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Humboldt-Universität zu Berlin - Mathematisch-Naturwissen­schaft­liche Fakultät - Experimentelle Elementarteilchenphysik

Michael Gajdus (2015)

The Vela pulsar in very high energy gamma-rays with H.E.S.S. II

PhD thesis, Humboldt Universität zu Berlin.

In the presented thesis the analysis of the first pulsar to be detected with the H.E.S.S. array is described. The high significance detection at the 11.4 σ level down to a hitherto unachievable 15 GeV energy threshold is a result of 24 h of observations on the Vela pulsar with the new CT 5 telescope introduced into the array in 2012. This is only the second pulsar to be detected in the very high energy (VHE) γ-ray regime. The capabilities of the advanced, template-based ImPACT reconstruction software are analysed and optimised in order to perform best for sources with steeply falling spectra in the lowest energy band (< 50 GeV) accessible with CT 5. This ensures that the maximum detection significance is obtained when analysing sources. The software was also optimised for two other source classes differing in the steepness of spectra to be used on other γ-ray sources by the H.E.S.S. Collaboration, thus providing customised analysis configurations for a broad range of source types. Of particular interest are the very high energy (VHE) spectra of pulsars as this is an almost completely unexplored energy domain. This does however entail smaller signals as the rate of γ-rays generally drops according to a power law function. New weighted statistical tests are introduced to improve the sensitivity to weak pulsations for use with future observations of pulsars. These tests are modifications of currently used tests and are applicable to any discrete data in which a single or many pulses are expected; they are powerful when the phase profile is unknown and with a basic weighting provide up to a 10 % boost in detection significance. The optimised analysis chain contributed to the detection of the Vela pulsar with phase averaged energy flux in the energy range (15, 125) GeV of 4.29( −1.02 +1.14 ) stat ( −3.31 +5.50 ) sys ×10 −11 erg cm −2 s −1 . The single pulse is characterised with an asymmetric Lorentzian function with a narrower trailing edge. The photon flux falls as a power law with index −5.39 which is moderately consistent with an Annular Gap emission model but represents a steeper drop in emission than that measured with the Fermi-LAT. The emission efficiency of the Vela pulsar in the VHE band is also evaluated as 0.0025 %, which is comparable to that of the Crab pulsar in the same energy regime measured with the MAGIC telescope array. Comparisons to other candidate VHE pulsars are also drawn. A weak constraint is placed on the energy at which Lorentz invariance violation occurs in terms of the Planck energy as E LIV > 10 −4 E P .