Description

Sherpa includes the "additive" and "multiplicative" models of XSpec version 11.3. To use these models, simply prefix the original XSpec name by "xs"; for example, the name xsphabs represents the XSpec phabs model. To define a composite XSpec model such as

phabs * apec

do the following:

sherpa> xsphabs[abs1]
sherpa> xsapec[gas1]
sherpa> source 1 = abs1 * gas1

(Of course, one can use names other than "abs1" and "gas1" to represent these model components.)

Note that XSPEC models are always integrated over a bin and so require binned data.

Important Note:

XSpec models based on physical processes (e.g. line models such as raymond or absorption models such as wabs) assume that the dataspace is defined in keV. On the other hand Sherpa models are always calculated based on the input data scale. Thus when XSpec models are combined with Sherpa models, the user should be careful to ensure that both components have the same dataspace units; otherwise, calculated model amplitudes may be incorrect.

These models also expect that the x-values will always be energy bins. When the analysis setting is using non-energy bins (e.g., ANALYSIS WAVE) and an XSPEC model is defined, Sherpa converts the bins to energy before sending them to the XSPEC model. After the XSPEC model finishes, Sherpa converts back to the original units. Sherpa also scales the model values appropriately (e.g., if counts/keV came out of the XSPEC model, and Sherpa is working with wavelength bins, then Sherpa scales the output of the XSPEC model to counts/Angstrom).