The brightest known H2CO maser in the Milky Way: G339.88-1.26

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.