Ab-initio Comptonization in Reconnecting Current Sheets of X-ray Binary Coronae

Black hole accretion disks in X-ray binaries (XRBs) phenomenologically display two different X-ray emission states: a soft state with a low-energy thermal peak and a subdominant non-thermal component, and a hard state with a broad spectrum peaking in hard X-rays. The latter is of great interest from a plasma physics perspective, as the hard X-rays are believed to be powered by some form of plasma heating in the magnetized corona. The energization mechanism is not established, and one likely possibility is that it is fueled by the large-scale magnetic reconnection resembling Solar flares. We perform 2D particle-in-cell simulations of magnetic reconnection that self-consistently follow the production of hard X-rays. Our simulations follow ab-initio Compton scattering of seed photons in the reconnection region and include photon-photon pair creation, which affects the high-energy tail of the spectrum. We investigate the reconnection layer of a moderate optical depth and a high compactness parameter relevant to XRB coronae, and find that it is capable of producing the observed hard-state spectrum of Cyg X-1.