Rapid Variability of Radiative Signatures from Driven Relativistic Reconnection

Rapid variability in energetic flares is a distinctive feature of high-energy emission from many magnetically dominated astrophysical sources. We investigate the radiative variability of driven relativistic magnetic reconnection using Particle-in-Cell simulations of strongly magnetized flux tubes pushed to merge at driving speed v push. This setup captures the onset of reconnection and provides a controlled model for single reconnection-driven flares near compact objects, where magnetic structures can be twisted, sheared, and forced to merge by turbulent accretion flows or relativistic frame dragging. We find that the merger efficiently accelerates particles into nonthermal distributions, producing synchrotron emission whose variability tracks the reconnection phases. We identify two regimes: a quasi-steady, weakly driven regime for v push at most0.1,c, and strongly driven regime for v push>0.1,c. The transition occurs when the driving exceeds the typical inflow-to-outflow velocity ratio of about0.1 in steady collisionless relativistic reconnection, i.e., the average rate at which reconnection can process incoming flux, potentially providing an additional source of variability. The lightcurves rise on 1 -- 3 light-crossing times of the reconnecting region, and the particle spectra develop power laws with index p 1.8 -- 2.4. These timescales are compatible with observed variability in high-energy sources, including the very-high-energy flares of M87* and the γ-ray flares of the Crab Nebula, making this model a viable framework for rapid flaring in active galactic nuclei and pulsar wind nebulae.