  The X-ray pulsar 1A 0535+262 exhibited a giant outburst in 2020, offering us
a unique opportunity for X-ray polarimetry of an accreting pulsar in the
supercritical state. Measurement with PolarLight yielded a non-detection in 3-8
keV; the 99% upper limit of the polarization fraction (PF) is found to be 0.34
averaged over spin phases, or 0.51 based on the rotating vector model. No
useful constraint can be placed with phase resolved polarimetry. These upper
limits are lower than a previous theoretical prediction of 0.6-0.8, but
consistent with those found in other accreting pulsars, like Her X-1, Cen X-3,
4U 1626-67, and GRO J1008-57, which were in the subcritical state, or at least
not confidently in the supercritical state, during the polarization
measurements. Our results suggest that the relatively low PF seen in accreting
pulsars cannot be attributed to the source not being in the supercritical
state, but could be a general feature.
