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

¹ INAF–Ossevatorio Astronomico Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
e-mail: pilli@astropa.inaf.it
² Harvard-Smithsonian CfA, 60 Garden st, Cambridge, MA 02138, USA
³ Institut für Physik und Astronomie, Universität Potsdam, USA Karl-Liebknecht-Strasse 24/25, 14476 Potsdam-Golm, Germany

Received: 15 November 2016
Accepted: 20 March 2017

Abstract

We present the results of a 140 ks XMM-Newton observation of the B2 star ρ Oph 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 correspond 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 ~ 5 keV). From the analysis of its rise, we infer a magnetic field of ≥300 G and a size of the flaring region of ~ 1.4 − 1.9 × 10¹¹ cm, which corresponds to ~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 ≥2.5 yr, 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. In both cases ρ Oph would constitute an extreme system that is worthy of further investigation.