HARD X-RAY EMISSION IN THE STAR-FORMING REGION ON 2: DISCOVERY WITH<i>XMM-NEWTON</i>

We obtained X-ray XMM-Newton observations of the open cluster Berkeley 87 and the massive star-forming region (SFR) ON 2. In addition, archival infrared Spitzer Space Telescope observations were used to study the morphology of ON 2, to uncover young stellar objects, and to investigate their relationship with the X-ray sources. It is likely that the SFR ON 2 and Berkeley 87 are at the same distance, 1.23 kpc, and hence are associated. The XMM-Newton observations detected X-rays from massive stars in Berkeley 87 as well as diffuse emission from the SFR ON 2. The two patches of diffuse X-ray emission are encompassed in the shell-like H region GAL 75.84+0.40 in the northern part of ON 2 and in the ON 2S region in the southern part of ON 2. The diffuse emission from GAL 75.84+0.40 suffers an absorption column equivalent to AV ≈ 28 mag. Its spectrum can be fitted either with a thermal plasma model at T&amp;gt;∼30 MK or by an absorbed power-law model with γ≈−2.6. The X-ray luminosity of GAL 75.84+0.40 is LX≈1×1032 erg s−1. The diffuse emission from ON 2S is adjacent to the ultra-compact H (UCH ) region Cygnus 2N, but does not coincide with it or with any other known UCH region. It has a luminosity of LX≈6 × 1031 erg s−1. The spectrum can be fitted with an absorbed power-law model with γ≈−1.4. We adopt the view of Turner &amp;amp; Forbes (1982) that the SFR ON 2 is physically associated with the massive