Inspecting fit residuals

What you will learn

You will learn how you can inspect the residuals after model fitting.

Following a model fit it is good practice to inspect the fit residuals, i.e. the difference between the observed and the modelled event distributions. You do this using the comobsres script that projects the fit residuals along an event cone onto the sky. The example below show how a residual map is created for the fit of the Crab point source model.

$ comobsres
Input observation definition file [obs.xml] outobs.xml
Input model definition file [models.xml] results.xml
Residual map computation algorithm (SUB|SUBDIV|SUBDIVSQRT|SIGNIFICANCE) [SIGNIFICANCE]
Coordinate system (CEL - celestial, GAL - galactic) (CEL|GAL) [GAL]
Projection method (AIT|AZP|CAR|GLS|MOL|SFL|SIN|STG|TAN) [TAN]
Output residual map [resmap.fits]

On output comobsres creates a residual map that in the example above is named resmap.fits. The residual map is actually a map cube, containing a residual map for each energy bin. The map cube is displayed in the Figure below. The color scale ranges from -4 to +4 sigma. Some residuals appear to be present in the data, with the largest residuals being generally observed at low energies.

Residual maps for the four energy bins