First Detection of a THz Water Maser in NGC7538 IRS1

Despite numerous water line observations obtained with the Herschel Space Observatory, observations were not able to trace the emission from the hot core around the newly forming protostellar object in most of the observed massive protostars. We hence tried to probe the physical conditions and water abundance in the inner layers of the host protostellar object NGC7538-IRS1 using a highly excited H2O line, 8_{2,7}-7_{3,4} at 1296.4 GHz. We present SOFIA  observations of the o-H2O 8_{2,7}-7_{3,4} line and a 6_{16}-5_{23} 22 GHz e-MERLIN map of the region. In order to be able to constrain the nature of the emission - thermal or maser - we used near-simultaneous observations of the 22 GHz water maser performed with the Effelsberg radiotelescope and e-MERLIN. A thermal water model is presented based on HIFI pointed observations. Molecular water abundances are derived for the hot core. The o-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 the source velocity is consistent with thermal excitation and is excited in the innermost part of the IRS1a, in the closest circumstellar environment of the massive protostellar object. The other emission is very likely the first detection of a water THz maser line, pumped by shocks due to the IRS1b outflow, in a star-forming region. Assuming thermal excitation of the THz line, the water abundance in the hot core of NGC7538-IRS1 is estimated to be 5.2x10^-5 with respect to H2.