Design of a broadband soft x-ray polarimeter

Abstract

We describe an optical design and possible implementation of a broadband soft x-ray polarimeter. Our arrangement of gratings is designed optimally for the purpose of polarimetry with broadband focusing optics by matching the dispersion of the spectrometer channels to laterally graded multilayers (LGMLs). The system can achieve polarization modulation factors over 90%. We implement this design using a single optical system by dividing the entrance aperture into six sectors; high efficiency, blazed gratings from opposite sectors are oriented to disperse to a common LGML forming three channels covering the wavelength range from 35 to 75 Å (165 to 350 eV). The grating dispersions and LGML position angles for each channel are 120 deg to each other. CCD detectors then measure the intensities of the dispersed spectra after reflection and polarizing by the LGMLs, giving the three Stokes parameters needed to determine a source's linear polarization fraction and orientation. The design can be extended to higher energies as LGMLs are developed further. We describe examples of the potential scientific return from instruments based on this design.

A small satellite version of a soft x-ray polarimeter

“…Schematically, the design is the same as for the REDSoX Polarimeter. 47 Here, we outline various modifications for PiSoX.…”

“…Tolerances were determined using analytical formulae verified by raytracing. 47 Mechanical machining and assembly tolerances are sufficient for most aspects of the system. Aligning the gratings and the grating assemblies are carried out on an optical bench and in the MIT X-ray Polarimetry beamline; see sections 2.7.2 and 2.7.3 for details.…”

“…The X-ray background in the PiSoX bandpass is dominated by Galactic emission. A full ray-trace simulation including in particular gratings and the multi-layer mirror using the marxs code 47,59 show that off-axis X-rays are dispersed in such a way that their reflection is suppressed by the ML, in contrast to on-axis photons which are diffracted such that they hit the Bragg-peak. Using a measured flux for the Galactic emission and background AGN, 60 the ray-trace predicts an X-ray background rate of 2 counts per 1 Ms -clearly negligible.…”

The X-ray Polarization Probe mission concept

“…Each of these mirrors simultaneously illuminates a Low Energy Polarimeter (LEP), Medium Energy Polarimeter (MEP), and High Energy Polarimeter (HEP) as shown schematically in Figure 2. The LEP consists of gratings which select energy, variable spacing multilayer mirrors which are tuned to the Bragg angle where they are efficient polarizers and imaging detectors which map detector position to photon energy; the concept is described by Marshall et al 6 The gratings are Low Energy Polarimeter (0.2-0.8 keV)…”

“…More conceptual detail is provided in Marshall et al (2018). 6 Using LGMLs at three distinct orientations (e.g., 120 • to each other) provides enough information to measure the I, Q, and U Stokes parameters from which the linear polarization angle and magnitude can be determined. Gratings closer to the mirror provide greater dispersion (and thus potentially more precision in the energy determination), at the cost of requiring larger grating area and larger detectors.…”