Energy-dependent gamma-ray morphology Estimation Tool in Gammapy

Abstract: An understanding of the energy dependence of gamma-ray sources can yield important information on the underlying emission mechanisms. However, despite the detection of energy-dependent morphologies in many TeV sources, we lack a proper quantification of such measurements. We introduce an estimation tool within the Gammapy landscape, an open-source Python package for the analysis of gamma-ray data, to quantify the energy-dependent morphology of a gamma-ray source. The proposed method for this estimation tool fits the spatial morphology in a global fit across all energy slices (null hypothesis) and compares this to separate fits for each energy slice (alternative hypothesis). These are modelled using forward-folding methods, and the significance of variability is quantified by comparing the test statistic between the two hypotheses. We present a general tool to probe changes in the spatial morphology with energy, employing a full forward folding approach with 3D likelihood. We present its usage on a real dataset from H.E.S.S., and also on a simulated dataset to quantify the significance of energy dependence for sources of different sizes. In the first example, which utilises a subset of data from HESSJ1825-137, we observe extended emission at lower energies that becomes more compact at higher energies. The tool indicates very significant variability (9.8{\sigma}) in the case of the largely extended emission. In the second example, a source with a smaller extent (~0.1{\deg}), simulated using the CTAO response, shows the tool can still provide a statistically significant variation (9.7{\sigma}), despite the small scale.