ИСТИНА

The origin of supermassive black holes (SMBH) in galaxy centers still remains uncertain. They could have emerged either from massive “seeds” (100k-1M MSun) in the early Universe or from smaller (100 MSun) remnants of massive pop-III stars. The latter scenario would leave behind numerous intermediate-mass black holes (IMBHs, 100-100k MSun), while the former one would lead to a gap in the BH mass function. The largest published sample of bona-fide IMBH-powered AGN contains 14 objects confirmed in X-ray. Here we present X-ray confirmation of ten new optically selected IMBH candidates from a sample of 305 objects from Chilingarian et al. (2018): three with our own XMM-Newton observations, two from the source catalog 4XMM DR9, and five from Chandra archival data. With the expanded sample of bona fide IMBHs we probed BH accretion rates in low-mass AGN and identified five galaxies close to the Eddington limit, which is an unusually high fraction provided the total sample size of 20. If, on average, nuclear IMBHs indeed grow very fast once they start accreting, this will explain why IMBH search campaigns are challenging: after a few tens of Myrs of steady accretion they will overgrow the informal IMBH mass bracket.