Estimated growth history of PSO J0309+27 (red star). Solid lines represent the best fit cases, under which the corresponding λ_Edd and η are reported. The shaded horizontal regions correspond to the expected mass ranges of Pop III remnants BHs (M_seed ≤ 10² M_⊙, grey), stellar dynamical processes (M_seed ≥ 10³ − 10⁴ M_⊙, cyan), and direct collapse BHs (M_seed ∼ 10⁴ − 10⁶ M_⊙, dark cyan). The values of the different seed black holes are taken from Valiante et al. (2016).