AAS 203rd Meeting, January 2004
Session 55 AGN and Starburst Galaxies
Poster, Tuesday, January 6, 2004, 9:20am-6:30pm, Grand Hall

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[55.13] Measuring LLAGN Sizes Using Intraday Variability Studies

J. M. Anderson (NRAO/New Mexico Tech), J. S. Ulvestad (NRAO)

Although low-luminosity active galactic nuclei (LLAGNs) optically appear to be scaled down versions of more luminous AGNs, their properties at other wavelengths suggest that different physical processes dominate their behavior. They show unusual radio properties, the weakest having radio to optical ratios R = 102--106, and almost half of low-luminosity Seyferts have spectral indices \alpha > 0. Understanding the accretion and emission processes in these objects depends on measurements of the sizes of the emitting regions. Accretion disk models, such as advection dominated accretion flows (ADAFs), predict radio sizes of a few tens of microarcseconds, while jet models suggest emission regions of several hundred microarcseconds in size.

Intraday variability at radio wavelengths is caused by scintillation in the Galactic interstellar medium (ISM); this scintillation enables measurement of angular sizes in the range of a few to a few tens of microarcseconds, or about 100 to 1000 AU at a typical distance of 20 Mpc. Using the Very Large Array at 8.4 GHz, we have made measurements spread over ~14 days in both 2003 May and September searching for variability in a sample of 18 LLAGNs from the Palomar Bright Galaxy Sample. We use compact symmetric objects as calibrators to reduce systematic errors to ~ 1%. Assuming ADAF models and random lines of sight through the ISM, roughly 4 of the sources should show variability above the 5% level; detection of such variability would effectively rule out jets as the origin of the radio emission. This sample is also large enough to statistically eliminate current accretion-based models if no variability is found. Preliminary results suggest that variability above 5% on a few days timescale is present in a few objects.

JMA is supported by the NRAO Pre-Doctoral Research Program.

The author(s) of this abstract have provided an email address for comments about the abstract: janderso@nrao.edu

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Bulletin of the American Astronomical Society, 35#5
© 2003. The American Astronomical Soceity.