Stokes imaging of AM Her systems using 3D inhomogeneous models-II. Modelling X-ray and optical data of CP Tucanae

TL;DR

This study develops a comprehensive 3D model to simultaneously fit optical and X-ray data of the polar CP Tuc, revealing the importance of absorption effects and the primary emission region's location.

Contribution

An extended CYCLOPS code was created to model both optical and X-ray emissions, incorporating absorption and bremsstrahlung, enabling consistent interpretation of CP Tuc data.

Findings

Absorption in the pre-shock region is essential to reproduce X-ray orbital modulation.

The primary emission region is near the rotation pole, pointing towards the observer.

Self-eclipse alone cannot explain the X-ray observations.

Abstract

The viewing geometry of the polar CP Tuc that better explains its optical and X-ray light curves is controversial. Previous modelling of white-light polarimetric data considered the partial self-eclipse of an extended inhomogeneous emitting region. Alternatively, phase-dependent absorption has been used to reproduce the X-ray data. This paper presents new optical polarimetric data of CP Tuc and a model that consistently explains its optical and X-ray data. The model was based on an extension of the CYCLOPS code that added X-ray bremsstrahlung emission and pre-shock region absorption to the original version, which only accounted for cyclotron emission. The new code creates the possibility of simultaneous optical and X-ray fitting. We show that self-eclipse and absorption data have distinct signatures on the X-ray spectra. Although we were able to reasonably fit the CP Tuc optical data to cases of absorption and self-eclipse, we were only able to reproduce the X-ray orbital modulation after considering the absorption in the pre-shock region. Specifically, we were unable to reproduce the X-ray observations in the self-eclipse case. We found that the primary emitting region in CP Tuc is located near the rotation pole that approximately points to the observer.