A reduced orbital period for the supermassive black hole binary candidate in the quasar PG 1302-102?

• Published in: Monthly notices of the Royal Astronomical Society 2015-09, Vol.452 (3), p.2540-2545
• Main Authors: D'Orazio, D. J., Haiman, Z., Duffell, P., Farris, B. D., MacFadyen, A. I.
• Format: Article
• Language: English
• Subjects: Astronomy; Black holes; Black holes (astronomy); Discs; Double stars; Emission; Mathematical models; Orbitals; Periodic variations; Quasars; Red shift; Simulation
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/stv1457

Graham et al. have detected a 5.2 yr periodic optical variability of the quasar PG 1302-102 at redshift z = 0.3, which they interpret as the redshifted orbital period (1 + z)t bin of a putative supermassive black hole binary (SMBHB). Here, we consider the implications of a 3–8 times shorter orbital period, suggested by hydrodynamical simulations of circumbinary discs (CBDs) with nearly equal-mass SMBHBs (q ≡ M 2/M 1 ≳ 0.3). With the corresponding 2–4 times tighter binary separation, PG 1302 would be undergoing gravitational wave dominated inspiral, and serve as a proof that the BHs can be fuelled and produce bright emission even in this late stage of the merger. The expected fraction of binaries with the shorter t bin, among bright quasars, would be reduced by one to two orders of magnitude, compared to the 5.2 yr period, in better agreement with the rarity of candidates reported by Graham et al. Finally, shorter periods would imply higher binary speeds, possibly imprinting periodicity on the light curves from relativistic beaming, as well as measurable relativistic effects on the Fe K α line. The CBD model predicts additional periodic variability on time-scales of t bin and ≈0.5t bin, as well as periodic variation of broad line widths and offsets relative to the narrow lines, which are consistent with the observations. Future observations will be able to test these predictions and hence the binary+CBD hypothesis for PG 1302.