2020ApJ...898..157M

We report a systemic search for nonmetastable NH₃ masers toward a high-mass star-forming region Sgr B2(N) using the Shanghai 65 m Tianma radio telescope with its Ku-band (12-18 GHz) and K-band (18-26.5 GHz) receivers. A total of 18 maser emission transitions were detected, and eight of these are newly identified NH₃ maser transitions in interstellar space. Most (five) of the new maser transitions were found from the Ku-band observations which were absent from previous searches. Rotation diagram analysis for the 18 quasi-thermal transitions derived a high gas kinematic temperature (∼400 K) and an extremely high H₂ column density (∼5 x 10²⁵ cm^–2). Such conditions suggest that the NH₃ masers are likely pumped by vibrational excitation. Compared with quiet maser activity observed more than 20 yr ago, NH₃ masers may be bursting in our observation time when the gas kinematic temperature might increase from ∼150 to ∼400 K. As extreme conditions (either high gas kinematic temperature and/or high gas column density) are required to excite nonmetastable NH₃ masers, it can be argued that the detected NH₃ maser outburst may be linked to a more intensive accretion-burst event from an high-mass young stellar object in the Sgr B2(N) region.