Representative XRT images (in equatorial coordinates) showing the evolution of the rings, along with the WISE 22 μm data plotted as contours (at normalized levels of 0.9885, 0.9940, 0.9995, 1.0050 and 1.0105 from dark-grey to white). The XRT data were smoothed by convolving with a Gaussian of width σ = 7.1 arcsec. The mid-point observation times of the XRT data are marked in the upper-left corner of each image. The arrows in the top right identify the Galactic coordinate north and west directions. Note, the azimuthal angle ϕ = 0° direction points towards the west (i.e. right). The XRT image is animated in the online version of this paper.