Evolution of the primary BH spin during the collapse of the stellar envelope. Blue lines are for a_1,init = 0.1 and red lines for a_1,init = 0.85. The solid and dashed lines show the models with the envelope’s angular momentum normalized with x = l_spec/l_crit = 1.5 and 7, respectively. The model assumes homologous accretion of the envelope shells onto the BH and no wind. The envelope is spun up by the companion BH of a mass M₂ = 3  M_⊙. The time is given as the free-fall timescale, so it scales with M₁.