Hans M. Günther

REDSoX

Monte-Carlo ray-tracing for a soft X-ray spectroscopy polarimeter

Hans M. Günther, Mark Egan, Ralf K. Heilmann, Sarah N. T. Heine, Tim Hellickson, Jason Frost, Herman L. Marshall, Norbert S. Schulz, and Adam Theriault-Shay

Outline

  • Polarimety with Bragg reflection
  • REDSoX layout
  • Ray-tracing to verify the concept
  • An observation of Mk 421
  • Trade-off studies
  • Summary

Basic idea

Brewster's Angle

Bragg reflection

Brester angle Bragg reflection
By Pajs (wikipedia)

REDSoX basics

  • Rocket Experiment Demonstration for Soft X-ray polarimitry
  • Sounding rocket proposal - about 5 min above atmosphere
  • Imaging X-ray optics
eng overview
eng gratings eng focplane eng overview

Raytracing

  • MARXS - https://github.com/Chandra-MARX/marxs
  • Python code, GPL license, under active development
  • Material properties are an input from laboratory data or external programs
  • for REDSoX simulation: simplified mirror model

Ray-trace: single energy

https://tinyurl.com/redsoxpol

Details on grating placement

  • Reflection at Brewster angle reflects only one polarization direction
  • X-rays are hard to reflect at large angle, but Bragg reflection works for 2-8 nm
  • Bragg reflection only works for $$n\lambda = 2 d \sin \alpha$$
  • use laterally graded multi-layer mirror with $ d(x) = d_0 x $ and diffraction grating to disperse light onto different positions on the the mirror
  • Place gratings at $r_g = \frac{P}{\sqrt{2}D (\sin\beta_g + \cos\beta_g) \sin^3\gamma_g}$

Sub-aperturing

  • Mirror scattering larger in plane of reflection than perpendicular
  • Multi-layer mirrors have narrow bandpass, but this helps
observed spectrum

Effective area and figure of merit

  • Modulation factor: $M = 0\dots1$
  • Figure of merit: $F_m \equiv M \sqrt{A_\mathrm{eff}}$
observed spectrum

A simulated observation of Mk 421

  • Spectrum and flux taken from Chandra observation
  • Assuming 20% polarization
  • 5 min integration time
observed spectrum

Trade study: Mirror quality

mirror

Trade study: Pointing jitter

jitter

REDSoX Summary

  • Sounding rocket experiment for soft X-ray polarimetry
  • CAT gratings and laterally graded multi-layer mirrors
  • Ray-trace confirms concept works
  • Design is forgiving to alignment, jitter etc.
  • Polarimetry for brightest sources is within reach