February 18
3D Particle-in-Cell Simulations of Reconnection-Driven Particle Acceleration and Their Implications for Sagittarius A* Flares
Speaker: Dr. Sophia Sánchez-Maes , IREAP, UMD
Abstract: The brightest X-ray flares from Sagittarius A* (Sgr A*) reveal extreme episodes of particle acceleration. Explaining observed spectra requires electrons accelerated to Lorentz factors in the range of hundreds of thousands to millions, which is beyond what traditional approaches have demonstrated in this context. Magnetic reconnection offers a compelling mechanism, capable of efficiently accelerating particles to high energies. However, two-dimensional (2D) particle-in-cell (PIC) simulations have intrinsic limitations: particle motion is confined within plasmoids, restricting energy gain. Acceleration in 2D reconnection typically saturates near the plasma magnetization limit, falling short of the energies required to explain the brightest X-ray flares. In contrast, 3D reconnection introduces additional degrees of freedom, allowing particles to escape plasmoids and enhancing acceleration efficiency.
To connect reconnection physics with observed Sgr A* flare spectra, we performed 3D PIC simulations spanning a range of magnetic guide field strengths (Bg/B0), and investigated the resulting particle acceleration and synchrotron spectra. We find that the guide field plays an important role: a strong guide field limits particle energies and softens the spectrum, while a weak or absent guide field in 3D produces overly hard power-law slopes. Intermediate guide field strengths yield particle distributions more consistent with those observed during X-ray flares from Sgr A*. These results suggest that reconnection-mediated acceleration can plausibly power the IR and X-ray variability observed near the Galactic Center’s supermassive black hole. The strength of the guide magnetic field may further regulate whether flare emission extends into the X-ray, consistent with the fact that all Sgr A* X-ray flares have IR counterparts, but not all IR flares produce observed X-rays.
Host: Ankita Bera