Multi-component study of extended sources with a likelihood method

The spectral and morphological analysis for gamma-ray sources with multiple emission components remains a major challenge for Cherenkov telescopes due to background emission from diffuse gamma rays. Current methods of background suppression, usually based on the bin-by-bin subtraction of OFF-source data do not allow an analysis of the various background components. As an alternative, we present an approach based on an event-by-event likelihood fit of ON-source data, using a combined spectral model for the source emission as well as for the gamma-like background obtained from fits of the OFF-source data. Multiple emission components are separated by successive fits in different energy regimes and spectral variations inside the extended source is derived. The performance of this approach is evaluated with toy Monte-Carlo studies. For the application to real data, two well-studied H.E.S.S. sources are re-examined: the extragalactic point-source PKS 2155-304 and the extended pulsar wind nebula HESS J1825-137. For the latter, radial variation of the emission spectral index was evaluated, confirming earlier findings by the H.E.S.S. collaboration.