Extended Data Table 3 Sky density of sources from the wavdetect source detection algorithm

From: A white dwarf accreting planetary material determined from X-ray observations

  1. We present results in each of the three standard science bands considered (soft, soft + medium and broad), for two values of the significance threshold parameter, sigthresh, where s1 and s2 are significance thresholds of 1 × 10−6 and 5 × 10−7. From the documentation, the significance threshold should be set to s = 1/npx, which typically produces one false detection. The ACIS-S S3 CCD has 1024 × 1024 pixels, so s1 should be a sufficiently low threshold. However, to align with the world coordinate system (WCS), the CCD image is rotated relative to the bounding box of the image, meaning that the image on which wavdetect was run has 1414 × 1401 pixels, hence the value of s2 = 1/(1414 × 1401) ≈ 5 × 10−7. The sky density is computed as the number of sources (excluding the target, if detected), divided by the field of view, which is estimated as (10242 × 0.49202), where the second number is the pixel size in arcsec. Multiplying the sky density by the area defined by the 90% confidence limit on the radial offset of ACIS-S targets (π0.722 arcsec2), we provide the confidence, CLalign, with which we can reject the hypothesis that our source counts could originate from a background source.
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