Caveats and Limitations
![[NOTE]](imgs/note.png)
This page describes the caveats and limitations associated with Release 2.2 of the Chandra Source Catalog. While we recommend using the latest release, users working with data from earlier releases should refer to the corresponding caveats and limitations pages for Releases 2.1, 2.0, and 1.1.
Catalog Data
CSC users should be aware that there may be fundamental and significant selection effects that restrict the source content of the catalog and may therefore limit scientific studies that require an unbiased source sample. The CSC is constructed from pointed observations obtained using the Chandra X-ray Observatory; it is not an all-sky catalog, and does not include sources detected to a uniform depth. Users should be aware that sources with spatial extents greater than ~30 arcseconds are detected using a separate algorithm from that used for compact sources. These sources have names ending in an 'X' to distinguish them from compact sources.
Note that for some sources in observations with short exposures, there are zero counts in the local background region for some bands. In these cases, the source flux is not computed.
Specific Caveats and Limitations applicable through CSC release 2.2
- The true astrometric position error should be consistent with the reported position error for ~90% of sources. The ~10% of sources for which this is not the case are not randomly distributed. There are a small number of stacked observations for which the absolute position error may be significantly larger than reported, up to a maximum of ~2.0 arcseconds.
- Users of the XSPEC X-ray spectral fitting package may need to manually split the source region PHA spectrum file (pha3) into separate source region and background region spectra prior to analysis in XSPEC; see the page "Using the L3 Data Products" for instruction. Users of Sherpa can just read in the file with load_pha or load_data, which will read in the background automatically, and so do not need to split up the file.
- The model independent flux_aper values are computed using the observed energies of the events. In narrow energy bands, especially the ultra-soft (0.3–0.5keV) band, the spectral response can change rapidly with energy. As a result, a small difference between an event's observed energy, as described by the RMF, can produce a large change in the inferred flux. When only a small number of events are present, this can result in large, undetermined systematic errors. This is less of a concern in other energy bands, where the spectral response generally changes more slowly with energy. It also becomes less significant when a large number of events are present, because the Gaussian-like RMF is well sampled and the uncertainties tend to average out.
Anomalous ObsID-Level Aperture Photometry Fluxes
In CSC 2.2, there are a number of ObsID-level detections in which the broad band energy fluxes appear anomalously high in comparison to the sum of the s, m, and h band fluxes, often by factors of 10 or more. The effect is demonstrated in Figure 1, which shows the marginalized posterior distributions for energy flux for a detection in ObsID 29384, that contributes to master source 2CXOJ173524.2-295459.
Figure 1: Anomalously high broad-band marginalized posterior distribution fluxes
![[In CSC 2.2, there are a number of ObsID-level detections where the broad band energy fluxes appear anomalously high in comparison to the sum of the s, m, and h band fluxes, often by factors of 10 or more. This may be seen in the marginalized posterior distributions for energy flux in a detection in ObsID 29384, that contributes to master source 2CXOJ173524.2-295459.]](imgs/CSC2.2_mPDF_flux.png)
Figure 1: Anomalously high broad-band marginalized posterior distribution fluxes
The effect has been traced to s band events used to compute energy fluxes in aperture photometry——with the CIAO tool eff2evt—returns anomalous values for quantum efficiency and flux. This is due to the fact that the tool uses the nominal energy, from the diagonal element of the RMF, to compute flux. In regions where the effective area or quantum efficiency changes rapidly, small differences between the nominal and true energies can lead to large errors in flux (see the discussion of the RMFIMG parameter). This can occur for events with energies near the oxygen K-shell edge at ~543 eV.
The histogram in Figure 2 illustrates the extent of the issue for CSC 2.1 and CSC 2.2.
Figure 2: Aperture energy flux ratio histograms
![[In regions where the effective area or quantum efficiency changes rapidly, small differences between the nominal and true energies can lead to large errors in flux.]](imgs/CSC2.2_respdeviate.png)
Figure 2: Aperture energy flux ratio histograms
Here,
\[ flux\_ratio = \frac{b_{\mathrm{mid}}}{s_{\mathrm{mid}} + m_{\mathrm{mid}} + h_{\mathrm{mid}}}\ , \]and
\[ b_{\mathrm{mid}} = \frac{flux\_aper\_hilim\_b + flux\_aper\_lolim\_b}{2}\ , \]and similarly for the other bands.
Approximately 0.4% of the ObsID-level detections with measurable fluxes or upper limits in CSC 2.2 have flux ratios, as defined above, greater than or equal to 2.0. In comparison, approximately 0.15% in CSC 2.1 do.
Anomalous ObsID-level fluxes may affect master-level fluxes. Approximately 1,500 (or 300 with unique match_type) master sources in CSC 2.2 include ObsID-level detections with a flux ratio greater than or equal to 2.0. A list of these sources is provided as a CSV file.
This issue impacts both the s- and b-band fluxes in CSC 2.2.
It should be noted that aperture model energy fluxes, such as flux_powlaw_aper, etc., do not suffer from this issue, as the Figure 3 histogram illustrates. For users whose sources are affected, it may be possible to substitute flux_〈mode〉_aper_〈band〉 for flux_aper values. Aperture model energy fluxes should be available for all flux blocks except those comprised of only non-detections. In rare cases, this may include the best block.
Figure 3: Aperture model energy flux ratio histograms
![[Aperture model energy fluxes do not suffer from this issue. For users whose sources are affected, it may be possible to substitute flux_〈mode〉_aper_〈band〉 for flux_aper values.]](imgs/CSC2.2_model_flux_ratio.png)