The Physics of Hybrid Thermal/Non-Thermal Plasmas

Models of the continuum radiation from accreting hot plasmas typically assume that the plasma heating mechanism produces energetic particles distributed in energy either as a Maxwellian (the “thermal” models) or as an extended power law (the “non-thermal” models). The reality, however, is that neither description is probably accurate. In other astrophysical contexts where we have been able to observe the actual particle energy distributions, e.g. solar system plasmas, and in many particle acceleration theories, the heating mechanism supplies only some fraction of the available energy to very energetic particles. The remainder goes into producing lower energy particles which settle into a quasi-Maxwellian energy distribution. Here, I review the arguments for “thermal” versus “non-thermal” plasmas in accreting black hole systems and discuss the physics and emission properties of “hybrid” plasmas, where the particle distribution energy is approximately a Maxwellian plus a power law tail. Using results from a new emission code, I then show that such plasmas may be relevant to explaining recent observations, particularly those of Galactic black hole candidates in their soft state.