Hard X-ray emission in the star-forming region ON2: discovery with XMM-Newton

TL;DR

This study reports the discovery of diffuse hard X-ray emission in the star-forming region ON 2 and Berkeley 87 using XMM-Newton, suggesting multiple possible origins including synchrotron radiation, cluster winds, or unresolved sources.

Contribution

First detection of diffuse X-ray emission in ON 2 with analysis of its spectral properties and potential origins, linking it to the star-forming environment and massive star cluster.

Findings

Diffuse X-ray emission detected in ON 2 and Berkeley 87.

Spectral fits suggest thermal plasma or power-law models for the emission.

Possible origins include synchrotron radiation, cluster winds, or unresolved sources.

Abstract

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. 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 II 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 A_V approx. 28 mag. Its spectrum can be fitted either with a thermal plasma model at T < 30 MK or by an absorbed power-law model with gamma; approx. -2.6. The X-ray luminosity of GAL 75.84+0.40 is L_X approx. 6 10^31 erg/s. The diffuse emission from ON 2S is adjacent to the ultra-compact H II (UCHII) region Cygnus 2N, but does not coincide with it or with any other known UCHII region. It has a luminosity of L_X approx. 4 10^31 erg/s. The spectrum can be fitted with an absorbed power-law model with gamma; approx.-1.4. We adopt the view of Turner and Forbes (1982) that the SFR ON 2 is physically associated with the massive star cluster Berkeley 87 hosting the WO type star WR 142. 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.