Concise Spectrotemporal Studies of Magnetar SGR J1935+2154 Bursts

SGR J1935+2154 has truly been the most prolific magnetar over the last decade: it has been entering into burst active episodes once every 1-2 yr since its discovery in 2014, it emitted the first Galactic fast radio burst associated with an X-ray burst in 2020, and it has emitted hundreds of energetic short bursts. Here, we present the time-resolved spectral analysis of 51 bright bursts from SGR J1935+2154. Unlike conventional time-resolved X-ray spectroscopic studies in the literature, we follow a two-step approach to probe true spectral evolution. For each burst, we first extract spectral information from overlapping time segments, fit them with three continuum models, and employ a machine-learning-based clustering algorithm to identify time segments that provide the largest spectral variations during each burst. We then extract spectra from those nonoverlapping (clustered) time segments and fit them again with the three models: the cutoff power-law model, the sum of two blackbody functions, and the model considering the emission of a modified blackbody undergoing resonant cyclotron scattering, which is applied systematically at this scale for the first time. Our novel technique allowed us to establish the genuine spectral evolution of magnetar bursts. We discuss the implications of our results and compare their collective behavior with the average burst properties of other magnetars.