In the local Universe as well as in more distant galaxies, detected black holes (using electromagnetic or gravitational waves) mostly fall into two categories in terms of their mass: stellar black holes (<100 Msun) and massive black holes (>10^5 Msun). The intermediate-mass black holes (IMBHs, 100 Msun < M < 10^5 Msun) appear to be rather rare, which is most likely due to several characteristics, such as the environment where they form and their subsequent accretion mode. I will start the talk by reviewing IMBH formation mechanisms, with the focus on their occurrence in galactic nuclei, which are the densest systems in galaxies. Then I will list several specific examples, where supposedly IMBHs could be present, including the Galactic center. In the rest of my talk, I will describe in more detail the extragalactic source first identified by the ASASSN survey, which exhibited quasiperiodic enhancements in the soft X-ray absorption every 8.5 days. In other words, the source has a quasiperiodic ultrafast outflow that can obscure the inner accretion flow. Using analytical as well as GRMHD simulations, we show that the source properties are best described by an IMBH orbiting around the SMBH.

Location: ASU Praha-Spořilov