Tips for Farster Spectral Fits of XIS Data
1. Rebinning a RMF
Because the standard RMF of Suzaku XIS has 7900 energy bins (2 eV step, 0.2 - 16 keV) times 4096 PI bins, XSPEC needs much memory to read the RMF and time to calculate a spectral fit model. This fine-step matrix is usually over-sampled for moderate flux sources with featureless X-ray spectra (e.g., AGNs).
You can rebin the RMF in both channel- and energy-spaces with the rbnrmf ftools. Note that you must rebin the PHA spectrum too, when the RMF is rebinned in the channel-space. You can also specify the channel-space rebin factor by the "rebin" parameter of the xisrmfgen.
- There is a bug in the previous (HEAsoft v6.3.2) version of rbnrmf when doing binning on energies. The version in the latest (HEAsoft v6.4) release does not have this bug. Alternatively, patches are available via the HEAsoft 6.3.2 Bugs page.
The RMF energy bins are determined with the default set of parameters
ebin_lowermost=0.20, ebin_uppermost=16.0, and ebin_width=2.0. If you
are only interested in the soft band spectrum, e.g. ebin_uppermost=12.0
will reduce the RMF size by about 25%. When the spectral model to fit
is featureless (no strong emission lines), ebin_width=4.0 or
ebin_width=8.0 will give almost the same fit result. Older version of
RMF, e.g., ae_xi0_20050916.rmf, in the CALDB has non-equal energy
bin with 4096 steps in 0.2-12.0 keV. You can use this energy steps, by
ebin_mode=1 ebinfile=ae_xi0_20050916.rmfARFs must be re-created when the RMF energy bins are changed.
2. Combining XIS0, (XIS2,) and XIS3
The XIS team recommends adding the spectra and response for the units with the frontside illuminated (FI) chips. XIS1 spectrum, however, must be fitted separate since its (backside illuminated, or BI chip) response is distinctly different from those of FI chips.
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