Water-vapor maser survey for active galactic nuclei: A megamaser in NGC 6926

Restricted Access.We made a survey of water-vapor maser emission for 93 AGNs with the Nobeyama 45-m and Mopra 22-m telescopes from 1999 to 2002. A megamaser was detected in a Seyfert 2 galaxy, NGC 6926, at a distance of 80Mpc, in 2002 June. [Greenhill et al. (2003a) have also reported a detection of the megamaser at the close date.] The peak flux density was 110mJy, and the total isotropic luminosity was 340 Lȯ. The maser shows triply peaked spectrum, suggesting an edge-on disk. A narrow-line feature of the maser components at VLSR = 6001 km s-1 was strongly variable with a time scale of a few tens of days, and the variation should be of intrinsic origin. We also showed a possibility of variability of water-vapor maser emission of a megamaser previously detected in a Seyfert/ultraluminous FIR galaxy, NGC 6240