Testing magnetically confined wind shock models for Beta Cep using XMM-Newton and Chandra phase-resolved X-ray observations

Abstract

(abridged) We have performed a set of phase-resolved X-ray observations of the magnetic B star Beta Cep, for which theoretical models predict the presence of a confined wind emitting X-rays from stationary shocks. We obtained four observations spaced in rotational phase with XMM-Newton and with Chandra. A detailed analysis of the data was performed to derive both photometric and spectral parameters from the EPIC data, searching for rotational modulation, and to derive the location of the X-ray plasma from the line ratios in the He-like triplets of N, O and Ne from the RGS data. The LETG data were used to constrain the presence of bulk motions in the plasma. The strong rotational modulation predicted by the early, static magnetically confined wind model for the X-ray emission is not observed in Beta Cep. The small modulation present goes in the opposite direction, pointing to the absence of any optically thick disk of neutral material, and showing a modulation consistent with the later, dynamic models of magnetically confined wind models in B stars. The lack of observed bulk motion points to the plasma being confined by a magnetic field, but the low plasma temperature and lack of any flaring show that the plasma is not heated by magnetic reconnection. Therefore, the observations point to X-ray emission from shocks in a magnetically confined wind, with no evidence of an optically thick, dense disk at the magnetic equator