Particle acceleration in relativistic magnetic reconnection with strong inverse-Compton cooling in pair plasmas

Particle-in-cell (PIC) simulations have shown that relativistic collisionless magnetic reconnection drives nonthermal particle acceleration (NTPA), potentially explaining high-energy (X-ray/$\gamma$-ray) synchrotron and/or inverse Compton (IC) radiation observed from various astrophysical sources. The radiation back-reaction force on radiating particles has been neglected in most of these simulations, even though radiative cooling considerably alters particle dynamics in many astrophysical environments where reconnection may be important. We present a radiative PIC study examining the effects of external IC cooling on the basic dynamics, NTPA, and radiative signatures of relativistic reconnection in pair plasmas. We find that, while the reconnection rate and overall dynamics are basically unchanged, IC cooling significantly influences NTPA: the particle spectra still show a hard power law (index $\geq -2$) as in nonradiative reconnection, but transition to a steeper power law that extends to a cooling-dependent cutoff. The steep power-law index fluctuates in time between roughly $-$3 and $-$5. The time-integrated photon spectra display corresponding power laws with indices $\approx -0.5$ and $\approx -1.1$, similar to those observed in hard X-ray spectra of accreting black holes.