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Integral Spectrometer/Coded Mask Instrument Support
The INTEGRAL Spectrometer (SPI) is a coded-mask telescope, with a 19-element Germanium detector array. There are several complications regarding the spectral analysis of coded-aperture data. For XSPEC the most obvious problem is the source confusion issue; as there may be multiple sources in the FoV leading to different degrees of shadowing on different detectors. Thus, a separate instrumental response must be applied to a spectral model for each possible source, for each detector. If there are multiple sources in the FoV, then additional spectral models can be applied to an additional set of response matrices, enumerated as before over detector and dither pointing. This capability – to model more than one source at a time in a given minimization procedure – did not exist in XSPEC prior to v12. The other unique aspect of the INTEGRAL analysis is that the background is modeled along with the source(s).
XSPEC analysis of INTEGRAL/SPI data is very different from other
instruments is the manner in which the response matrices are
handled. Since there are a large number of responses involved in the
de-convolution problem, memory use becomes a concern. To load the
required response matrices (as XSPEC normally does), would require
floating-point memory locations
per source. This could become quite large for high-spectral resolution
and/or long observation scenarios. To address this problem, a
methodology has been developed to reconstruct the required 2-D
response matrices from a basis set, consisting of a small number (3)
of 2-D objects (template RMFs), and a larger number of 1-D objects
(component ARFs). The full matrices can then be reconstructed “on the
fly” at the minimization step of the calculation, and discarded after
each use. This, in principle, occurs all very transparently to the
user.
A fuller description of INTEGRAL data analysis appears in Chapter 2 of this manual.