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[88.04] Is the Core of BL Lac Precessing?

R. L. Mutel (University of Iowa), G. R. Denn (Sweet Briar College)

We investigate the recent remarkable claim of Stirling et al. (MNRAS, 341, 450 ,2003) that the inner radio core of BL Lac has a 2.3 year periodic oscillating nozzle structure. We have analyzed 43~GHz VLBA maps made at nine epochs overlapping the time span analyzed by Stirling et al. using a similar Gaussian model fitting scheme for the inner core. Our results are not inconsistent with a periodic oscillation with a 2.3 yr period, but they are equally consistent with the null hypothesis that there is no precession. The most important difference in our analysis and that of Stirling et al. is in the uncertainty estimation of the position angle of the inner core subcomponents, on which evidence for precession critically depends. We use simulated data to investigate the uncertainties expected from radiometer noise and systematic effects, and suggest a more definitive test of the precession hypothesis.

The author(s) of this abstract have provided an email address for comments about the abstract: robert-mutel@uiowa.edu

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