What primarily powers the observable activity of a magnetar?

Decay of its magnetic field

Unlike typical neutron stars, whose observable energy output and luminosity are primarily powered by the continuous slowdown of their rotation, the activity of a magnetar is fundamentally dictated by the decay of its gargantuan magnetic field. The magnetic energy reservoir within a magnetar is so substantial that its gradual restructuring and decay dictate the object's visible signature over time. This process involves immense internal stress, where twisting magnetic field lines generate powerful currents within the star's crust, leading directly to observable phenomena such as energetic flares and bursts of radiation as stored magnetic energy is suddenly released.