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Guangzhong Xie, Benzhong Dai, Enwei Liang, Li Ma, Zejun Jiang, The Spectral Energy Distributions of Blazars and the Connections among XBLs, RBLs, and OVV Quasars, Publications of the Astronomical Society of Japan, Volume 53, Issue 3, 25 June 2001, Pages 469–474, https://doi.org/10.1093/pasj/53.3.469
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Abstract
In this paper, we introduce two composite spectral indices, |$\alpha_{\mathrm{xox}}=\alpha_{\mathrm{ox}}- \alpha_{\rm {x}}$| and |$\alpha_{\mathrm{oro}}=\alpha_{\mathrm{or}}- \alpha_{\mathrm{o}}$|. In the composite color–color (|$\alpha_{\mathrm{xox}}$|–|$\alpha_{\mathrm{oro}}$|) diagram, there are three regions: (1) For XBLs and XBL-like RBLs, the values of |$\alpha_{\mathrm{xox}}$| is in the range of |$-1.0$| -|$0.0$|; (2) For RBLs, |$\alpha_{\mathrm{xox}}$| is in 0.0–1.5, and |$\alpha_{\mathrm{oro}} \lt -0.75$|; (3) For OVV quasars, |$\alpha_{\mathrm{xox}}$| is in 0.0–1.5 and the |$\alpha_{\mathrm{oro}} \gt-0.75 \alpha_{\mathrm{oro}} \gt-0.75$|. Our results support the conclusion that RBLs are intermediate between XBLs and OVV quasars, which has been given by Sambruna et al. (1996, AAA 65.159.120). On the other hand, based on the idea that a group of extragalactic objects which can be regarded as a category with similar physical properties must fit the same relation, using the HST new observational data, we found that there is a strong correlation between the apparent magnitude of the host galaxies and log |$z$| for all BL Lac objects (XBLs + RBLs), but OVV quasars lie about 4 magnitude above the Hubble line of 86 BL Lacs. This implies that all XBL-like and XBLs belong to a single category with similar physical properties, while OVV quasars and BL Lacs belong to different populations.
