Probing the obscuring medium around active nuclei using masers: The case   of 3C403
  We report the first detection of a water megamaser in a radio-loud galaxy, 3C403, and present a follow-up study using the VLA. 3C403 has been observed as a part of a small sample of FRII galaxies with evidence of nuclear obscuration. The isotropic luminosity of the maser is ~1200 solar luminosities. With a recessional velocity of cz~17680 km/s it is the most distant water maser so far reported. The line arises from the densest (> 10^(8) cm^(-3)) interstellar gas component ever observed in a radio-loud galaxy. Two spectral features are identified, likely bracketing the systemic velocity of the galaxy. Our interferometric data clearly indicate that these arise from a location within 0.1" (~110 pc) from the active galactic nucleus. We conclude that the maser spots are most likely associated with the tangentially seen parts of a nuclear accretion disk, while an association with dense warm gas interacting with the radio jets cannot yet be ruled out entirely. 
A. Tarchi, C. Henkel, M. Chiaberge, K.M. Menten, A. Brunthaler, L.   Moscadelli