F_P ∝ F_SXR relations for the four integral energy bands of the SEP events: E  >  10 (top left); E  >  30 (top right); E  >  60 (bottom left); and E  >  100 MeV (bottom right). The log-log relations are obtained with the RMA regression fitting (solid black line). The estimated upper limits of F_P in terms of F_SXR based on Takahashi et al. (2016) are depicted as solid orange lines in each panel. The dashed black lines are similar to the RMA line scaled to the uppermost point of the sample.