A New X-Ray and Radio Burst Activity from the Magnetar SGR 1935+2154

Abstract

SGR 1935+2154 has emerged as the most active magnetar in recent years, exhibiting X-ray burst activity nearly annually and emitting a fast radio burst (FRB 200428 (FRB1)) in its 2020 April activity, for the first time from a Milky Way source, accompanied by an X-ray burst (X_FRB1). The source was active again in 2021 September and 2022 January, but without emitting radio bursts. In 2022 October, the magnetar entered a new activity that included radio bursts, three of them reported to be associated with X-ray bursts. We present a comprehensive study of the activity, as observed by the Fermi Gamma-Ray Burst Monitor, and offer a comparison with the previous activities. The mean burst duration of the observed 113 X-ray bursts (∼122 ms) is ∼30% shorter than in 2020 April but longer than most other activities. The mean spectral parameters (Γ_CPL ∼ 0.82, E_{p CPL} ∼ 26 keV and kT_BBs ∼ 5.10 keV, kT_BBh ∼ 8.96 keV) are generally comparable to the previous activities but with a softer Γ_CPL and larger R_BBh than in 2020 April. The parameter space shows a marked contrast where two distinctive, nonoverlapping, and smaller soft and hard blackbody regions are manifested in 2020 April but not in 2022 October and the other episodes. The fluence–duration power-law correlation is ∼60% steeper in 2022 October and the other episodes than in 2020 April. The three new X-ray bursts associated with radio bursts stand out in being ∼2–10-fold longer than the mean duration, with X-ray-to-radio duration, flux, and fluence ratios of ∼10^2–3, 10^3–4, and 10^5–7, respectively, comparable to the X_FRB1/FRB1 ratio for duration but larger for flux and fluence due to fainter radio bursts in 2022. We discuss the results in the context of the new emission models that propose physical mechanisms for the X-ray and radio burst emission from magnetars.