2014A&A...568A.117F

Very-high-energy (VHE, E>0.1TeV) γ-ray emission regions with angular extents comparable to the field-of-view of current imaging air-Cherenkov telescopes (IACT) require additional observations of source-free regions to estimate the background contribution to the energy spectrum. This reduces the effective observation time and deteriorates the sensitivity. A new method of reconstructing spectra from IACT data without the need of additional observations of source-free regions is developed. Its application is not restricted to any specific IACT or data format. On the basis of the template background method, which defines the background in air-shower parameter space, a new spectral reconstruction method from IACT data is developed and studied, the Template Background Spectrum (TBS); TBS is tested on published H.E.S.S. data and H.E.S.S. results. Good agreement is found between VHE γ-ray spectra reported by the H.E.S.S. collaboration and those re-analysed with TBS. This includes analyses of point-like sources, sources in crowded regions, and of very extended sources down to sources with fluxes of a few percent of the Crab nebula flux and excess-to-background ratios around 0.1. However, the TBS background normalisation introduces new statistical and systematic errors which are accounted for, but may constitute a limiting case for very faint extended sources. The TBS method enables the spectral reconstruction of data when other methods are hampered or even fail. It does not need dedicated observations of VHE γ-ray-free regions (e.g. as the On/Off background does) and circumvents known geometrical limitations to which other methods (e.g. the reflected-region background) for reconstructing spectral information of VHE γ-ray emission regions are prone to; TBS would be, in specific cases, the only feasible way to reconstruct energy spectra.