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 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 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 similar to 36% in December 2011. This spin-down-rate transition 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 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) mG in the PWN.