On the Difference of Quasars and BL Lac Objects

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Approaching Micro-Arcsecond Resolution with VSOP-2: Astrophysics and Technology ASP Conference Series, Vol. 402, c 2009 Y. Hagiwara, E. Fomalont, M. Tsuboi, and Y. Murata, eds.

On the Diﬀerence of Quasars and BLLac Objects

Xiang Liu Urumqi Observatory of NAOC, 40-5 South Beijing Rd, Urumqi 830011, China

Abstract. We analyze the two diﬀerences between radio-loud quasars and BLLacs: the lack of emission lines in BLLacs, and the diﬀerent position angles of the jets. We propose that BLLacs have much less gas and are possibly optically-thin near the central engine in nuclei. This could be attributed to the orbiting of binary black holes in a very late merging stage.

1. Introduction

Blazars are one of the most extreme classes of active galactic nuclei (AGN), characterized by high luminosity, rapid variability, high polarization, radio core- dominance and apparent superluminal speeds. The two blazar classes, flat- spectrum radio quasars (FSRQ) and BLLacertae objects, are thought to be the beamed counterparts of high- and low-luminosity radio galaxies, respectively. The main difference between the two blazar classes is shown in their emission lines, which are strong and quasar-like for FSRQs and weak or in some cases absent in BLLacs (Padovani et al. 2007). BLLac objects are often divided into two classes according to the position of the location of the peak in their synchrotron distribution: low-energy peaked BLLacs (LBLs), with the peak located at IR/optical wavelengths, and high- energy peaked BLLacs (HBLs) with the synchrotron emission peaking in the UV/X-ray energy band (e.g. Giommi & Padovani 1994). It is also noted that BLLacs have cosmological properties different than those of FSRQs and other type of AGNs. Although based on a small sample, LBLs have consistent properties of a non-evolving population (Stickel et al. 1991), while HBLs are less numerous and/or less luminous at high redshift (Piranomonte et al. 2007). The broad-band emission from blazars is dominated by non-thermal pro- cesses, emitted by a relativistic jet pointed close to our line of sight. The properties of misdirected blazars are consistent with those of radio galaxies, which form the so-called ‘parent population’ within the unified schemes (e.g. Urry & Padovani 1995). It is generally agreed that BLLacs are radio galaxies with their jets oriented close to the line-of-sight. Our knowledge of BLLacs is based mainly with the studies of two samples: the radio selected 1-Jy sample and the X-ray selected Einstein Medium Sensitivity Survey (EMSS) sample. Recently, new samples of BLLacs have been found by cross-correlating existing radio and X-ray source catalogues, searching for compact sources which are then classified based on optical identification. These searches tend to find BLLacs with properties in- 307 308 Liu termediate between those of the 1-Jy and EMSS samples. Detailed studies of intermediate samples are expected to provide a more complete picture of the BLLacs class of objects and their relation to radio galaxies. One such sample was drawn from the Deep X-ray Radio Blazar Survey (DXRBS)(Padovani et al 2007, Bignall et al. 2007). Because of these extreme physical characteristics and their differences with quasars, BLLacs have been the subject of intense research activity and obser- vation campaigns from radio to TeV energies. In this early stage of study, it is first important to analyze the observational differences between BLLacs and quasars and to question the underline physics of them, especially in the low energy-peaked synchrotron emission regime. We have summarized the properties of BLLacs and the differences with quasars, and propose possible underline physics.

2. A model for BLLacs

37 of the 518 radio sources of 1-Jy sample are BLLACs (Kuhr et al. 1981, Stickel et al. 1991), about 7%, and quasars consist of 52% in this sample. The majority of BLLacs in the 1-Jy sample are LBLs (Bignall et al. 2007), while BLLacs in X-ray selected are mostly HBLs (Padovani et al. 2007). As mentioned in the introduction, the main difference of BLLacs and quasars is that BLLacs have weak or absent emission lines, and they are thought to be the beamed counterparts of low-luminosity radio galaxies. We conclude that BLLacs lack much gas in their nuclei. Two possibilities: that BLLacs have much less gas than FSRQs in their nuclei, and that the featureless of emission lines occur because they are extremely face-on, as suggested in the framework of unified scheme. In the first case, BL Lacs in the radio selected samples are FR-1 radio sources which have a featureless spectrum and low accretion rate. In the second case, the BLLac jets are pointed nearly toward us so that the gas would be pushed away with little line emission, in contrast to FSRQs whose jets are pointed over a large range of viewing angles so that more intensive emission lines would be observable from the gas outside of the jet cone. For case 1, why is there much less gas in BLLacs? We propose that the evolution of binary black holes (BBH) system has reduced the amount of gas in BLLacs. In the merging scenario for the formation of blazars, they have a binary black hole system, and we suggest that these BLLacs may be at an evolutionary phase of a close pair of BBH at an orbital separation less than 1 pc. The gas in both the narrow and broad line regions are absorbed in the process of BBH evolution, and the fueling rate is also reduced. In contrast, FSRQs may be at a phase of a wide pair of BBH with an orbital separation of a few pc, so the interaction of BBH with stars and gas would increase the felling rate, and the BBH at this stage would produce pc scale, FR-2 scale jets and strong broad line region (Lobanov 2004). For case 2, the jets of BLLacs are pointed closer to the line-of-sight than that for FSRQs, and this can partly account for the differences of BLLacs and FSRQs. Quasars and BLLac Objects 309

Hence, we propose the combined model from the case 1 and case 2: BLLacs are the subclass of extremely beamed blazars with a close pair of BBH which has decreased the amount of gas in both the narrow and broad line regions. FSRQs are beamed blazars associated with a wider pair of BBH and signiﬁcant gas has remained. In this model, BLLacs are at a very late stage of galaxy-galaxy merging since they have a close pair of BBH, and this is consistent with a non-evolving population. Furthermore, the secondary BH of the close BBH in some BLLacs, could be interacting (even impacting) with the accretion disk of the primary BH, and producing the HBLs which show a synchrotron emission peak in the UV/X-ray band. In contrast, the wide pair of BBH in FSRQs produces a low- energy peaked synchrotron emission. Also, BLLacs may have an optically-thin broad line region (BLR) due to the lack of gas, and this enhances the probability of detections of Tev gamma-ray from BLR. That 12 out of 13 extragalactic Tev sources detected so far are BLLacs (Padovani 2006) supports this assertion. On the contrary, FSRQs may have an optically-thick BLR, so that the ‘blazar sequence’ from the ‘FSRQs to BLLacs’ is still valid.

3. Outlook

Taylor et al. (2006) reported a BBH binary system in the radio galaxy 0402+379 that has a projected separation of 7.3 pc. This may be an example of the wide pair of BBH. A BLLacs associated with a close pair of BBH could be the source OJ287 (Shi et al.). If it contained close double radio core associated with a BBH, it may be detectable with the VSOP-2 program at 22 and 43 GHz. Acknowledgments. I wish to thank Leonid Gurvits for useful comment. This work was supported by the National Natural Science Foundation of China under grant No.10773019.

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