Magnetic fields in X-ray emitting A-type stars

Abstract

A common explanation for the observed X-ray emission of A-type stars is the presence of a hidden late-type companion. While this hypothesis can be shown to be correct in some cases, there is also evidence suggesting that low-mass companions cannot be the proper cause for the observed X-ray activity in all cases. Babel & Montmerle (1997) presented a theoretical framework to explain the X-ray emission for magnetic Ap/Bp stars, focusing on the A0p star IQ Aur. We test if this theoretical model is capable to explain the observed X-ray emissions. We present observations of 13 A-type stars that have been associated with X-ray emission detected by ROSAT. To determine the mean longitudinal magnetic field strength we measured the circular polarization in the wings of the Balmer lines using FORS 1. Although the emission of those objects with magnetic fields fits the prediction of the Babel & Montmerle model, not all X-ray detections are related to the presence of a magnetic field. Additionally, the strengths of magnetic fields do not correlate with the X-ray luminosity and thus the magnetically confined wind shock model cannot explain the X-ray emission from all investigated stars.