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Register a new userDid ASAS-SN Kill the Supermassive Black Hole Binary Candidate PG1302-102?
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Tingting Liu
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Graham et al. (2015a) reported a periodically varying quasar and supermassive black hole binary candidate, PG1302-102 (hereafter PG1302), which was discovered in the Catalina Real-Time Transient Survey (CRTS). Its combined Lincoln Near-Earth Asteroid Research (LINEAR) and CRTS optical light curve is well fitted to a sinusoid of an observed period of $approx 1,884$ days and well modeled by the relativistic Doppler boosting of the secondary mini-disk (DOrazio et al. 2015). However, the LINEAR+CRTS light curve from MJD $approx 52700$ to MJD $approx 56400$ covers only $sim 2$ cycles of periodic variation, which is a short baseline that can be highly susceptible to normal, stochastic quasar variability (Vaughan et al. 2016). In this Letter, we present a re-analysis of PG1302, using the latest light curve from the All-Sky Automated Survey for Supernovae (ASAS-SN), which extends the observational baseline to the present day (MJD $approx 58200$), and adopting a maximum likelihood method which searches for a periodic component in addition to stochastic quasar variability. When the ASAS-SN data are combined with the previous LINEAR+CRTS data, the evidence for periodicity decreases. For genuine periodicity one would expect that additional data would strengthen the evidence, so the decrease in significance may be an indication that the binary model is disfavored.
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The photometric light curve of PG1302-102 shows periodic variability which makes this object one of the most plausible supermassive black hole binary candidate. Interestingly, the most recent study of its updated optical light curve reports a decrease in significance of periodicity which may suggest that the binary model is less favorable. Here, we model the PG 1302-102 light curve, spanning almost 20 years, with a supermassive black hole binary system in which a perturbation in the accretion disk of more massive component is present. Our model reproduces well the observed light curve with a slight perturbation of a sinusoidal feature and predicts that a slightly larger period than previously reported, of about 1899 days, could arise due to a cold spot in the disk of {bf more massive} component of a close, unequal-mass ($frac{mathrm{m}_{1}}{mathrm{m}_{2}}=0.1$) black hole binary system. The light curve resembles the pattern of sinusoid-like shape within a few years, which could be confirmed by future observations. In addition, using our hybrid method for periodicity detection, we show that the periods in the observed ($1972pm 254$ days) and modeled ($1873 pm 250$ days) light curves are within one-sigma, which is also consistent with our physical model prediction and with previous findings. Thus, both the periodic nature and its slight fluctuation of the light curve of PG1302-102 is evident from our physical model and confirmed by the hybrid method for periodicity detection.
PG1302-102 is thought to be a supermassive binary black hole (BBH) system according to the periodical variations of its optical and UV photometry, which may be interpreted as being due to the relativistic Doppler boosting of the emission mainly from the disk around the secondary black hole (BH) modulated by its orbital motion. In this paper, we investigate several broad emission lines of PG1302-102 using archived UV spectra obtained by IUE, GALEX, and Hubble, to reveal the broad-line region (BLR) emission properties of this BBH system under the Doppler boosting scenario. We find that the broad lines Ly$alpha$, NV, CIV, and CIII] all show Gaussian profiles, and none of these lines exhibits obvious periodical variation. Adopting a simple model for the BLR, we perform Markov chain Monte Carlo fittings to these broad lines, and find that the BLR must be viewed at an orientation angle of $sim33^{circ}$, close to face-on. If the Doppler boosting interpretation is correct, then the BLR is misaligned with the BBH orbital plane by an angle of $sim51^circ$, which suggests that the Doppler boosted continuum variation has little effect on the broad-line emission and thus does not lead to periodical line variation. We further discuss the possible implications for such a BLR configuration with respect to the BBH orbital plane.
We identify SDSS J153636.22+044127.0, a QSO discovered in the Sloan Digital Sky Survey, as a promising candidate for a binary black hole system. This QSO has two broad-line emission systems separated by 3500 km/sec. The redder system at z=0.3889 also has a typical set of narrow forbidden lines. The bluer system (z=0.3727) shows only broad Balmer lines and UV Fe II emission, making it highly unusual in its lack of narrow lines. A third system, which includes only unresolved absorption lines, is seen at a redshift, z=0.3878, intermediate between the two emission-line systems. While the observational signatures of binary nuclear black holes remain unclear, J1536+0441 is unique among all QSOs known in having two broad-line regions, indicative of two separate black holes presently accreting gas. The interpretation of this as a bound binary system of two black holes having masses of 10^8.9 and 10^7.3 solar masses, yields a separation of ~ 0.1 parsec and an orbital period of ~100 years. The separation implies that the two black holes are orbiting within a single narrow-line region, consistent with the characteristics of the spectrum. This object was identified as an extreme outlier of a Karhunen-Loeve Transform of 17,500 z < 0.7 QSO spectra from the SDSS. The probability of the spectrum resulting from a chance superposition of two QSOs with similar redshifts is estimated at 2X10^-7, leading to the expectation of 0.003 such objects in the sample studied; however, even in this case, the spectrum of the lower redshift QSO remains highly unusual.
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