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تسجيل مستخدم جديدExtended Gamma-ray Emission from the G25.0+0.0 Region: A Star Forming Region Powered by the Newly Found OB Association?
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We report a study of extended $gamma$-ray emission with the Large Area Telescope (LAT) onboard the Fermi Gamma-ray Space Telescope, which is likely to be the second case of a $gamma$-ray detection from a star-forming region (SFR) in our Galaxy. The LAT source is located in the G25 region, $1.7^{circ} times 2.1^{circ}$ around $(l, b) = (25.0^{circ}, 0.0^{circ})$. The $gamma$-ray emission is found to be composed of two extended sources and one point-like source. The extended sources have a similar sizes of about $1.4^{circ} times 0.6^{circ}$. An $sim 0.4^{circ}$ diameter sub-region of one has a photon index of $Gamma = 1.53 pm 0.15$; and is spatially coincident with HESS J1837$-$069, likely a pulsar wind nebula. The other parts of the extended sources have a photon index of $Gamma = 2.1 pm 0.2$ without significant spectral curvature. Given their spatial and spectral properties, they have no clear associations with sources at other wavelengths. Their $gamma$-ray properties are similar to those of the Cygnus cocoon SFR, the only firmly established $gamma$-ray detection of an SFR in the Galaxy. Indeed, we find bubble-like structures of atomic and molecular gas in G25, which may be created by a putative OB association/cluster. The $gamma$-ray emitting regions appear confined in the bubble-like structure; similar properties are also found in the Cygnus cocoon. In addition, using observations with the the XMM-Newton we find a candidate young massive OB association/cluster G25.18+0.26 in the G25 region. We propose that the extended $gamma$-ray emission in G25 is associated with an SFR driven by G25.18+0.26. Based on this scenario, we discuss possible acceleration processes in the SFR and compare them with the Cygnus cocoon.
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