2017ApJ...851L...3C

We report the detection of the strongest 1₀₁–1₁₁ 6 cm H₂CO emission in our Galaxy. The detection toward the massive star-forming region G339.88-1.26 was made using the Shanghai Tianma radio telescope (TMRT). The G339.88-1.26 star formation region hosts one of the strongest 6.7 GHz methanol masers and has an accompanying collimated, ionized jet seen in radio continuum free-free emission. The peak flux density of the detected H₂CO emission is ∼19 Jy, one order of magnitude stronger than the nine previously known H₂CO maser sources. The corresponding luminosity is also brighter than has been observed in previous H₂CO maser sources, even those detected in the Central Molecular Zone of our Galaxy. A TMRT on-the-fly map of the region shows a point-like source structure and this, combined with the spectral characteristics of the H₂CO emission (multiple, narrow components), leads us to conclude that the detected H₂CO emission is masing (with a brightness temperature in excess of 10⁴ K). The detection of a very strong H₂CO maser in G339.88-1.26 suggests that ionized jet/outflow environments might provide efficient maser pumping for this transition, with the ionized jet/outflow-driven shock causing collisional excitation, and are also able to provide additional seed photons for maser amplification of the radio continuum emission.