Posterior probability density distribution of the accretion-to-jet energy conversion efficiency η = E_j/M_ac² in GW170817 (solid red line), assuming f_w = 0.3 and f_γ = 0.15. The dashed orange line shows the same quantity, but with the disc mass in GW170817 required to be equal or greater than 0.04 M_⊙ (Radice & Dai 2019). Black error bars show the expected efficiency for the Blandford & Znajek (1977) and annihilation mechanisms, according to different prescriptions (see text).