New Evidence for the Unified Scheme of BL Lac Objects and Fri Radio Galaxies

Abstract

In this paper, we collect radio and X-ray observations for most Fanaroff-Riley I (FRI) radio galaxies in the Zirbel-Baum radio galaxy sample, and investigate the distribution of the radio-to-X-ray effective spectral index, $α_{rx}$, to test the unified scheme of BL Lac objects and FRI radio galaxies. It is found that the range of $α_{rx}$ for FRI radio galaxies is almost the same as that for BL Lac objects, that the distribution of $α_{rx}$ probably peaks at the same position as BL Lac objects, and that the distribution of $α_{rx}$ for FRIs is similar to that for BL Lac objects. These suggest that there exist two subclasses of FRI radio galaxies: one is HBL-like, and the other is LBL-like, corresponding to high-energy-peaked (HBL) and low--energy-peaked (LBL) BL Lac objects, respectively. This result is consistent with previous VLA observations, and supports the unified scheme of BL Lac objects and FRI radio galaxies.

5 pages, 1 figure