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The formation of massive stars is still not well understood. Accumulating alarge amount of mass infalling within a single entity in spite of radiationpressure is possible if, among several other conditions, enough thermal energyis released. Despite numerous water line observations, with the Herschel SpaceObservatory, in most of the sources observations were not able to trace theemission from the hot core around the newly forming protostellar object. Wewant to probe the physical conditions and water abundance in the inner layersof the host protostellar object NGC7538-IRS1 using a highly excited H2O line.Water maser models predict that several THz water masers should be detectablein these objects. We present SOFIA observations of the o-H2O 8(2,7)-7(3,4) lineat 1296.41106 GHz and a 6(1,6)-5(2,3) 22 GHz e-MERLIN map of the region(first-ever 22 GHz images made after the e-MERLIN upgrade). In order to be ableto constrain the nature of the emission - thermal or maser - we usenear-simultaneous observations of the 22 GHz water maser performed with theEffelsberg radiotelescope and e-MERLIN. A thermal water model using the RATRANradiative transfer code is presented based on HIFI pointed observations.Molecular water abundances are derived for the hot core. The H2O 8(2,7)- 7(3,4)line is detected toward NGC7538-IRS1 with one feature at the source velocity(-57.7 km/s) and another one at -48.4 km/s. We propose that the emission at thesource velocity is consistent with thermal excitation and is excited in theinnermost part of the IRS1a massive protostellar object's closest circumstellarenvironment. The other emission is very likely the first detection of a waterTHz maser line, pumped by shocks due to IRS1b outflow, in a star-formingregion. Assuming thermal excitation of the THz line, the water abundance inNGC7538-IRS1's hot core is estimated to be 5.2x10^{-5} with respect to H2.

First detection of THz water maser in NGC7538-IRS1 with SOFIA and new 22 GHz e-MERLIN maps