The nucleus of the Seyfert galaxy NGC 1068 is believed to host a supermassive black hole .
Evidence for the presence of a massive central object is provided by water maser emission , which displays a linear pattern in the sky , suggestive of a rotating disk .
The rotating disk hypothesis is further strengthened by the declining shape of the derived rotation curve .
Similar maser emission from NGC 4258 has led to a reliable estimate of the mass of the central black hole , because in this case the rotation curve is Keplerian .
In the case of NGC 1068 the rotation curve traced by the water maser is non-Keplerian .
In this paper we provide an interpretation of the non-Keplerian rotation in NGC 1068 by means of a self-gravitating accretion disk model .
We obtain a good fit to the available data and derive a black hole mass M _ { \bullet } \approx ( 8.0 \pm 0.3 ) \leavevmode \nobreak 10 ^ { 6 } M _ { \sun } .
The resulting disk mass is comparable to the black hole mass .
As an interesting by-product of our fitting procedure , we are able to estimate the viscosity parameter , which turns out to be \alpha \approx 10 ^ { -2 } , in line with some theoretical expectations .