An acceleration-radiation model for nonthermal flares from Sgr A$^\star$

Abstract: (Abridged) Sgr A$^\star$ is the electromagnetic counterpart of the accreting supermassive black hole in the Galactic center. Its emission is variable in the near-infrared (NIR) and X-ray wavelengths on short timescales. The physical origin of NIR and X-ray flares is still under debate. We introduce a model for the production of NIR and X-ray flares from an active region in Sgr A$^\star$, where particle acceleration takes place intermittently. In contrast to other radiation models for Sgr A$^\star$ flares, the particle acceleration is not assumed to be instantaneous. We studied the evolution of the particle distribution and the emitted electromagnetic radiation from the flaring region by numerically solving the kinetic equations for electrons and photons. Our calculations took the finite duration of particle acceleration, radiative energy losses, and physical escape from the flaring region into account. To gain better insight into the relation of the model parameters, we complemented our numerical study with analytical calculations. Flares are produced when the acceleration episode has a finite duration. The rising part in the light curve of a flare is related to the particle acceleration timescale, while the decay is controlled by the cooling or escape timescale of particles. Bright X-ray flares, such as the one observed in 2014, have $\gamma$-ray counterparts that might be detected by the Cherenkov Telescope Array Observatory. Our model for NIR and X-ray flares favors an interpretation of diffusive nonresonant particle acceleration in magnetized turbulence. If direct acceleration by the reconnection electric field in macroscopic current sheets causes the energization of particles during flares in Sgr A$^\star$, then models considering the injection of preaccelerated particles into a blob where particles cool and/or escape would be appropriate to describe the flare.