The Nature Of An AGN Accretion Disk Within The Central Parsec: Strengthened Evidence For Spiral Structure

Details The Nature Of An AGN Accretion Disk Within The Central Parsec: Strengthened Evidence For Spiral Structure The Nature Of An AGN Accretion Disk Within The Central Parsec: Strengthened Evidence For Spiral Structure

Dec 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007aas...21111801h&link_type=abstract

American Astronomical Society, AAS Meeting #211, #118.01; Bulletin of the American Astronomical Society, Vol. 39, p.946

Astronomy and Astrophysics

Astronomy

Scientific paper

A major issue for understanding the accretion process of AGN is determining the structure of the inner accretion disk. The only way to map the disk at distances < 1 pc from the central supermassive black hole is by using radio interferometric observations of nuclear water masers. In a new study of the nuclear masers of nearby Seyfert 2/LINER NGC 4258, we have imaged a sub-parsec (0.11 -- 0.28 pc) portion of the disk at 18 epochs using VLBI. The observations have enabled a first measurement of the thickness of the maser medium to be 5 μas or 0.0002 pc (1σ). Assuming that this corresponds to the thickness of the accretion disk, hydrostatic equilibrium requires a gas temperature of ≈ 600 K i.e., consistent with the physical conditions required to pump the masers. We confirm that warping of the disk could result in obscuration of the central engine, and that there is a characteristic radial scale of 0.027 pc for maser emission along the disk midline (the diameter perpendicular to the line-of-sight to the black hole) consistent with a model of spiral density waves in the disk. We find a trend in centripetal accelerations of masers in the line-of-sight to the black hole, as a function of disk impact parameter, that is long-lived with respect to the transit time of masers orbiting in front of the black hole. The trend could be due to (i) a spiral arm of mass 105 M&sun; of pattern speed << Keplerian rotation; (ii) disk geometry and orientation that results in a preferred region for maser emission; or, (iii) eccentricity in maser orbits. A preliminary investigation of case (iii) indicates that maser emission in front of the black hole that arises from a single, eccentric orbit is unlikely to be the cause.

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