Diffuse X-ray emission in the Cygnus OB2 association
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abstract
We present a large-scale study of diffuse X-ray emission in the nearby
massive stellar association Cygnus OB2 as part of the Chandra Cygnus OB2 Legacy
Program. We used 40 Chandra X-ray ACIS-I observations covering $\sim$1.0
deg$^2$. After removing 7924 point-like sources detected in our survey,
background-corrected X-ray emission, the adaptive smoothing reveals large-scale
diffuse X-ray emission. Diffuse emission was detected in the sub-bands Soft
[0.5 : 1.2] and Medium [1.2 : 2.5], and marginally in the Hard [2.5 : 7.0] keV
band. From X-ray spectral analysis of stacked spectra we compute a total [0.5 :
7.0 keV] diffuse X-ray luminosity of L$_{\rm x}^{\rm
diff}\approx$4.2$\times$10$^{\rm 34}$ erg s$^{-1}$, characterized with plasma
temperature components at kT$\approx$ 0.11, 0.40 and 1.18 keV, respectively.
The HI absorption column density corresponding to these temperatures has a
distribution consistent with N$_{\rm H}$ = 0.43, 0.80 and 1.39
$\times$10$^{22}$ cm$^{-2}$. The extended medium band energy emission likely
arises from O-type stellar winds thermalized by wind-wind collisions in the
most populated regions of the association, while the soft band emission
probably arises from less energetic termination shocks against the surrounding
Interstellar-Medium. Super-soft and Soft diffuse emission appears more widely
dispersed and intense than the medium band emission. The diffuse X-ray emission
is generally spatially coincident with low-extinction regions that we attribute
to the ubiquitous influence of powerful stellar winds from massive stars and
their interaction with the local Interstellar-Medium. Diffuse X-ray emission is
volume-filling, rather than edge-brightened, oppositely to other star-forming
regions. We reveal the first observational evidence of X-ray haloes around some
evolved massive stars.
