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Periodic or regular flaring of class II methanol masers in nine high massstar forming regions is now a well established phenomenon. Amongst the ninestar forming regions, G37.55+0.20 is the only case at present where apart fromthe presence of a periodic class II methanol maser, correlated flaring ofanother masing species, formaldehyde in this case, has been detected. Weperform numerical calculations to investigate under which conditions the 4.8GHz transition of ortho-formaldehyde is inverted in order to address thequestion of the correlated flaring of the 6.7 GHz methanol and 4.8 GHzformaldehyde masers in G37.55+0.20. We developed a numerical code to study thepopulation inversion of o-formaldehyde. Equilibrium solutions for the levelpopulations are found by integrating the rate equations using Heun's method. Itis found that collisional excitation with H$_2$ as well as radiative excitationby the free-free radio continuum radiation from a nearby ultra- orhyper-compact HII region can invert the 4.8 GHz transition. It is also foundthat radiative excitation by the dust infrared radiation field does not lead toan inversion of the 4.8 GHz transition. The 14.5 GHz and 28.9 GHz transitionsare inverted only in the presence of the free-free continuum radiation field ofa very compact HII region. Due to the different pumping mechanisms of theformaldehyde and methanol masers it is unlikely that the near simultaneousflaring of the methanol and formaldehyde masers in G37.55+0.20 is due tochanges in the pumping of the masers.

Pumping of the 4.8 GHz H$_{\text{2}}$CO masers and its implications for the periodic masers in G37.55+0.20