The changing UV and X-ray properties of the Of?p star CPD -28 2561

TL;DR

This study monitors the Of?p star CPD -28 2561 across X-ray and UV wavelengths, revealing phase-dependent variability linked to magnetic confinement, with new insights into wind asymmetries and emission properties.

Contribution

It provides the first detailed phase-resolved X-ray and UV analysis of CPD -28 2561, highlighting unique variability patterns and wind asymmetries not previously observed in similar stars.

Findings

X-ray brightness varies by ~55% with rotational phase.

No significant hardness variation except at the second minimum.

UV line profiles show large phase-dependent variations and asymmetries.

Abstract

The Of?p star CPD -28 2561 was monitored at high energies with XMM-Newton and HST. In X-rays, this magnetic oblique rotator displays bright and hard emission that varies by ~55% with rotational phase. These changes occur in phase with optical variations, as expected for magnetically confined winds; there are two maxima and two minima in X-rays during the 73d rotational period of CPD -28 2561. However, contrary to previously studied cases, no significant hardness variation is detected between minima and maxima, with the exception of the second minimum which is slightly distinct from the first one. In the UV domain, broad-band fluxes remain stable while line profiles display large variations. Stronger absorptions at low velocities are observed when the magnetic equator is seen edge-on, which can be reproduced by a detailed 3D model. However, a difference in absorption at high velocities in the CIV and NV lines is also detected for the two phases where the confined wind is seen nearly pole-on. This suggests the presence of strong asymmetries about the magnetic equator, mostly in the free-flowing wind (rather than in the confined dynamical magnetosphere).