Testing the relativistic Doppler boost hypothesis for supermassive binary black holes candidates via broad emission line profiles

Abstract

Optical periodicity QSOs found by transient surveys are suggested to be sub-parsec supermassive binary black holes (BBHs). An intriguing interpretation for the periodicity of some of those QSOs is that the continuum is radiated from the accretion disk associated with the BBH secondary component and modulated by the periodical rotation of the secondary via Doppler-boost effect. Close to edge-on orbital orientation can lead to more significant Doppler-boost effect and thus are preferred for these systems, which is distinct from those normal type-1 QSOs with more or less face-on orientations. Therefore, the profiles of broad lines emitted from these Doppler-modulated systems may be significantly different from other systems that are not Doppler-modulated. We investigate the properties of the broad emission lines of optical-periodicity QSOs, including both a sample of QSOs that can be interpreted by the Doppler-modulated effects and a sample that cannot. We find that there is no obvious difference in the profiles and other properties of various (stacked) broad emission lines of these two samples, though a simple broad line region model would suggest significant differences. Our finding raises a challenge to the Doppler boost hypothesis for some of those BBHs candidates with optical periodicity.

Variation of Broad Emission Lines from QSOs with Optical/UV Periodicity to Test the Interpretation of Supermassive Binary Black Holes

“…However, there may be some deviations of the observed light curves (or line profiles) from those predicted by the Doppler-boosting interpretation as pointed out by Qian et al (2018) and Kovačević et al (2019, or Song et al 2020, 2021. Such sinusoidal light curves may also be directly due to the periodical variation of the accretion rate modulated by the BBH orbital motion as suggested by numerical simulations (e.g., Hayasaki et al 2008;MacFadyen & Milosavljević 2008;Roedig & Sesana 2014;Farris et al 2015;Bowen et al 2017Bowen et al , 2018.…”

Variation of Broad Emission Lines from QSOs with Optical/UV Periodicity to Test the Interpretation of Supermassive Binary Black Holes

“…For the geometry of a circumbinary BLR outside the BBH system (R BLR /a BBH >> 1), we have also discussed the possible variation caused by different setup of BBH orbital period (T orb , continuum variation period (T var ), and spectral index α in the far-ultraviolet and optical bands (see PAPER I for details). As predicted by comparing the modelling of light curves and BEL profile fittings (e.g., Song et al 2020Song et al , 2021, the misaligned BLRs from BBH orbital plane make the emissivity of BLR clouds more complex than the coplanar case.…”

“…With the interpretation on detailed response of BLRs to the central varying continuum at different offset angles, we can explore detailed features of their periodic profile variations, which can be applied for fitting observed BELs in mostly UV, optical, and infrared bands, such as Lyα, N V, C III], C IV, Mg II, Hβ, Hα, and Paα broad lines (e.g., Pancoast et al 2014b;Gravity Collaboration et al 2018;Song et al 2020Song et al , 2021. As to the periodic variation of profile shapes, the edgeon viewed BLR present double-peaked or strongly asymmetric shapes with the blue and red parts raising up or going down alternatively.…”

“…In order to identify or rule out these BBH candidates, therefore, it may be necessary to find other BBH signatures for these QSOs and investigate whether their spectral properties are consistent with the BBH interpretation. Song et al (2020) analyzed the BEL properties of periodicity QSOs in the sample of Charisi et al (2016) and found that the BELs of all these QSOs are circum-binary and viewed at an inclination angle of i BLR < 45 • while at least half of these BBH candidates are viewed at an orientation close to edge-on if its periodicity is due to the DB effect. These apparent contradictory results suggest that the DB effect is not the main reason for the periodicity or the BLR is mis-aligned with BBH orbital plane in those BBH systems viewed at an orientation close to edge-on (e.g., Song et al 2021).…”

The Gravitational-wave physics II: Progress

“…We have investigated the detection of individual SMBBHs by current and future PTAs and its possible cosmological application to constrain cosmological parameters [634][635][636]. We have found a number of SMBBH candidates [637,638], investigated the detailed observational properties of some SMBBH candidates [639,640], and introduced several new methods to identify BBH candidates and distinguish it from alternative interpretations [641][642][643][644], investigated the cosmic evolution of SMBBHs and predicted the SGWBs resulting from inspiralling SMBBHs and SMBBH merger rate [645].…”