On the origin of spectral states in accreting black holes

Abstract

The origin of dramatically different electron distributions responsible for Comptonization in black hole X-ray binaries (BHBs) in their various states is discussed. We solve the coupled kinetic equations for photons and electrons without approximations on the relevant cross-sections accounting for Compton scattering, synchrotron radiation, and Coulomb collisions. In the absence of external soft photons, the electrons are efficiently thermalized by synchrotron self-absorption and Coulomb scattering even for pure nonthermal electron injection. The resulting quasi-thermal synchrotron self-Compton spectra have very stable slopes and electron temperatures similar to the hard states of BHBs. The hard spectral slopes observed in the X-rays, the cutoff at 100 keV, and the MeV tail together require low magnetic fields ruling out the magnetic dissipation mechanism. The motion of the accretion disk toward the black hole results in larger Compton cooling and lower equilibrium electron temperature. Our self-consistent simulations show that in this case both electron and photon distributions attain a power-law-dominated shape similar to what is observed in the soft state. The electron distribution in the Cyg X-1 soft state might require a strong magnetic field, being consistent with the magnetically dominated corona.