Discovery of the First Antiglitch Event in the Rotation-powered Pulsar PSR B0540-69

Using data from the Neutron star Interior Composition ExploreR observatory, we identified a permanent spin frequency decrease of Δ ν = −(1.04 ± 0.07) × 10 ^−7 Hz around MJD 60132 in the rotation-powered pulsar PSR B0540-69, which exhibits a periodic signal at a frequency of ν ∼ 19.6 Hz. This points to an antiglitch event, a sudden decrease of the pulsar’s rotational frequency without any major alteration in the pulse profile or any significant increase of the pulsed flux. Additionally, no burst activity was observed in association with the antiglitch. To date, observations of the few known antiglitches have been made in magnetars or accreting pulsars. This is the first antiglitch detected in a rotation-powered pulsar. Given its radiatively quiet nature, this antiglitch is possibly of internal origin. Therefore, we tentatively frame this event within a proposed mechanism for antiglitches where the partial “evaporation” of the superfluid component leads to an increase in the normal component’s moment of inertia and a decrease in the superfluid one.