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It is believed that an isolated pulsar loses its rotational energy mainly through a relativistic wind consisting of electrons, positrons and possibly Poynting flux 1-3 . As it expands, this wind may eventually be terminated by a shock, where particles can be accelerated to energies of X-ray synchrotron emission, and a pulsar wind nebula (PWN) is usually detectable surrounding a young energetic pulsar 1-3 . However, the nature and/or energetics of these physical processes remain very uncertain, largely because they typically cannot be studied in a timeresolved fashion. Here we show that the X-ray PWN around the young pulsar PSR B0540-69 brightens gradually up to 32±8% over the mean previous flux, after a sudden spin-down rate (ν) transition (SRT) by ∼ 36% in December 2011, which has very different properties from a traditional pulsar glitch 4 . No evidence is seen for any change in the pulsed X-ray emission. We conclude that the SRT results from a sudden change in the pulsar magnetosphere that increases the pulsar wind power and hence the PWN X-ray emission. The X-ray light curve of the PWN suggests a mean life time of the particles of 397 ± 374 days, corresponding to a magnetic field strength of 0.78 +4.50 −0.28 mG in the PWN.

We report results on the timing analysis of the 2020 giant outburst of 1A 0535+262, using broadband data from Insight-HXMT. The analysis of the pulse profile evolution from the subcritical-luminosity to the supercritical-luminosity regime is presented for the first time. We found that the observed pulse profile exhibits a complex dependence on both energy and luminosity. A dip structure at the energy of the cyclotron resonant scattering features is found for the first time in the pulse fraction–energy relation of 1A 0535+262, when the outburst evolves in a luminosity range from 4.8 × 1037 to 1.0 × 1038 erg s−1. The observed structure is luminosity dependent and appears around the source critical luminosity (∼6.7 × 1037 erg s−1).