Abstract
We present an instrument design capable of measuring linear X-ray polarization over a broad-band using conventional spectroscopic optics. A set of multilayer-coated flats reflects the dispersed X-rays to the instrument detectors. The intensity variation with position angle is measured to determine three Stokes parameters: I, Q, and U - all as a function of energy. By laterally grading the multilayer optics and matching the dispersion of the gratings, one may take advantage of high multilayer reflectivities and achieve modulation factors >90% over the entire 0.2 to 0.8 keV band. This instrument could be used in a small suborbital mission or adapted for use in an orbiting satellite to complement measurements at high energies. We present progress on laboratory work to demonstrate the capabilities of key components.
Highlights
The X-ray spectra are much steeper than the optical spectra, indicating that the X-rays are produced by the highest energy electrons, accelerated closest to the base of the jet or to shock regions in the jet
Brindle (1986) showed that the polarization of blazars increases from the IR to the optical band and can be as high as 25%, indicating that the X-ray polarization should be greater than 30%
Theoretical work indicates that active galactic nuclei (AGN) accretion disks and jets should be 10-20% polarized (McNamara et al 2009; Schnittmann & Krolik 2009, Dovciak, et al 2011) and that the polarization angle and magnitude should change with energy in a way that depends on the system inclination
Summary
The detectors provide linear polarization information but for other bandpasses, special steps are required, so an observer has to choose whether to obtain the additional Stokes parameters. Polarization studies in the optical and radio bands have been very successful. Perhaps the most important contribution of optical polarimetry led Antonucci & Miller (1985) to develop the seminal “unified model” of Seyfert galaxies, a subset of active galactic nuclei (AGN). Over the entire history of X-ray astronomy, there has never been a mission or instrument flown that was designed to measure the polarization of soft Xrays. We describe a few potential scientific studies to be performed with an X-ray polarimetry mission with sensitivity in the 0.1-1.0 keV band that would complement observations with an instrument such as GEMS
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