Electron and Proton Heating in Transrelativistic Magnetic Reconnection

Hot collisionless accretion flows, such as the one in Sgr A* at our Galactic center, provide a unique setting for the investigation of magnetic reconnection. Here protons are nonrelativistic, while electrons can be ultrarelativistic. By means of 2D particle-in-cell simulations, we investigate electron and proton heating in the outflows of transrelativistic reconnection (i.e., sigma(w) similar to 0.1-1, where the magnetization sigma(w) is the ratio of magnetic energy density to enthalpy density). For both electrons and protons, we find that heating at high beta(i) (here beta(i) is the ratio of proton thermal pressure to magnetic pressure) is dominated by adiabatic compression (adiabatic heating), while at low beta(i) it is accompanied by a genuine increase in entropy (irreversible heating). For our fiducial sigma(w) = 0.1, the irreversible heating efficiency at beta(i) <= 1 is nearly independent of the electron-to-proton temperature ratio T-e/Ti (which we vary from 0.1 up to 1), and it asymptotes to similar to 2% of the inflowing magnetic energy in the low-beta(i) limit. Protons are heated more efficiently than electrons at low and moderate beta(i) (by a factor of similar to 7), whereas the electron and proton heating efficiencies become comparable at beta(i) similar to 2 if T-e/T-i = 1, when both species start already relativistically hot. We find comparable heating efficiencies between the two species also in the limit of relativistic reconnection (sigma(w) >= 1). Our results have important implications for the two-temperature nature of collisionless accretion flows and may provide the subgrid physics needed in general relativistic MHD simulations.