Introducing The SHell misAlignment Detection for straylight Estimation (SHADE) algorithm: the case of XMM-Newton

TL;DR

This work tackles stray-light contamination in Wolter-I X-ray telescopes, where single reflections off hyperbolic mirror segments produce arc-like patterns whose geometry depends on both source off-axis angle and shell alignment. The authors introduce SHADE (Shell misAlignment Detection for straylight Estimation), a two-parameter per-shell model ($γ$ for tilt amplitude and $ξ$ for tilt orientation) implemented in an analytical framework to reproduce stray-light arcs and infer misalignments from observed patterns. By applying SHADE to XMM-Newton observations of GX5-1, the method yields quantitative tilt estimates for shell 16 (e.g., $γ ≈ 18.3''$, $ξ ≈ 5.99$ rad with uncertainties) and demonstrates the ability to extend to multi-shell and double-arc configurations, including a case with shells 10 and 11. Time-invariance of stray-light patterns across epochs supports pooling data for improved calibration, highlighting SHADE’s potential to enhance stray-light modeling for current and future Wolter-I telescopes and to inform pre/post-launch calibration and design considerations.

Abstract

When performing X-ray observations with a Wolter-I telescope, the presence of bright off-axis sources can introduce unfocused rays, known as straylight, which contaminate the detector and compromise the scientific analysis. Among the different components of straylight, single reflections off the hyperboloid section of the mirror shells often produce arc-like patterns on the detector. These arcs depend not only on the off-axis angle of the source but also on the geometrical alignment of the individual shells. In this paper, we introduce the SHell misAlignment Detection for straylight Estimation (SHADE) algorithm, a novel and flexible tool designed to infer the misalignment parameters of individual shells, reproduce the geometry of straylight arcs and predict its pattern on the detector. SHADE allows us to model each shell displacement with two parameters: $(γ,ξ)$ that represents the tilt amplitude and direction. While the algorithm is general and applicable to any Wolter-like telescope, we demonstrate its effectiveness using a set of XMM-Newton observations of the low-mass X-ray binary GX5-1. As a proof of concept, we recover the best-fit misalignment parameters for a selected shell, obtaining $γ= 21.9''^{+10.3}_{-9.02}$ and $ξ= 5.88^{+1.02}_{-0.97}$ rad. SHADE represents a new approach to diagnosing mirror misalignments from straylight patterns and can support both pre and post-launch calibration efforts and future telescope designs.

Figures (25)

• Figure 1: Observation 0932201101 performed around the LMXB GX5-1, showing an example of the presence of the stray light arcs for X-ray observations performed by Wolter I telescope with an off-axis source. The axes refer to pixel coordinates.
• Figure 2: Representation of a sketch of a tilted optical axis. The tilted shell orientation is specified by the angles $\gamma$ and $\xi$, in cylindrical coordinates. The source direction is specified by the angles $\theta$, $\iota$ in the same reference frame.
• Figure 3: Observations considered in the analysis of the stray light arcs. The quality of these observations depends on the shared aimpoint and relationship between the PA values. Also, the different time span covered helps to explore the time variability of the phenomenon.
• Figure 4: Visual representation of the geometrical features of the arcs reproduced in this study, including bifurcations and different spacing among shells.
• Figure 5: Stray light pattern generated with parametric Eqs \ref{['params_eq_1']} and \ref{['params_eq_2']} considering a source at $\theta = 1^\circ$ and $\iota = 3\pi/2$. The number of shells generated is limited due to the geometric configuration of the telescope.