A detailed X-ray investigation of ζ Puppis

Abstract

Context. One of the optically brightest and closest massive stars, ζ Pup, is also a bright X-ray source. Previously, its X-ray emission was found to be variable with light curves harbouring “trends” with a typical timescale longer than the exposure length, i.e. >1 d. The origin of these changes was proposed to be linked to large-scale structures in the wind of ζ Pup, but further characterization of the variability at high energies was needed to investigate this scenario. Aims. Since the previous papers of this series, a number of new X-ray observations have become available. Furthermore, a cyclic behaviour with a 1.78 d period was identified in long optical photometric runs, which is thought to be associated with the launching mechanism of large-scale wind structures. Methods. We analysed these new X-ray data, revisited the old data, and compared the X-ray light curves with the optical data, notably those taken simultaneously. Results. The behaviour of ζ Pup in X-rays cannot be explained in terms of a perfect clock because the amplitude and shape of its variations change with time. For example, ζ Pup was much more strongly variable between 2007 and 2011 than before and after this interval. Comparing the X-ray spectra of the star at maximum and minimum brightness yields no compelling difference beyond the overall flux change: the temperatures, absorptions, and line shapes seem to remain constant, well within errors. The only common feature between X-ray datasets is that the variation amplitudes appear maximum in the medium (0.6–1.2 keV) energy band. Finally, no clear and coherent correlation can be found between simultaneous X-ray and optical data. Only a subgroup of observations may be combined coherently with the optical period of 1.78 d, although the simultaneous optical behaviour is unknown. Conclusions. The currently available data do not reveal any obvious, permanent, and direct correlation between X-ray and optical variations. The origin of the X-ray variability therefore still needs to be ascertained, highlighting the need for long-term monitoring in multiwavelengths, i.e. X-ray, UV, and optical.

Surprises in the simultaneous X-ray and optical monitoring of π Aquarii

“…Following recommendations of the XRT team for such bright sources (see also Nazé et al 2018), the spectral fitting Bjorkman et al (2002). The maps correspond to a slice at v z = − 10 km s −1 , the mean value in RV of the system; values of +10 or 0 km s −1 provide similar results, however.…”

Variations on a theme: the puzzling behaviour of Schulte 12

“…Hence the presence of CIRs in Schulte 12 would not be a surprising occurrence. In this context, several cases of rotational modulation of the X-ray flux of O-stars have been reported in recent years and were attributed to such largescale structures (Oskinova et al 2001;Nazé et al 2013Nazé et al , 2018bMassa et al 2014;Rauw et al 2015b). However, their variation timescales are shorter (1.8-4.2 d) than for Schulte 12 and the peak-to-peak amplitude of their modulations is ∼10% of the mean flux, which is much smaller than for Schulte 12.…”

X-ray spectroscopy of massive stellar winds: previous and ongoing observations of the hot star ζ Pup

“…XMM shows variability for ζ Pup on a timescale of days in the total band, which may reflect the presence of CIRs (Nazé et al 2018. Temporal variation in the X-ray emission from hot stars might also be expected due to the growth and fading of shocks.…”

“…XMM has long used ζ Pup as a reference target, so 18 years of observations are available from this spacecraft. These observations have been extensively analyzed in Nazé et al (2012), Nazé et al (2013), Hervé et al (2013), and Nazé et al (2018). One early Chandra HETG spectrum with a 67 kilosecond exposure is also currently available (Cassinelli et al 2001).…”