Several mechanisms for producing hard ionizing radiation have been proposed in the literature, such as WR stars [65], primordial (zero-metallicity) stars, high-mass X-ray binaries (HMXB; [66]), radiative shocks [67], and O stars at low metallicity [62].
We discuss, in [54], the possibility that the He II [lambda]4686 emission line, in HS2236 + 1344, is associated with WR stars, high-mass X-ray binaries (HMXBs), O stars at low metallicities, and/or a low-luminosity Active Galactic Nucleus.