Non-thermal emission from the plunging region: a model for the high-energy tail of black hole X-ray binary soft states

ABSTRACT X-ray binaries exhibit a soft spectral state comprising thermal blackbody emission at 1 keV and power-law tail above 10 keV. Empirical models fit the high-energy to radiation from non-thermal electron distribution, but physical location of electrons reason for their index cut-off are still largely unknown. Here, we propose that originate within black hole’s innermost stable circular orbit (the ‘plunging region’). Using an analytic model plunging region dynamics distribution function properties particle-in-cell simulations, outline steady-state can reproduce observed features. In particular, our reproduces photon indices ? ? 2 luminosities order few per cent disc luminosity strong magnetic fields, consistent with observations state. Because originates so close hole, predict should strongly depend on system inclination angle hole spin. This could be extended tails 400 in hard binaries.