On using the HRC-I RMF v2 for spectral analysis of HRC-I data
Users who want to analyze HRC-I data for any spectral information have previously been strongly advised to do this only for hardness ratios and for quantile color-color diagrams, as per the HRC-I web page instrutions. (See the URLs below.) HRC-I does provide PHA information, but does not have adequate energy resolution for standard spectral modeling analysis with Sherpa/XSPEC. It has come to our attention that further information concerning the use of the HRC-I RMF is necessary.
The HRC-I RMF is constructed with an EBOUNDS extension that is physically meaningful for only a small range of PI channels. For extreme PI values, the corresponding values in the EBOUNDS are set to extreme energy values so that using them will flag as an obvious error in the analysis. This was deemed necessary due to the poor spectral resolution of the HRC, where events recorded at any given PI value have large probabilities of having occured over any energy that the Chandra mirrors are sensitive to. In other words, even though the nominal range of energies listed in the EBOUNDS is 0 to 75 keV, all the events recorded at all PI values can only be within approximately 0.2-10 keV.
The proper method for using the HRC-I RMF (in Sherpa) is to avoid setting spectral filters by energy ranges but to use PI channel filters instead. In fact, the HRC team recommends this as part of a strategy for background reduction. However, the low spectral resolution is not sufficiently constraining to achieve a good fit with reasonable errors in standard spectral fitting analysis.
Hence, we continue to advise users against using the latest PHA-based RMF in the CalDB for spectral fitting in Sherpa or XSPEC.
See the following web pages for more information on the proper use of HRC-I RMF files:
CIAO Why page: http://cxc.harvard.edu/ciao/why/hrc_rmf.html
CalDB Caveats page at http://cxc.harvard.edu/caldb/caveats/index.html.
HRC-I QCCD information pages:
http://cxc.harvard.edu/cal/Hrc/RMF/index_200611.html#i (for the PHA-based PI)
http://cxc.harvard.edu/cal/Hrc/RMF/samp_rmf.html#i (for the SAMP-based PI)