We present the results of the first X-ray polarimetric observation of the
low-mass X-ray binary 4U 1957+115, performed with the Imaging X-ray Polarimetry
Explorer in May 2023. The binary system has been in a high-soft spectral state
since its discovery and is thought to host a black hole. The ∼571 ks
observation reveals a linear polarisation degree of 1.9%± 0.6% and a
polarisation angle of -41^∘.8 ± 7^∘.9 in the 2-8 keV energy range.
Spectral modelling is consistent with the dominant contribution coming from the
standard accretion disc, while polarimetric data suggest a significant role of
returning radiation: photons that are bent by strong gravity effects and forced
to return to the disc surface, where they can be reflected before eventually
reaching the observer. In this setting, we find that models with a black hole
spin lower than 0.96 and an inclination lower than 50^∘ are disfavoured.
