tokens: regions described in Section 4.3 . In Figure 6 , we present again the oxygen abundance and ionization parameter gradients and color-code the data points based on the different groups defined in Figure 5 . Figure 6 further demonstrates the azimuthal variation visible on the oxygen abundance map (Figure 3 ). The H ii regions that belong to the spiral arms have systematically higher oxygen abundances than the other regions at the same galactocentric distances. The upstream regions have oxygen abundances in between the main spiral and downstream H ii regions. A total of 73 of the 96 (76%) main spiral H ii regions are above the best-fit abundance gradient. In contrast, only 13/76 (17%) of the downstream H ii regions are above the best-fit abundance gradient. The distributions of the residuals, after the best-fit radial gradient is subtracted, are statistically different (Figure 6 histogram). Two-sample Kolmogorov–Smirnov tests suggest that the null hypotheses that the residuals of the spiral-downstream, spiral-upstream, and downstream-upstream regions are drawn from the same distributions can effectively be ruled out (, , and , respectively). Figure 6. Oxygen abundance (left panels) and ionization parameter (right panels) gradients in NGC 1365. Each color point corresponds to one H ii region in Figure 3 . The different color symbols correspond to the different groups of H ii regions shown in Figure 5 . The black solid lines are the overall best fits. The red, blue, and green dashed lines are the linear best fits to the “Spiral,” “Downstream,” and “Upstream” H ii regions, respectively. The bottom panels show the residuals, i.e., after subtracting the black solid lines. The best-fit parameters are tabulated in Table 2 . We also perform the same least-squares linear fits to the spiral arm, downstream and upstream H ii regions. The best-fit oxygen abundance gradients shown in Figure 6 demonstrate the systematic differences between the different groups of H ii regions. The best-fit parameters are tabulated in Table 2 . It is worth pointing out that the majority of the upstream H ii regions (21/27; 78%) are in between the best-fit gradients defined by the spiral arm and downstream H ii regions. In Figure 7 , we present the azimuthal variations more clearly by subtracting the best-fit radial gradient from the oxygen abundance map. If there are no azimuthal variations, the residual map should present only random fluctuations. However, Figure 7 shows that the residuals correlate strongly with the spiral pattern. Figure 7. Oxygen abundance map after subtracting the best-fit radial abundance gradient (i.e., Figure 6 ). The azimuthal variations of oxygen abundance are now clearly visible. To summarize, while overall there is a negative oxygen abundance gradient in NGC 1365, we find pronounced azimuthal variations. The H ii regions on the spiral arms present, on average, about 0.2 dex higher oxygen abundance than the downstream H ii regions. The upstream H ii
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