The intrinsic energy resolution of the HRC is poor compared with the ACIS, but it does have some ability to discriminate between hard and soft spectra.
Using observations of HR1099 carried out with the HRC-I/LETG at various locations on the detector, the calibration team has constructed a response matrix for the HRC-I. This PHA-derived HRC-I RMF was first released in CALDB 3.2.1 (15 December 2005), which has since been superseded and replaced, was updated with a SAMP-derived RMF in CALDB 4.2.0 (15 December 2009). This SAMP RMF is required for use with event data that have PI values derived from the HRC-I gain maps, also released in CALDB 4.2.0. A revision on the SAMP RMF (N0002) was released in CALDB 4.8.4 (03 September 2019) where the response curve is identical to the earlier N0001 version, but uses a finer energy grid.
The first HRC-S RMF was released in CALDB 4.4.1 (15 December 2010) and updated in CALDB 4.8.4, defined on a finer energy grid. This HRC-S RMF files are required for use with event data that have PI values derived from the SAMP HRC-S gain maps.
For details on the RMF files, refer to the HRC Spectral Response calibration page.
The files are distributed in the CALDB:
$CALDB/data/chandra/hrc/rmf/hrciD1999-07-22samprmfN0001.fits $CALDB/data/chandra/hrc/rmf/hrciD1999-07-22samprmfN0002.fits $CALDB/data/chandra/hrc/rmf/hrcsD1999-07-22samprmfN0001.fits $CALDB/data/chandra/hrc/rmf/hrcsD1999-07-22samprmfN0002.fits
The RMF can be used to calibrate hardness ratios or quantile color-color diagrams (QCCDs) to distinguish between gross differences in the spectra. QCCDs are useful if the user is unsure of which PI ranges to choose. While hardness ratios are easier to compute and understand, QCCDs are less sensitive towards residual systematic errors, since the width of spectra are less dependent on precise values of the gain. Note that "QCCD" is a slight misnomer, as it is really a plot of (suitably normalized) mid-point vs width for spectra.
We do not advocate using this RMF in spectral fits; the spectral response is not sufficiently constraining to achieve a good fit with reasonable errors. Unlike instruments with high intrinsic spectral resolution (like the ACIS), specific energy ranges cannot be assigned to PI ranges. Therefore, when using these RMFs, one should always work in channel space, not energy space. To set the analysis to channel space in Sherpa:
Because the correspondence between PI bins and energy values is not well-defined and is not monotonic, the energy grid in the EBOUNDS extension of the RMF has been deliberately set to be meaningless, which matters only when spectra are plotted or data are filtered in energy space.