The early B-type star Rho Oph A is an X-ray lighthouse

Abstract

We present the results of a 140 ks XMM-Newton observation of the B2 star $\rho$ Ophiuchi A. The star has exhibited strong X-ray variability: a cusp-shaped increase of rate, similar to that which we partially observed in 2013, and a bright flare. These events are separated in time by about 104 ks, which likely corresponds to the rotational period of the star (1.2 days). Time resolved spectroscopy of the X-ray spectra shows that the first event is caused by an increase of the plasma emission measure, while the second increase of rate is a major flare with temperatures in excess of 60 MK ($kT\sim 5$ keV). From the analysis of its rise, we infer a magnetic field of $\ge 300$ G and a size of the flaring region of $\sim 1.4-1.9\times 10^{11}$ cm, which corresponds to $\sim 25\%-30\%$ of the stellar radius. We speculate that either an intrinsic magnetism that produces a hot spot on its surface or an unknown low mass companion are the source of such X-rays and variability. A hot spot of magnetic origin should be a stable structure over a time span of $\ge$2.5 years, and suggests an overall large scale dipolar magnetic field that produces an extended feature on the stellar surface. In the second scenario, a low mass unknown companion is the emitter of X-rays and it should orbit extremely close to the surface of the primary in a locked spin-orbit configuration, almost on the verge of collapsing onto the primary. As such, the X-ray activity of the secondary star would be enhanced by its young age, and the tight orbit as in RS Cvn systems and $\rho$ Ophiuchi would constitute an extreme system that is worthy of further investigation.

10 pages, 7 figures, 2 tables, A&A accepted, this is the version after the language editor corrections

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• Detection of magnetic field in the B2 star $\rho$ Oph A with ESO FORS2
• Detection of a centrifugal magnetosphere in one of the most massive stars in the $\rho$ Oph star-forming cloud