The First FRB-associated X-Ray Bursting Episode of SGR 1935+2154: Swift-BAT Observations and Multi-instrument Comparisons with Fermi GBM, INTEGRAL/ISGRI, and Insight-HXMT

Abstract SGR 1935+2154 has emerged as the most prolific magnetar of the past decade and the first galactic source to establish that magnetars can produce fast radio bursts (FRBs), alongside their canonical short, luminous X-ray bursts. We present the observations of SGR 1935+2154 with the Swift Burst Alert Telescope (BAT) during its 2020 April activity, which exhibited FRB 200428 (FRB1), accompanied by an X-ray counterpart (X FRB1 ). Forty X-ray bursts are detected with a mean duration of 127 ms. Their energy spectra are well represented by a cutoff power law (CPL) and two blackbody (2BB) models, with a mean CPL photon index and peak energy of −0.73 and 21 keV, respectively, and 2BB mean soft and hard temperatures of 4.47 and 9.52 keV, respectively. The fluence follows a dN / dS ∝ S −0.86 distribution and correlates positively with the duration at S ∝ T 1.59 . The BB soft and hard temperatures and emission areas exhibit anticorrelation ( R BB 2 ∝ k T BB α ) with indices of −3.96 and −4.41, respectively. We complement the Swift-BAT results with observations from three additional instruments that observed the activity in a comparable energy range—Fermi Gamma-ray Burst Monitor, INTEGRAL IBIS/ISGRI, and Insight-HXMT—enabling a four-instrument comparison. We further examine X FRB1 in the context of the burst sample detected across all four instruments, finding that it exhibits a distinctive combination of long duration, hard spectrum, small blackbody radii, and is a clear outlier in the R BB 2 − k T BB parameter space—properties unmatched by any other burst in the sample. These distinctive characteristics point to a separate emission channel for FRB-associated X-ray bursts, and strongly support the origin of the (X FRB1 , FRB1) pair in a strong field region, such as the polar caps or a multipolar field configuration. This makes SGR 1935+2154 particularly valuable for the emerging prospect of probing exotic strong field physics signatures such as vacuum birefringence and polarization, and axion-photon conversion, known to imprint observable features on the multiwavelength magnetar spectra.