Volume 602, June 2017
|Number of page(s)||10|
|Section||Stellar structure and evolution|
|Published online||20 June 2017|
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
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
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.
Key words: stars: activity / stars: individual: Rho Ophiuchi / stars: early-type / stars: magnetic field / starspots / X-rays: stars
© ESO, 2017
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