D. Göring

Morphometric analysis in gamma-ray astronomy using Minkowski functionals - Source detection via structure quantification

Aims. H.E.S.S. observes an increasing number of large extended sources. A new technique based on the structure of the sky map is developed to account for these additional structures by comparing them with the common point source analysis. Methods. Minkowski functionals are powerful measures from integral geometry. They can be used to quantify the structure of the counts map, which is then compared with the expected structure of a pure Poisson background. Gamma-ray sources lead to significant deviations from the expected background structure. The standard likelihood ratio method is exclusively based on the number of excess counts and discards all further structure information of large extended sources. The morphometric data analysis incorporates this additional geometric information in an unbiased analysis, i.e., without the need of any prior knowledge about the source. Results. We have successfully applied our method to data of the H.E.S.S. experiment. The morphometric analysis presented here is dedicated to detecting faint extended sources.

Shape analysis of counts maps

A novel approach for source detection via structural deviations from the typical features of a random background counts map is presented. Minkowski functionals, powerful tools from integral geometry, quantify the shape of level sets of a counts map. Compared to standard techniques, which use the total number of counts only, additional morphometric information is incorporated without the need for any prior knowledge about the source. Minkowski sky maps quantify local structural deviations; they localize and visualize potential sources.

The H.E.S.S. data acquisition system

The High Energy Stereoscopic System (H.E.S.S.) is an array of five Imaging Atmospheric Cherenkov Telescopes located in the Khomas Highland in Namibia. It measures cosmic gamma-rays with very high energies (> 100 GeV) using the Earths atmosphere as a calorimeter. The H.E.S.S. experiment has entered Phase II in September 2012 with the inauguration of a fifth telescope that is larger and more complex than the other four. The very large mirror area of 600 m2 in comparison to the 100 m2 of the smaller telescopes results in a lower energy threshold as well as an increased overall sensitivity of the system. Moreover, the huge effective area, due to the large mirror size, is crucial in the detection of short time scale low energy transient events. This paper will give a brief overview of the design principles of the current H.E.S.S. data acquisition and array control system. Particular emphasis is given to the new Target of Opportunity alert system that has recently been introduced to the array and allows the instrument to react to such an alert within 60 s.

Discovery of gamma-ray emission from the extragalactic pulsar wind nebula N157B with the High Energy Stereoscopic System

We present the significant detection of the first extragalactic pulsar wind nebula (PWN) detected in gamma rays, N157B, located in the large Magellanic Cloud (LMC). Pulsars with high spin-down luminosity are found to power energised nebulae that emit gamma rays up to energies of several tens of TeV. N157B is associated with PSRJ0537-6910, which is the pulsar with the highest known spin-down luminosity. The High Energy Stereoscopic System telescope array observed this nebula on a yearly basis from 2004 to 2009 with a dead-time corrected exposure of 46 h. The gamma-ray spectrum between 600 GeV and 12 TeV is well-described by a pure power-law with a photon index of 2.8 \pm 0.2(stat) \pm 0.3(syst) and a normalisation at 1 TeV of (8.2 \pm 0.8(stat) \pm 2.5(syst)) \times 10^-13 cm^-2s^-1TeV^-1. A leptonic multi-wavelength model shows that an energy of about 4 \times 10^49erg is stored in electrons and positrons. The apparent efficiency, which is the ratio of the TeV gamma-ray luminosity to the pulsars spindown luminosity, 0.08% \pm 0.01%, is comparable to those of PWNe found in the Milky Way. The detection of a PWN at such a large distance is possible due to the pulsars favourable spin-down luminosity and a bright infrared photon-field serving as an inverse-Compton-scattering target for accelerated leptons. By applying a calorimetric technique to these observations, the pulsars birth period is estimated to be shorter than 10 ms.