Black hole mass is one of the fundamental physical parameters of active galactic nuclei (AGNs), for which many methods of estimation have been proposed. One set of methods assumes that the broad-line region (BLR) is gravitationally bound by the central black hole potential, so the black hole mass can be estimated from the orbital radius and the Doppler velocity. Another set of methods assumes the observed variability timescale is determined by the orbital timescale near the innermost stable orbit around the Schwarzschild black hole or the Kerr black hole, or by the characteristic timescale of the accretion disk. We collect a sample of 21 AGNs, for which the minimum variability timescales have been obtained and their black hole masses (M σ) have been well estimated from the stellar velocity dispersion or the BLR size-luminosity relation. Using the minimum variability timescales we estimated the black hole masses for 21 objects by the three different methods, the results are denoted by M s , M k and M d , respectively. We compared each of them with M σ individually and found that: (1) using the minimum variability timescale with the Kerr black hole theory leads to small differences between M σ and M k , none exceeding one order of magnitude, and the mean difference between them is about 0.53 dex; (2) using the minimum variability timescale with the Schwarzschild black hole theory leads to somewhat larger difference between M σ and M s : larger than one order of magnitude for 6 of the 21 sources, and the mean difference is 0.74 dex; (3) using the minimum variability timescale with the accretion disk theory leads to much larger differences between M σ and M d , for 13 of the 21 sources the differences are larger than two orders of magnitude; and the mean difference is as high as about 2.01 dex.
Using a large sample of blazars of the Fermi observations presented by Abdo et al., we constructed a sample of blazars including high energy peaked BL Lac objects (HBLs), low energy peaked BL Lac objects (LBLs) and flat-spectral radio quasars (FSRQs). These unique characteristics make it possible to unambiguously address the question of how HBLs, LBLs and FSRQs are related. In this paper, we investigated the relationship between X-ray and γ-ray spectral indices (αx−αγ), as well as the relationship between the broadband spectral indices (αro−αrx, αro−αox, αro−αxγ and αrx−αxγ) for this sample. The color-color diagram shows that there is a significant correlation between both quantities when all three subclasses of blazars are considered, which suggests that there is a unified scheme for blazars. On the other hand, the αx−αγ diagram reveals that three kinds of blazars have different spectral energy distributions: the trend of HBLs is different from that of FSRQs and LBLs, whereas FSRQs and LBLs have a similar trend, which hints that FSRQs and LBLs have similar spectral properties, but HBLs have distinct spectral properties. In addition, the broadband energy distributions also reveal the similar spectral properties with that of the αx−αγ diagram. The spectral properties revealed from the Fermi sample do not support the blazar sequence reported by Fossati et al. and Ghisellini et al.
We compiled the radio, optical, and X-ray data of blazars from the Sloan Digital Sky Survey (SDSS) database, and presented the distribution of luminosities and broad band spectral indices. The distribution of luminosities shows that the averaged luminosity of flat-spectral radio quasars (FSRQs) is larger than that of BL Lacs objects. On the other hand, the broad band spectral energy distribution reveals that FSRQs and low energy peaked BL Lac objects (LBLs) objects have similar spectral properties, but high energy peaked BL Lac objects (HBLs) have a distinct spectral property. This may be due to that different subclasses of blazars have different intrinsic environments and are at different cooling levels. Even so, a unified scheme also is revealed from the color-color diagram, which hints that there are similar physical processes operating in all objects under a range of intrinsic physical conditions or beaming parameter.
Many astronomers have discussed the property of BL Lacertae objects, including the variation of spectrum, the correlation of multi-wave bands and the property of polarization, which could give good information for studying intrinsic correlation of components and position of BL Lacertae objects. In the paper, we investigated the properties of RBLs and XBLs and RBLs/XBLs (these objects can also be found by radio survey as by X-ray survey). Firstly, we collected the light curve of 28 BL Lacertae objects, and gained their short timescales by Structure Function (SF). Secondly, we analyzed the distribution of the short timescales of some BL Lacertae objects, including 28 objects' timescales calculated by SF, as well as the distribution of the redshift and the black hole mass and the flux densities of the multiwavebands (radio, near-infrared, optical and Gamma-ray). Based on statistical analysis, the result of the paper support the unification model of RBLs and XBLs.
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