The Fast Radio Burst-emitting Magnetar SGR 1935+2154—Proper Motion and Variability from Long-term Hubble Space Telescope Monitoring

Abstract

Abstract We present deep Hubble Space Telescope (HST) near-infrared (NIR) observations of the magnetar SGR 1935+2154 from 2021 June, approximately 6 yr after the first HST observations, a year after the discovery of fast-radio-burst-like emission from the source, and in a period of exceptional high-frequency activity. Although not directly taken during a bursting period the counterpart is a factor of ∼1.5–2.5 brighter than seen at previous epochs with F140W(AB) = 24.65 ± 0.02 mag. We do not detect significant variations of the NIR counterpart within the course of any one orbit (i.e., on minutes to hour timescales), and contemporaneous X-ray observations show SGR 1935+2154 to be at the quiescent level. With a time baseline of 6 yr from the first identification of the counterpart we place stringent limits on the proper motion (PM) of the source, with a measured PM of μ = 3.1 ± 1.5 mas yr−1. The direction of PM indicates an origin of SGR 1935+2154 very close to the geometric center of SNR G57.2+08, further strengthening their association. At an adopted distance of 6.6 ± 0.7 kpc, the corresponding tangential space velocity is ν T = 97 ± 48 km s−1 (corrected for differential Galactic rotation and peculiar solar motion), although its formal statistical determination may be compromised owing to few epochs of observation. The current velocity estimate places it at the low end of the kick distribution for pulsars, and makes it among the lowest known magnetar kicks. When collating the few-magnetar kick constraints available, we find full consistency between the magnetar kick distribution and the much larger pulsar kick sample.