CSC Sensitivity Map Service
One of the interfaces available for determining CSC limits in identifying X-ray counterparts to sources of interest is the CSC Sensitivity Map Service, an on-line tool which returns approximate sensitivity at specified celestial locations.
The approximate limiting sensitivity is given as photons cm-2 s-1 for each of CSC science energy bands, including the ACIS ultrasoft, soft, medium, hard, and broad bands, as well as the HRC wide band. Visit the "Sky Coverage" FAQs to learn about the other interfaces available for searching the catalog for Chandra counterparts to sources of interest.
The limiting sensitivity returned by this tool is based on mapping Chandra sensitivity to a sky grid using the HEALPix pixelization algorithm (Gorski, et al. 2005, ApJ, 622, 759). HEALPix indices were computed for the centers of all sensitivity map pixels with non-zero limiting sensitivity, for all observations in CSC Release 1. We rebinned the sensitivity maps to 32"x32" prior to doing this to keep the number of indices manageable, and used the HEALPix parameters NSIDE=8192 and ORDERING=RING. If more than one sensitivity map pixel contributed to the same HEALPix pixel, we kept only the more sensitive value. In certain cases (see Caveats, below) we were unable to use the 32" x 32" sensitivity map and used a coarser map, instead.
Individual R.A. and Dec. positions in decimal degrees can be input to the tool to return a sensitivity value, if one is found (take caution when cutting and pasting values into the R.A. and Dec. fields; a sensitivity value will not be returned if a space exists after either of the entered numbers). Alternatively, a text file of tab-delimited R.A.'s and Dec.'s can be input and the tool will return either a table display or a file for download. The input file may contain other tab-delimited columns after R.A. and Dec., which will be included in the output table or file. The output table or file will contain no additional appended columns if no sensitivity is found, or will contain tab-delimited HEALPix pixel R.A., Dec., and sensitivity. In all cases the HEALPix pixel number is prepended as the first column in the output. Note also that the input source position list is sorted in HEALPix index prior to matching, so output source order may not match input order.
If no sensitivity was found at the original HEALPix index but a subsequent search of the coarser map did return a local average sensitivity, a message is displayed for the individually input R.A. and Dec. case. For an uploaded file, matches to the coarser HEALPix map will still have the original HEALPix index prepended to each output row. However, rows that have relied on the coarser HEALPix map to determine sensitivity can be identified by HEALPix R.A. and Dec. columns which exactly match input R.A. and Dec. values.
There are two important caveats with this procedure. First, binning the original Chandra sensitivity maps and choosing the more sensitive value when multiple observations overlap a HEALPix pixel can alter and may slightly overstate sensitivity at a given location. Second, in cases where no 32"x32" sensitivity map pixel center overlaps a HEALPix pixel, a more coarse HEALPix map was used to provide average sensitivity for an approximately 100"x100" area.
This coarser map further alters local sensitivity from the original Chandra maps, and may display a sensitivity up to approximately 100" outside of the Chandra field-of-view (FOV) where no observations were, in fact, made. If you believe your source to be near the edge of the FOV, you can confirm this by downloading the CSC "fov3" data products for observations in the vicinity of the input source positions. You can then determine, e.g. with a display tool such as DS9, whether the sources are included in the FOVs.
We expect future refinements to reduce or eliminate both of these effects.