2017ApJS..232...29A

We present results of a multi-epoch monitoring program on variability of 6 cm formaldehyde (H₂CO) masers in the massive star-forming region NGC 7538 IRS 1 from 2008 to 2015, conducted with the Green Bank Telescope, the Westerbork Radio Telescope , and the Very Large Array. We found that the similar variability behaviors of the two formaldehyde maser velocity components in NGC 7538 IRS 1 (which was pointed out by Araya and collaborators in 2007) have continued. The possibility that the variability is caused by changes in the maser amplification path in regions with similar morphology and kinematics is discussed. We also observed 12.2 GHz methanol and 22.2 GHz water masers toward NGC 7538 IRS 1. The brightest maser components of CH₃OH and H₂O species show a decrease in flux density as a function of time. The brightest H₂CO maser component also shows a decrease in flux density and has a similar LSR velocity to the brightest H₂O and 12.2 GHz CH₃OH masers. The line parameters of radio recombination lines and the 20.17 and 20.97 GHz CH₃OH transitions in NGC 7538 IRS 1 are also reported. In addition, we observed five other 6 cm formaldehyde maser regions. We found no evidence of significant variability of the 6 cm masers in these regions with respect to previous observations, the only possible exception being the maser in G29.96-0.02. All six sources were also observed in the H₂¹³CO isotopologue transition of the 6 cm H₂CO line; H₂¹³CO absorption was detected in five of the sources. Estimated column density ratios [H₂¹²CO]/[H₂¹³CO] are reported.