Phosphorus-bearing molecules in the L1157-B1 protostellar shock. Left: emission map of the PN N = 2–1 line obtained with NOEMA in the southern lobe of the L1157 outflow. The emission (colour) is integrated in the velocity range −10 to +5 km s⁻¹. The first contour and contour interval (blue) are 3σ (36 mJy beam⁻¹ km s⁻¹) and 2σ, respectively. The dashed white circle shows the primary beam of the PN image (53″). The CO J = 1–0 outflow emission, as imaged by Gueth et al. (1996) is superimposed in white contours. The white triangle at the top points to the position of the L1157-mm protostar. Right: magnified view of the L1157-B1 central region. The maps are centred at: α(J2000) = 20^h39^m 10^s.2, δ(J2000) = +68°01′10.″5. The PN N = 2–1 map is compared with the stacked intensity distribution of PO J = 5/23/2, F = 3–2, ef (cyan thick contours). The first contour and contour interval of the PO map are 3σ (21 mJy beam⁻¹ km s⁻¹) and 2σ respectively. The 3σ level of the p-H₂CO J = 2_0,2−1_0,1 integrated emission (Benedettini et al. 2013; grey contours) is here used to define the whole structure of the B1 shock. The cyan ellipse draws the synthesised beam of the PO observations, and shock knots B1a-B1b-B1c-B0e are marked by yellow triangles.