2017MNRAS.466.4364C

We report the detections of H₂CO emission at the 2 cm transition towards Galactic star-forming regions with known 6 cm counterpart using the Shanghai Tianma Radio Telescope. One significant detection (in NGC 7538) and two possible detections (in G23.01-0.41 and G29.96-0.02) were made. Comparing with previous observations, we found that there is a time lag of appearance of 2 and 6 cm emissions detected in NGC 7538, contradicting with the prediction of radiative pumping via radio continuum radiation. Combinations of the variability of 6 cm masers in NGC 7538 suggest that collisional pumping via high-velocity shocks could better explain the 6 cm H₂CO maser emission. Under this scheme, excitation of the 2 cm maser may require a higher collision energy compared to the 6 cm transition.