tokens: in the F125LP image shown in Figure 1 (top left). The FUV bow shock appears to be asymmetric, being thicker and brighter east of the pulsar. We measure the apex distance (from the pulsar to the leading edge of the bow shock) to be ≈25, consistent with the measurements in H α by Brownsberger Romani ( 2014 ), Gaensler et al. ( 2002 ). To extract the bow-shock flux, we defined the source and two background regions, “inside” and “outside” of the bow shock, shown in Figure 1 (top right). The background regions were selected based on the F125LP image, in which the bow shock was most clearly detected. The regions are close to the bow-shock extraction region to mitigate effects from the inhomogeneous detector background while excluding the pulsar. Because of the larger apparent thickness of the east side of the bow shock, more space was left between the bow-shock extraction region and the background regions. We took the background variations into account by selecting background regions on both sides of the bow shock: one “inside” and one “outside” the bow shock. These regions were used for flux measurements in both F125LP and F140LP. A combined background from the two regions was extracted. The net source count rate was calculated similarly to the pulsar, with the area A b being the combined area of the “outside” and “inside” background regions. The F125LP and F140LP count rates and their uncertainties are given in Table 2 . From the measured count rates we calculated the absorbed fluxes in the F125LP and F140LP wavelength ranges: F (1250–2000 Å) = (8.1 ± 0.4) × 10 −15 erg s −1 cm −2 and F (1350–2000 Å) = (7.0 ± 0.7) × 10 −15 erg s −1 cm −2 , for a flat f λ spectrum. The unabsorbed luminosity can be estimated as L (1250–2000 Å) ≈ 1.9 × 10 29 erg s −1 , for E ( B − V ) = 0.03, and d = 410 pc. The bow shock is not detected in the F475X images (see Figure 8 ). Because of the apparent non-uniformities in the background, which significantly exceed the statistical fluctuations, we chose to measure the upper limits by sampling the background counts from ten different regions in the vicinity of the bow shock and calculating the standard deviation from the mean value. The areas of the background regions were equal to that of the bow-shock region used for FUV flux measurements. The standard deviation can be considered as a conservative 1 σ upper limit. The corresponding 3 σ upper limit on the bow-shock flux in the area of 52 arcsec 2 is F (3750–6000 Å) ≈ 2.9 × 10 −15 erg s −1 cm −2 , for a flat f λ spectrum and E ( B − V ) = 0.03. Figure 8. UVIS/F475X image with the bow-shock extraction regions (the same as in Figure 1 ), used for the data analysis. The pulsar and the direction of its proper motion are shown with a circle and an arrow, respectively. Similar to our analysis of the FUV bow shock around J0437 (Rangelov et al. 2016 ), we measured the ratio of the count rates in F125LP and F140LP filters for the J2124 bow shock. We obtained a ratio of C 125 / C 140
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