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

Aims. We have performed a set of highand low-spectral resolution phase-resolved X-ray observations of the magnetic B star β Cep, for which theoretical models predict the presence of a confined wind emitting X-rays from stationary shocks. Some of the models predict, given the peculiar geometry of β Cep, strong rotational modulation of the X-ray emission, while other models predict a much lower amplitude modulation at 90 deg phase shift from the modulation predicted from the first group of models. Our observations were designed to provide a stringent test of such models. Methods. We obtained four observations spaced in rotational phase with XMM-Newton (using both the EPIC cameras and the RGS spectrograph) and with Chandra (using the LETG spectrograph). 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. Results. The strong rotational modulation predicted by the early, static magnetically confined wind model for the X-ray emission is not observed in β 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.