Simple filters enable x-ray polarization measurements

Abstract

Measurements of the degree and angle of polarization of emission from astronomical sources enable physical characterization of the objects and environmental conditions fromwhich the photons originate. However, a lack of suitable instrumentation in the x-ray and gamma-ray regimes has been frustrating astronomers for over three decades. To date, only one unambiguous astrophysical x-ray polarization measurement has been obtained: the Crab Nebula was observed to exhibit 19% linear polarization at 2.6keV.1 Conventional x-ray polarimetry relies on Bragg reflection or Thomson scattering, but instruments operating on the basis of either effect tend to suffer from low sensitivity. The next generation of large x-ray observatories currently under study—such as the International X-ray Observatory2 (IXO) and Astro-H3— have triggered renewed interest in practical x-ray polarization measurements. Device efficiency improvements have been facilitated by the development of novel gaseous detectors that can operate in the x-ray regime.4, 5 We have been pursuing an alternative approach to achieve x-ray polarimetry by developing dichroic filters that work in an analogous way to standard optical Polaroid R © film.6–8 These low-mass devices could add a polarimetry capability to the existing focal-plane microcalorimeter instruments proposed for these new observatories, while avoiding the cost and complexity of developing and operating additional detectors solely dedicated to x-ray polarimetry.6 At visible wavelengths, polarization can be detected using highly ordered materials such as H-sheet Polaroid, a polyvinyl alcohol polymer impregnated with iodine. Polarization of 33.17keV x-rays using ordinary H-sheet material was successfully analyzed for the first time in 1997.9 However, the Figure 1. Energy spectra for an inclusion compound of 1-bromoadamantane in a thiourea host. The red and black curves are for χ = 90 and 0◦, respectively, where χ is the angle between the crystal’s dichroic axis and the polarization direction of the photons. Such measurements were first reported by Chao and collaborators.10