A photoelectric polarimeter based on a Micropattern Gas Detector for X-ray astronomy

Abstract

Polarimetry of cosmic X-rays is one of the possible observational approaches, together with spectroscopy, photometry and imaging, to study celestial sources. It can provide a general tool to explore the structure of compact sources and derive information on mass and angular momentum of supermassive objects. Comparing with the other three modalities, the development of X-ray polarimetry is modest, due to the inefficiency of traditional X-ray polarimeters. In this paper a new instrument (the Micropattern Gas Detector) to measure the linear polarization of X-ray sources with high efficiency is presented. It is based on the photoelectric effect. Angle and amount of polarization is computed from the angular distribution of the photoelectron tracks, reconstructed by a finely segmented gas detector. The device has pixel read-out allowing a full 2-D image reconstruction. The improvement in sensitivity is nearly two orders of magnitude with respect to traditional polarimeters. At the focus of a large X-ray telescope, in orbit, it can detect low level of polarization in galactic and extragalactic sources.