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HEASoft and XSPEC are now available as conda packages. See details ....


Background models in xspec

xspec v11

In v11 additive model components can be specified with the /b qualifier. eg 
   XSPEC> model phabs(pow + ga) + pow/b
These flagged components are multiplied only by the RMF and not by the ARF. If there is no ARF read in (ie the effective areas are included in the response matrix) then the flagged components are multiplied by a version of the response each row of which sums to unity.

xspec v12

In v12 the /b qualifier is no longer supported and is replaced by a more general mechanism. Separate model components can now have different response matrices. So, suppose the response is example.rmf and the arf is example.arf then : 
   XSPEC12> model phabs(pow + ga)
   XSPEC12> response 2:1 example.rmf
   XSPEC12> model 2:myback pow
sets up the background model identical to that in the v11 example above. myback is the name assigned for the background model in this example. However, unlike v11, the background model can be any allowed xspec model combination. Parameters in the background model can be changed by eg : 
   XSPEC12> newpar myback:1 2.0

If multiple datasets are in use then the additional responses are defined by eg : 

   XSPEC12> response 2:1 example.rmf
   XSPEC12> response 2:2 example2.rmf
   XSPEC12> response 2:3 example3.rmf
If the datasets are in separate datagroups then the second model has one set of parameters for each datagroup in the same way as the first model.

If there is no ARF in use and the response contains the effective area then in order to match the v11 /b algorithm it is necessary to create a new response matrix where each row sums to unity.