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A. Young, L. Rudnick (University of Minnesota), D. M. Katz-Stone (U.S. Naval Academy), G. Giovannini (Istituto di Radioastronomia, CNR, Bologna)
We examine the synchrotron spectrum of Fanaroff-Riley I (FRI) sources. In particular, we identify and analyze two spectral components of FRI sources: the jet and the sheath. We explore the question of whether these components exist as independent relativistic electron populations or originate from a single electron population. We observed 1116+28, 1243+26, and 1553+24 with the Very Large Array at 20 cm, 6 cm, and 3.6 cm. Our analysis makes use of spectral tomography and color-color diagrams (Katz-Stone et al. 1999 ApJ 516:716, 1993 ApJ 407:549). Spectral tomography is used to isolate the two spectral components while color-color diagrams are used to discern the spectral history and behavior of the jet and the sheath. From spectral tomography, we separate the jet material from the sheath material for 1116+28 and 1243+26. Early results on the spectral behavior of the jet material for 1116+28 and 1553+24 point toward a power law. Using color-color diagrams, 1116+28 as a whole shows spectral steepening away from a power law. The Jaffe-Perola model and Kardashev-Pacholczyk model can account for some, but not all, of the spectral steepening seen in 1116+28. We will present our results and discuss the implications on these aging models. This work is supported at the University of Minnesota by grant AST96-16964 from the National Science Foundation.