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.