Background simulations of the wide field imager of the ATHENA X-ray observatory

The ATHENA X-ray Observatory-IXO is a planned multinational orbiting X-ray observatory with a focal length of 11.5 m. ATHENA aims to perform pointed observations in an energy range from 0.1 keV to 15 keV with high sensitivity. For high spatial and timing resolution imaging and spectroscopic observations the 640 × 640 pixels² large DePFET-technology based Wide field Imager (WFI) focal plane detector, providing a field of view of 18 arcsec will be the main detector. Based on the actual mechanics, thermal and shielding design we present estimates for the WFI cosmic ray induced background obtained by the use of Monte-Carlo simulations and possible background reduction measures.

I. Introduction

THE European Space Agency-ESA is currently investigating the ATHENA L-class mission for a next generation X-ray observatory. ATHENA is based on a simplified IXO¹

International X-ray Observatory: the planned larger predecessor of ATHENA, which would have been jointly built by ESA and NASA.

[1], [2] design with the number of instruments and the focal length of the Wolter optics being reduced. One of the two instruments, the Wide Field Imager (WFI) is a DePFET [3], [4] based focal plane pixel detector, allowing spectroscopy in combination with high time and spatial resolution in the energy-range between 0.1 and 15 keV. In order to fulfill the mission goals a high sensitivity is essential, especially to study faint and extended sources. To achieve the required sensitivity a background rate of cts kev^™1 cm²s is required, making a detailed understanding of the detector background induced by cosmic ray particles crucial. During mission design generally extensive Monte-Carlo simulations are used to estimate the detector background in order to optimize shielding components and software rejection algorithms. The Geant4 tool-kit [5], [6] is frequently the tool of choice for this purpose. In the context of our previous work for SIMBOL-X [7], [8], [9], [10], [11] and IXO [12], [13], [14] we present recent results of our estimates for the ATHENA WFI cosmic ray induced background, which demonstrate that DEPFET-technology based detectors are able to achieve the required sensitivity.