The brightening of the pulsar wind nebula of PSR B0540−69 after its spin-down-rate transition

It is believed that an isolated pulsar loses its rotational energy mainly through a relativistic wind consisting of electrons, positrons and possibly Poynting flux1–3. 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 pulsar1–3. However, the nature and/or energetics of these physical processes remain very uncertain, largely because they typically cannot be studied in a time-resolved 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 change in the spin-down rate of ~36% in December 2011. This spin-down-rate transition has very different properties from a traditional pulsar glitch4. No evidence is seen for any change in the pulsed X-ray emission. We conclude that the spin-down-rate transition 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 lifetime of the particles of 397 ± 374 d, corresponding to a magnetic field strength of $$0.78_{-0.28}^{+4.50}\ {\mathrm{mG}}$$ 0.7 8 - 0.28 + 4.50 mG in the PWN.