Data Caveats


1)    CCD readout streak. When the central AGN is very bright, the CCD readout may cause streaks along the CCD column (see column 10 in Table 2). In this case, we determine the streak region by acis_streak_map for each obsid and remove this region from the image and event files for adaptive binning (section 3.3) and spectral analysis (section 3.4-3.5). Effectively we treat the streak the same way as the point sources removed in section 3.2. However, unlike the point sources, we do not refill the excluded region for the diffuse hot gas image, because the streaks are often in the middle of complex hot gas structures.

2)    CCD gaps and boundaries. The gaps between chips and node boundaries are often less exposed and are most evident in raw images before the exposure correction, but sometimes still visible even after the exposure corrections. They are least visible in the spectral maps (e.g., in a T map), because the different exposures are appropriately treated bin by bin. One good example is NGC 1550 which was observed four times and the significant exposure variation is clearly visible near the southern edge of the D25 ellipse. Although the raw data and binned image (counts/area) show a sharp discontinuity, the T map and EM (normalization/area) map are generally smooth with no clear artifact at that location. We note that the gap may appear more significant in CB than other binning methods because the CB spatial bin is determined in a region with similar counts/area. Users should be cautious about the gaps when considering any physical changes across the gap.

3)    Strong nuclear sources. When an AGN at the center dominates the entire X-ray emission with a strong nuclear X-ray source and little amount of hot gas which is confined near the central region, the hot gas properties are difficult to measure. In this release, we adopt a power-law with a spectral index of 1.7 for the nuclear source. If the AGN X-ray spectrum is significantly different, the measured hot gas properties are subject to systematic uncertainties. We will carefully investigate the circum-nuclear hot gas as one of our focused studies in the near future.