We present detailed spectral and temporal characteristics of the persistent X-ray emission of SGR J1935+2154 based on our XMM-Newton and Chandra observations taken in the aftermath of its April 2020 burst storm, during which hundreds of energetic X-ray bursts were emitted, including one associated with an extraordinary fast radio burst. We clearly detect the pulsed X-ray emission in the XMM-Newton data. An average spin-down rate of 1.6×10−11 s s−1 is obtained using our spin period measurement combined with three earlier values reported from the same active episode. Our investigations of the XMM-Newton and Chandra spectra with a variety of phenomenological and physically-motivated models, concluded that the magnetic field topology of SGR J1935+2154 is most likely highly non-dipolar. The spectral models indicate that surface field strengths in somewhat localized regions substantially exceed the polar value of 4.4×1014 G inferred from a spin-down torque associated with a rotating magnetic dipole.
@article{arxiv.2012.01871,
title = {Persistent Emission Properties of SGR J1935+2154 During Its 2020 Active Episode},
author = {Ersin Gogus and Matthew G. Baring and Chryssa Kouveliotou and Tolga Guver and Lin Lin and Oliver J. Roberts and George Younes and Yuki Kaneko and Alexander J. van der Horst},
journal= {arXiv preprint arXiv:2012.01871},
year = {2021}
}