is desirable foreventually detecting signals polarized at the $\approx0.1\%$ level from typicalradio afterglows.

Radio linear polarization of GRB afterglows: Instrumental Systematics in ALMA observations of GRB 171205A