Temporal analysis of the least energetic events in pulsar data from observations with the high energy stereoscopic system

Abstract

It has been more than 60 years since astronomers turned their attention towards the 7֊ray window (> 100 TeV). Nowadays, 7֊ray astronomy has won its place as a separate branch of astronomy in its own right. The present thesis introduces the reader to 7-ray observations in the 〜 100 GeV-100 TeV energy window, but focuses, in particular, on the efforts to describe and detect the pulsed, Very High-Energy (VHE) 7-ray emission from pulsars. Pulsars are highly magnetised {B 〜 101շ G) ， rapidly rotating (P ~ 10—2 s) neutron stars. Periodic radio emission from pulsars has been detected in more than 1,500 cases, in contrast to their 7-ray signature which has been confirmed for only six of them and only up to a few GeV. There are many models in existence which attempt to reproduce the observed pulsed profiles and energy spectra in high energies (optical, X and ๆ rays). Nevertheless, two classes of models are the most popular: the Polar Cap and the Outer Gap models. Both predict spectral cut-offs at tens of GeV, which are consistent with previous upper limits in the VHE range. The six most energetic pulsars have been detected with the EGRET (Energetic Gamma Ray Experiment Telescope) instrument on-board the с GRO (Compton Gamma Ray Observatory) satellite. Probing the universe at higher energies requires a different detection technique. The Imaging Atmospheric Technique (lACT) exploits the Earth's atmosphere with the use of large, ground-based reflectors that are very sensitive to Cherenkov light (300-600 nm). The latter is produced during electromagnetic particle cascades, triggered by the interaction of VHE 7 rays with the top atmospheric layers. So far there has not been a confirmed pulsar detection using Cherenkov astronomy. The High Energy Stereoscopic System (H.E.S.S.) in Namibia is an array of four telescopes, which is sensitive above 100 GeV. H.E.S.S. uses the lACT to reject the lO3 times more abundant cosmic-ray events that suppress the 7-ray signal. The system is capable of stereoscopic observations of the same source with all four telescopes, which further eliminates background events. Despite the fact that imaging with H.E.S.S. is not effective below 100 GeV, lower energy events can still be recorded, along with a large portion of the background. The present thesis deals with the least energetic events (< 100 ĢeV) detectable with H.E.ร.ร., where pulsar 7-ray emission is likely to be present. A very sensitive temporal analysis has been performed in order to identify the potentially periodic events in the large background. The necessary procedures and parameters of the analysis are described in detail, prior to the results. The author has analysed data from two 7-ray pulsars, the Crab and PSR B1706-44, which were seen with EGRET up to ~ 20 GeV, as well as the binary radio pulsar PSR B1259-63, which has not been detected at high energies (> 1 eV). The data were optimised for the lowest energies, and the lowest energy threshold achieved was 75 GeV (in the case of PSR B1706-44). In all cases studied, the author coded and applied a number of periodicity tests that check for significant deviations from random noise. The resulting probabilities were not significantly low to support signal presence. Based on the background levels in the data sets, the author derived upper limits on the integral and differential flux. These upper limits were consistent with the Polar Cap and Outer Gap scenarios, within statistical errors, but constrain the alternative model of a spectrum with a simple exponential cut-off in the case of PSR B1706—44. Despite the lack of detection, these results represent the lowest energies explored with H.E.S.S.， yet. [brace not closed]