Presentation #116.34 in the session Stellar/Compact Objects.
Magnetars are the extremes of isolated neutron stars, powered by the decay of their immense internal and external magnetic fields which often exceed 1.0e14 G. They are unique in their variability timescales, from milliseconds in the form of short bursts to years-long outburst epochs when their quiescent X-ray flux increases by as many as three orders of magnitude accompanied by drastic spectral and temporal changes to their surface thermal and magnetospheric emission. In a first so far, one magnetar has displayed a fast radio burst (FRB), revealing the nature of at least some extragalactic FRBs. NICER, with its large effective area, timing accuracy, and observing flexibility has played a transformative role in deciphering these variability patterns in several sources so far. In this talk, I will present three years in the life of the FRB-emitting magnetar SGR 1935+2154, emphasizing the importance of heavy cadence X-ray observations around the times of major X-ray and radio bursting episodes. Our observations reveal extreme timing phenomena associated with these epochs not yet seen in such a detail before. I will discuss these results in the context of crustal mass shedding and plasma dynamics in magnetar magnetospheres.