A detailed X-ray investigation of ζ Puppis.
II. The variability on short and long timescales
Yael Naze, Lidia M. Oskinova, Eric Gosset
Stellar winds are a crucial component of massive stars, but their exact
properties still remain uncertain. To shed some light on this subject,
we have analyzed an exceptional set of X-ray observations of ζ Pup,
one of the closest and brightest massive stars. The sensitive
lightcurves that were derived reveal two major results. On the one hand,
a slow modulation of the X-ray flux (with a relative amplitude of up to
15% over 16h in the 0.3-4.0keV band) is detected. Its characteristic
timescale cannot be determined with precision, but amounts from one to
several days. It could be related to corotating interaction regions,
known to exist in ζ Pup from UV observations. Hour-long changes,
linked to flares or to the pulsation activity, are not observed in the
last decade covered by the XMM observations; the 17h tentative period,
previously reported in a ROSAT analysis, is not confirmed either and is
thus transient, at best. On the other hand, short-term changes are
surprisingly small (<1% relative amplitude for the total energy band).
In fact, they are compatible solely with the presence of Poisson noise
in the data. This surprisingly low level of short-term variability, in
view of the embedded wind-shock origin, requires a very high
fragmentation of the stellar wind, for both absorbing and emitting
features (>105 parcels, comparing with a 2D wind model). This is the
first time that constraints have been placed on the number of clumps in
an O-type star wind and from X-ray observations.
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