Computation of the off-axis effective area of the New Hard X-ray Mission modules by means of an analytical approach

TL;DR

This paper introduces an analytical method to efficiently compute the off-axis effective area of X-ray telescope mirrors with multilayer coatings, improving over traditional ray-tracing techniques.

Contribution

The authors develop a novel analytical formalism for calculating the off-axis effective area of multilayer-coated X-ray mirrors, simplifying and speeding up the process.

Findings

Validated the analytical approach against ray-tracing results.

Applied the method to NHXM optical modules.

Demonstrated improved computational efficiency.

Abstract

One of the most important parameters determining the sensitivity of X-ray telescopes is their effective area as a function of the X-ray energy. The computation of the effective area of a Wolter-I mirror, with either a single layer or multilayer coating, is a very simple task for a source on-axis at astronomical distance. Indeed, when the source moves off-axis the calculation is more complicated, in particular for new hard X-ray imaging telescopes (NuSTAR, ASTRO-H, NHXM, IXO) beyond 10 keV, that will make use of multilayer coatings to extend the reflectivity band in grazing incidence. Unlike traditional single-layer coatings (in Ir or Au), graded multilayer coatings exhibit an oscillating reflectivity as a function of the incidence angle, which makes the effective area not immediately predictable for a source placed off-axis within the field of view. For this reason, the computation of the off-axis effective area has been so far demanded to ray- tracing codes, able to sample the incidence of photons onto the mirror assembly. Even if this approach should not be disdained, it would be interesting to approach the same problem from an analytical viewpoint. This would speed up and simplify the computation of the effective area as a function of the off-axis angle, a considerable advantage especially whenever the mirror parameters are still to be optimized. In this work we present the application of a novel, analytical formalism to the computation of the off-axis effective area and the grasp of the NHXM optical modules, requiring only the standard routines for the multilayer reflectivity computation.