WPOL, a future space Compton wide field polarimeter: Optimization for polarimetry

Abstract

Polarimetry in the hard X-ray/soft gamma-ray domain (20–1000keV) is a new area of astrophysics that started to unveil in the past few years, mainly with the ESA/INTEGRAL mission results. Following these pioneering observations, it is appropriate to prepare a new concept which will allow the astronomers to map the X-ray/gamma-ray polarized sources in our Galaxy. WPOL is a wide field polarimeter which aims at monitoring the X-ray and gamma-ray sources and measuring their polarimetric properties. This camera will be used in space to map our Galaxy and also to alert a main instrument in case of transient events such as gamma-ray bursts, black hole binaries state transition, supernovae. It will be proposed, as an accompanying instrument, within the context of the next medium mission ESA call (M4) in January 2015. This concept is based upon coded mask imaging, with a detector unit composed of two layers of Silicon Double Sided Strip Detectors (DSSD) and a tungsten mask.In this article, we will present the key scientific drivers and the instrumental concept for WPOL (imaging technique, energy reconstitution and polarimetric measurements). Then, we will present an optimization of the thickness of the first detection layer performed through MEGALIB Monte-Carlo simulations. Finally, we will present a MEGALIB simulation of WPOL׳s observation of a source at 100keV to compute the minimum detectable polarization reached by the instrument.