Dichroic Filters For Astronomical X-Ray Polarimetry

Abstract

We describe a programme of research to investigate materials which exhibit linear dichroism at X-ray wavelengths, and to assess their potential as astronomical X-ray polarimeters. The availability of polarising filters for X-rays would offer an efficient method of quantifying the polarisation of X-ray radiation from distant, cosmic sources. A polarising dichroic filter is expected to be a compact, low mass device which could add polarimetry capability to an instrument by being introduced into the optical path ahead of a detector. We provide estimates of sensitivity in the context of the proposed XEUS observatory, which compare favourably with those for the most promising alternative technology. We also discuss a number of astrophysical sources which are suited to polarimetric study at discrete energies offered by the X-ray dichroic filter design.

Narrow-band x-ray polarizing filters

“…We have shown 20 that the minimum detectable polarization for a number of sources could compare favorably to the micro-structured proportional counter instrument 24 based on the XEUS effective area and assuming the detectors are capable of resolving the edges of interest. We showed M DP values of a few percent for four active galactic nuclei (AGN) at 2.8 keV assuming the success of the bromine polarisers could be replicaeted at Cl-K (2.8 keV), we also showed much better than 1% M DP for these sources if a filter could be made to operate at lower energies (∼ 1 keV and below).…”

“…(2006) describe a novel way to exploit the polarization dependence of photo-excitation of core level electrons which yields a polarization sensitive transmission filter. 20 The polarization sensitivity arises from the difference in transmission of a family of materials which exhibit dichroism in narrow (∼ 10 eV) energy bands close to atomic absorption edges, where the electron is excited into a bound, molecular orbital. The simplicity of such a device makes it an ideal instrument for future X-ray observatories as it decouples the polarization sensitivity from the intrinsic efficiency of the detector.…”

Polarization of X-ray lines from galaxy clusters and elliptical galaxies - a way to measure the tangential component of gas velocity

“…From Table 4, we see that the following two key requirements should be improved to detect polarization from galaxy clusters: detector QE and energy resolution. One possibility of achieving good energy resolution is to use narrow‐band dichroic filters 8 (Bannister et al 2006; Martindale et al 2007). In such instruments the polarization sensitivity arises from the difference in transmission of a family of materials which exhibit dichroism in narrow (∼10 eV) energy bands close to atomic absorption edges, where the electron is excited into a bound, molecular orbital.…”

Simple filters enable x-ray polarization measurements

“…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][7][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.…”

“…[6][7][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.…”