We present a time-lapse sequence of 20 near-infrared ( J - and K ^ { \prime } -band ) snapshots of the central 20 \arcsec \times 20 \arcsec of the Crab pulsar wind nebula , taken at subarcsecond resolution with the Hokupa ’ a/QUIRC adaptive optics camera on the Gemini North Telescope , and sampled at intervals of 10 minutes and 24 hours .
It is observed that the equatorial wisps and polar knots in the termination shock of the pulsar wind appear to fluctuate in brightness on kilosecond time-scales .
Maximum flux variations of \pm 24 \pm 4 and \pm 14 \pm 4 per cent relative to the mean ( in 1.2 { ks } ) are measured for the wisps and knots respectively , with greatest statistical significance in J band where the nebula background is less prominent .
The J and K ^ { \prime } flux densities imply different near-infrared spectra for the nonthermal continuum emission from the wisps and outermost polar knot ( ‘ sprite ’ ) , giving F _ { \nu } \propto \nu ^ { -0.56 \pm 0.12 } and F _ { \nu } \propto \nu ^ { -0.21 \pm 0.13 } respectively .
The data are compared with existing optical and UV photometry and applied to constrain theories of the variability of the wisps ( relativistic ion-cyclotron instability ) and knots ( relativistic fire hose instability ) .