2005ApJ...633..931M

We present a time-lapse sequence of 20 near-infrared (J and K' band) snapshots of the central 20x20 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 hr. 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 timescales. Maximum flux variations of ±24%±4% and ±14%±4% relative to the mean (in 1.2 ks) are measured for the wisps and knots, respectively, with the greatest statistical significance in the J band, where the nebula background is less prominent. The J and K' flux densities imply different near-infrared spectra for the nonthermal continuum emission from the wisps and the outermost polar knot (the ``sprite''), giving F_ν∝ν^{–0.56±0.12} and F_ν∝ν^{–0.21±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).