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rgsspectrum creates an OGIP-compliant spectrum of counts for a single RGS exposure. The histogramming of events in this task is the same as in the instrument non-specific tasks, xmmselect and evselect, but rgsspectrum utilizes the RGS exposure maps found in the input event list file to produce a more sophisticated spectrum than is possible for those tasks. However, due to its alternative approach, this task is unable to benefit from a number of low-level SAS facilities. For instance, the user is limited to a single, hard-coded expression for selecting the events to be included in the spectrum: the ``canonical'' spectrum for a given source and reflection order. Further, no attempt is made to document the selections as a data sub-space within the output file. In these respects rgsspectrum is inferior to xmmselect and evselect. There is some possibility that these deficiencies will be addressed as the SAS continues to evolve. Nevertheless, rgsspectrum is the preferred spectrum-generator for RGS, and must be used for compatibility with rgsrmfgen and rgsfluxer.
Three types of spectrum are available, indicated by the value of the header attribute, HDUCLAS2:
| NET | background-corrected source spectrum |
| TOTAL | the source spectrum without background-correction |
| BKG | the background spectrum |
Note that although the spatial selection region for the background spectrum is not source-specific, the energy selection region is source-specific, and for this reason the background spectrum is technically source-specific. By default the TOTAL and BKG spectra are produced together as a pair. These two have the same format, which includes the new AREASCAL and BACKSCAL columns as specified under HDU Version 1.2.0 of the OGIP Spectral File Format (OGIP Memo OGIP/92-007). This format is supported as of LHEASOFT Version 5.1 (XSPEC Version 11.1). The background-corrected spectrum omits the BACKSCAL column, which is not needed (nor well-defined in that case), and instead provides the STAT_ERR column. In High Time Resolution (HTR) mode the entire cross-dispersion dimension is collapsed into one row, inextricably mixing source and background events together. In this mode the background region is not spatial; rather it is source and order specific in the plane of energy versus dispersion. The consequences of this detail are noted throughout this document.
In addition to the standard columns, AREASCAL and BACKSCAL, rgsspectrum will optionally include a non-standard column, FRAC_EXP. The differences between these three columns bears some explaining. AREASCAL is a channel-specific correction factor to the global EXPOSURE value recorded in the spectrum header. The product of EXPOSURE times AREASCAL is the exposure duration for any fully exposed pixels in each channel. Bad CCD elements and bad events reduce the realized exposure duration in the affected pixels from this full exposure value. Because of this the average realized exposure duration within the selection region at each channel may be less than the full exposure. FRAC_EXP records this average exposure duration as a correction factor to the full exposure at each channel; it represents the source selection region in NET and TOTAL spectra and the background selection region in BKG spectra. Note that in HTR mode the background selection region is not spatial, and so this column always refers to the source selection region. Finally, BACKSCAL records the effective number of fully exposed pixels in the selection region (source for TOTAL spectra and background for BKG spectra).
The above description of the BACKSCAL column applies only to
Spectroscopy mode data; in HTR mode this column has nothing to do
with exposure. The true meaning of the BACKSCAL column
derives from its role in the formula for background correction:
