Discovery of the binary nature of the magnetospheric B-type star ρ Oph A

Context. The nearby B-type star ρ Oph A was recently identified as a rapidly rotating magnetic B-type star with variable radio and X-ray emission consistent with a magnetospheric origin. Aims. We aim to refine constraints on ρ Oph A's magnetic field by using high-resolution spectropolarimetry, and to scrutinise its spectrum for indications of the expected Hα emission. Methods. We present a high-resolution spectropolarimetric time series obtained with ESPaDOnS, which we used to perform a magnetic analysis using least-squares deconvolution (LSD). The star's rotation period was revisited using Kepler-2 space photometry. Results. We find that ρ Oph A is a spectroscopic binary consisting of two B-type stars with masses of about 8 and 10 M_⊙ on a slightly eccentric 88-day orbit, with the magnetic field associated with the smaller Ab component. This leads to ρ Oph Ab's 4 kG surface magnetic dipole being approximately twice as strong as the previously reported 2 kG. The oblique rotator model derived from the longitudinal magnetic field curve agrees well with the LSD Stokes V profiles, indicating that the magnetic field is likely to be very nearly dipolar. The orbital and rotational axes appear to be aligned. We report for the first time ρ Oph Ab's magnetospheric Hα emission, which is consistent with an origin in a centrifugal magnetosphere. We also demonstrate that ρ Oph Ab's light curve can be recovered from the previously reported Kepler-2 light curve of ρ Oph C, and that it demonstrates prominent magnetospheric eclipses similar to those of ρ Ori E, which can be reproduced using a dipolar rigidly rotating magnetosphere model. Both Hα and the light curve are indicative of a strongly asymmetric magnetosphere. Conclusions. All indications are that ρ Oph Ab's magnetic field is essentially dipolar, meaning that contributions from higher-order multipoles probably cannot explain the strong asymmetry in the magnetosphere. Only two other stars show comparable degrees of asymmetry, both of which are also close binaries, suggesting that binarity can affect the magnetospheric plasma distributions.