Probing the star formation origin of gamma rays from 3FHL J1907.0+0713

Abstract

Star-forming (SF) regions embedded inside giant molecular clouds (GMCs) are potential contributors to Galactic gamma rays. The gamma-ray source 3FHL J1907.0+0713 is detected with a significance of roughly 13$\sigma$ in the 0.2 $-$ 300 GeV energy range after the removal of gamma-ray pulsation periods of PSR J1906+0722 from the Fermi-LAT data set of about 10 years. The energy spectrum of 3FHL J1907.0+0713 is best-fitted to a power law model with a spectral index of 2.26 $\pm$ 0.05. The CO($J$ = 1$-$0) data taken by NANTEN2 revealed that 3FHL J1907.0+0713 is overlapping with a GMC having a peak velocity of about 38 km s${}^{-1}$. The best-fitting location of 3FHL J1907.0+0713 is measured to be approximately 0.13 degrees away from the Galactic supernova remnant (SNR) 3C 397 and it overlaps with a star that is associated with a bow-shock nebula. We show that there is no physical connection between 3FHL J1907.0+0713, 3C 397, as well as any positional coincidence with the pulsar. The spectrum of 3FHL J1907.0+0713 is fitting to both hadronic and leptonic gamma-ray emission models and the total luminosity at a distance of 2.6 kpc is calculated to be 1.1 $\times$ 10${}^{34}$ erg s${}^{-1}$. We also discuss possible SF origins of gamma rays from 3FHL J1907.0+0713, where SNRs, massive protostar outflows, stellar winds from runaway stars, colliding wind binaries, and young stellar clusters are considered as candidate sources.

To be published in Monthly Notices of the Royal Astronomical Society (accepted in 2020 December 7)

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