tokens: a radius of 3 arcmin. The Galactic north/south line runs through azimuthal angles 35°/215°. Of the profiles in Fig. 8 , the maximum from ring 2 appears to align the closest with the Galactic north direction, as might be expected if the column density were increasing towards the Galactic plane. Further discussion is deferred to Section 4.1 . VP15 also noted the presence of azimuthal structure in the XRT halo data; however, they did not separate out the behaviour of rings 1 + 2 or 3 + 4, or proceed to discuss the findings in any depth. 2.3 Halo spectral analysis Spectra from the rings were extracted from the longest PC mode snapshots, or adjacent snapshots, using annular extraction regions tuned specifically for each case. Given their relative separations, spectra from rings 1 and 2 were extracted as one, which increased the statistical quality of the data. The same was done for rings 3 and 4. PC event grades 0–12 were selected for the spectra. As the rings dominate the field of view and leave limited space on the detector from which to extract a good quality background spectrum, the PC mode observations from 2012 (total exposure 132 ks) were used to obtain an accurate non-halo background spectrum, which was extracted using an annulus of size 2.95–7.66 arcmin centred on the source, after five faint background point sources were removed. (Note, the background sources were too faint to have any effect on the radial profiles in Section 2.1 .) Also, as the central source was often observed offset from the centre of the CCD, a given ring samples different parts of the telescope vignetting. To correct for this in the ancillary response files (ARFs), we followed the procedure outlined by Moretti et al. ( 2011 ). This involved using a vignetting map, created by dividing exposure maps made with and without the vignetting correction (calculated at 1.5 keV) applied, to work out the mean vignetting correction in each extraction annulus. Then, using the analytical description of the vignetting given by Tagliaferri et al. ( 2004 ), these were converted to mean off-axis angle estimates that were used as an input to the XRT software task xrtmkarf to apply the energy-dependent mean vignetting correction to the on-axis ARF. The non-vignetted exposure map was also used to estimate the mean exposure in the annulus and the spectral file exposure time was updated accordingly. The spectra were fit in xspec (version 12.8.2; Dorman Arnaud 2001 ), using its implementation of the Cash ( 1979 ) statistic (C-stat), which, when a background spectrum is supplied, is a profile likelihood statistic. The spectra for a given ring at different epochs were modelled using an absorbed power law, with the photon indices and column densities tied between spectra. The absorption was provided by the tbnew model, 3 using the abundances of Wilms, Allen McCray ( 2000 ) and cross-sections of Verner et al. ( 1996 ). The fitting was performed over the energy range 1–5 keV, where the low
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