Hard X-ray emission in the star-forming region on2: Discovery with xmm-newton

We obtained X-ray XMM-Newton observations of the open cluster Berkeley87 and the massive star-forming region (SFR) ON2. In addition, archival infrared Spitzer Space Telescope observations were used to study the morphology of ON2, to uncover young stellar objects, and to investigate their relationship with the X-ray sources. It is likely that the SFR ON2 and Berkeley87 are at the same distance, 1.23kpc, and hence are associated. The XMM-Newton observations detected X-rays from massive stars in Berkeley87 as well as diffuse emission from the SFR ON2. The two patches of diffuse X-ray emission are encompassed in the shell-like H II region GAL75.84+0.40 in the northern part of ON2 and in the ON2S region in the southern part of ON2. The diffuse emission from GAL75.84+0.40 suffers an absorption column equivalent to A _V 28mag. Its spectrum can be fitted either with a thermal plasma model at T ≳ 30MK or by an absorbed power-law model with γ -2.6. The X-ray luminosity of GAL75.84+0.40 is L _X 6 × 10³¹ergs^-1. The diffuse emission from ON2S is adjacent to the ultra-compact H II (UCH II) region Cygnus2N, but does not coincide with it or with any other known UCH II region. It has a luminosity of L _X 4 × 10³¹ergs ^-1. The spectrum can be fitted with an absorbed power-law model with γ -1.4. We adopt the view of Turner & Forbes that the SFR ON2 is physically associated with the massive star cluster Berkeley87 hosting the WO-type star WR142. We discuss different explanations for the apparently diffuse X-ray emission in these SFRs. These include synchrotron radiation, invoked by the co-existence of strongly shocked stellar winds and turbulent magnetic fields in the star-forming complex, cluster wind emission, or an unresolved population of discrete sources.