Dr. Liz Humphreys & Dr. Alice Argon (CfA)

"Analyzing Inner NGC 4258: Warped, Unstable, and Eccentric?"

Abstract: NGC 4258 provides a great opportunity to study an AGN accretion
disk < 1 pc from a supermassive black hole of ~40 million solar masses.
Using VLBI, we have recently added 18 epochs to a long-term study of 22 GHz H2O
maser emission originating from the disk. In this talk, we will show the
new data and a first measurement of the accretion disk thickness. We will
show latest models of the warped disk, and relate them to observed X-ray
variability. Centripetal acceleration data for low-velocity (systemic)
maser emission will be discussed in terms of possible orbital eccentricity
and disk spiral structure. Spatial periodicity in high-velocity maser emission
is consistent with the presence of spiral structure in an AGN accretion disk.