X-ray polarization properties of stellar-mass black holes in soft state: theory and first observations.

Abstract: Black holes serve as natural laboratories for investigating the effects of strong gravitational fields in the universe. Accreting stellar-mass black holes are highly variable sources that transition through different accretion states, exhibiting a range of X-ray spectral components. In sources in the thermally-dominated state, X-ray emission originates from the innermost regions of the accretion disk, where the black hole intense gravitational field significantly influences the radiation properties. The polarization of this emission is shaped by several effects, including scattering and absorption processes within the disk atmosphere, as well as relativistic effects during its propagation towards the observer. Since these effects depend on the black hole spin and the accretion disk inclination, X-ray polarimetric observations can provide valuable insights into these parameters. In this talk, I will discuss IXPE main findings on stellar-mass black holes in soft state and explore how these observations open new avenues for understanding these extreme systems.