Smooth X-ray variability fromρOphiuchi A+B

Abstract

X-rays from massive stars are ubiquitous yet not clearly understood. In an XMM-Newton observation devoted to observing the first site of star formation in the ρ Ophiuchi dark cloud, we detect smoothly variable X-ray emission from the B2IV+B2V system of ρ Ophiuchi. We tentatively assign the emission to the primary component. The light curve of the pn camera shows a first phase of low, almost steady rate, then a rise phase of duration of 10 ks, followed by a high rate phase. The variability is seen primarily in the band 1.0−8.0 keV while little variability is detected below 1 keV. The spectral analysis of the three phases reveals the presence of a hot component at 3.0 keV that adds up to two relatively cold components at 0.9 keV and 2.2 keV. We explain the smooth variability with the emergence of an extended active region on the surface of the primary star as being due to its fast rotation (v sin i ∼ 315 km s −1 ). We estimate that the region has a diameter in the range 0.5−0.6 R * . The hard X-ray emission and its variability hint at a magnetic origin, as suggested for a few other late-O through early-B type stars. We also discuss an alternative explanation based on the emergence from occultation of a young (5−10 Myr) low mass companion that is bright and hot in X-rays.

Evidence for radio and X-ray auroral emissions from the magnetic B-type star ρ Oph A

“…Two X-ray bursts from ρ Oph A, separated by ≈ 1.2 days, were detected by XMM-Newton during observations performed in 2016. The basal (quiescent) X-ray emission was instead observed almost steady and well in accordance with the XMM-Newton measurements performed in 2013 (Pillitteri et al 2014). The two observing epochs are separated by more than 2.5 yrs, suggesting a steady mechanism of the ρ Oph A X-ray quiescent emission.…”

“…The X-ray measurements were averaged within phase bins of 0.02. The 2013 observations (Pillitteri et al 2014) are marked using the filled squares. The open squares refer to the 2016 observations (Pillitteri et al 2017).…”

First stars of theρOphiuchi dark cloud

“…We detected X-ray emission from ρ Ophiuchi A+B (source 50, details in Pillitteri et al 2014), ρ Ophiuchi C (source 62, B5V star with a low mass companion, details in Sect. 3.5.2) and from Haro 1-4 (DoAr 16, spectral type K4, source 84) which is the only Classical T-Tauri star (CTT) in the XMM-Newton field of view.…”

“…ρ Ophiuchi A+B Variable X-ray emission from the ρ Ophiuchi A+B system has been detected, this variability has been studied in details and published in a separate paper (Pillitteri et al 2014). Briefly, smooth variability was observed in ρ Ophiuchi on a time scale of 10 ks.…”

On X-ray pulsations inβCephei-type variables

“…The MCWS predicts the X-ray variability of magnetic massive stars associated with the stellar rotation due to the occupation of X-ray emitting regions by the cold torus of matter accumulated in the equatorial plane (Donati et al 2001(Donati et al , 2002. Such X-ray modulations on a rotational time scale are sometimes observed (e.g., Gagné et al 1997;Pillitteri et al 2014); however, this is not ubiquitous behavior among magnetic massive stars -some of these wellknown objects, such as τ Sco, do not show any evidence of X-ray variability over the stellar rotation period (Ignace et al 2010).…”