Cold-chain Comptonization in black hole coronae

What powers the hard, non-thermal X-rays from accreting black holes is an unsolved mystery. We address this puzzle, and the underlying question of what energizes the electrons of the Comptonizing “corona” against the strong inverse Compton (IC) cooling losses. We perform first principle particle-in-cell simulations of magnetic reconnection in magnetically dominated (σ ≫ 1) electron-positron and mildly-magnetized (σ ~ 1) electron-ion plasmas subject to strong IC cooling. We find that the electron energy spectrum is dominated by a quasi-Maxwellian shaped peak at trans-relativistic energies (~100 keV), which results primarily from the bulk motions of “plasmoids.” In plasmoids, electrons are cooled down to non-relativistic energies, which makes the oft-invoked paradigm of “thermal Comptonization” by hot electrons unfeasible. We complement our particle-in-cell simulations with Monte-Carlo calculations of the transfer of seed soft photons through the reconnection layer, and produce synthetic X-ray spectra. We demonstrate that, regardless of the composition of the corona, Comptonization by the bulk motions of a chain of plasmoids containing IC-cooled electrons can naturally explain the hard-state spectra of accreting black holes.