Cosmic backgrounds from miniquasars

A large population of intermediate-mass black holes (IMBHs) might be produced at early cosmic times as a leftover of the evolution of the very massive first stars. Accretion on to IMBHs provides a source of (re) ionizing radiation. We show that the baryon mass fraction locked into IMBHs and their growth is strongly constrained by the observed residual soft Xray background (SXRB) intensity. Thus, unless they are extremely X-ray quiet, miniquasars must be quite rare and/or have a short shining phase. As a byproduct, we find that miniquasars cannot be the only source of reionization and that their alleged contribution to the near-infrared bands is completely negligible.