Abstract
The ctools open-source software package was developed for the scientific analysis of astronomical data from Imaging Air Cherenkov Telescopes (IACTs), such as H.E.S.S., VERITAS, MAGIC, and the future Cherenkov Telescope Array (CTA). To date, the software has been mainly tested using simulated CTA data; however, upon the public release of a small set of H.E.S.S. observations of the Crab nebula, MSH 15–52, RX J1713.7–3946, and PKS 2155–304 validation using real data is now possible. We analysed the data of the H.E.S.S. public data release using ctools version 1.6 and compared our results to those published by the H.E.S.S. Collaboration for the respective sources. We developed a parametric background model that satisfactorily describes the expected background rate as a function of reconstructed energy and direction for each observation. We used that model, and tested all analysis methods that are supported by ctools, including novel unbinned and joint or stacked binned analyses of the measured event energies and reconstructed directions, and classical On-Off analysis methods that are comparable to those used by the H.E.S.S. Collaboration. For all analysis methods, we found a good agreement between the ctools results and the H.E.S.S. Collaboration publications considering that they are not always directly comparable due to differences in the datatsets and event processing software. We also performed a joint analysis of H.E.S.S. and Fermi-LAT data of the Crab nebula, illustrating the multi-wavelength capacity of ctools. The joint Crab nebula spectrum is compatible with published literature values within the systematic uncertainties. We conclude that the ctools software is mature for the analysis of data from existing IACTs, as well as from the upcoming CTA.
Highlights
Gamma-ray photons are powerful probes for the most extreme and violent phenomena in the Universe
We examined the spectral residuals for nine spatial sub-regions, and the radial counts profiles and residuals as a function of offset angle θ for six energy bands
We have shown that it is possible to combine analysis techniques tailored to each instrument in a joint analysis, going beyond the approach illustrated in Nigro et al (2019) that analyse LAT data by unconventionally using the On-Off method to match the standard approach for Imaging Air Cherenkov Telescopes (IACTs) data
Summary
Gamma-ray photons are powerful probes for the most extreme and violent phenomena in the Universe. Studying these VHE gamma rays provides unique insights into the acceleration physics, the nature of the accelerated particles, relativistic particle propagation, the impact of the particles on the source environment, and the distribution of particle accelerators in the Universe. In classical IACT data analyses, the residual background is taken into account by estimating its contribution from gamma-ray source free regions in the same field of view, or, more rarely, from independent “Off” observations. We implemented these analysis methods in ctools, an open-source software package developed in the context of the Cherenkov Telescope Array (CTA) project (Knödlseder et al 2016a,b). The values of the safe energy thresholds are stored in the effective area component of the instrument response function of each observation, and limiting the analysis to energies above this threshold ensured that we used only events for which the instrument response information is accurate
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