@article {39988,
	title = {The Origin of Power-law Spectra in Relativistic Magnetic Reconnection},
	volume = {956},
	year = {2023},
	month = {2023/10/1},
	pages = {L36},
	publisher = {IOP},
	abstract = {Magnetic reconnection is often invoked as a source of high-energy particles, and in relativistic astrophysical systems it is regarded as a prime candidate for powering fast and bright flares. We present a novel analytical model-supported and benchmarked with large-scale three-dimensional kinetic particle-in-cell simulations in electron-positron plasmas-that elucidates the physics governing the generation of power-law energy spectra in relativistic reconnection. Particles with Lorentz factor γ ≳ 3σ (here, σ is the magnetization) gain most of their energy in the inflow region, while meandering between the two sides of the reconnection layer. Their acceleration time is ${t}_{\mathrm{acc}}\sim \gamma \,{\eta }_{\mathrm{rec}}^{-1}{\omega }_{{\rm{c}}}^{-1}\simeq 20\,\gamma \,{\omega }_{{\rm{c}}}^{-1}$ , where η <SUB>rec</SUB> ≃ 0.06 is the inflow speed in units of the speed of light and ω <SUB>c</SUB> = eB <SUB>0</SUB>/mc is the gyrofrequency in the upstream magnetic field. They leave the region of active energization after t <SUB>esc</SUB>, when they get captured by one of the outflowing flux ropes of reconnected plasma. We directly measure t <SUB>esc</SUB> in our simulations and find that t <SUB>esc</SUB> ~ t <SUB>acc</SUB> for σ ≳ few. This leads to a universal (i.e., σ-independent) power-law spectrum ${{dN}}_{\mathrm{free}}/d\gamma \propto {\gamma }^{-1}$ for the particles undergoing active acceleration, and ${dN}/d\gamma \propto {\gamma }^{-2}$ for the overall particle population. Our results help to shed light on the ubiquitous presence of power-law particle and photon spectra in astrophysical nonthermal sources.},
	keywords = {739, Astrophysics - High Energy Astrophysical Phenomena, High energy astrophysics, Physics - Plasma Physics},
	isbn = {0004-637X},
	url = {https://ui.adsabs.harvard.edu/abs/2023ApJ...956L..36Z},
	author = {Zhang, Hao and Sironi, Lorenzo and Giannios, Dimitrios and Petropoulou, Maria}
}
