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

TL;DR

This paper explores the possibility that the orbital period of the supermassive black hole binary in quasar PG 1302-102 is shorter than previously thought, which has implications for gravitational wave emission and observable signatures.

Contribution

It proposes that hydrodynamical simulations suggest a shorter orbital period, affecting the binary's evolution, observational signatures, and the estimated frequency of such systems.

Findings

Shorter orbital periods imply the binary is in a gravitational wave dominated inspiral phase.

Predicted periodic variability includes relativistic beaming and Fe Kα line effects.

Model aligns with observed periodicities and broad line variations in PG 1302-102.

Abstract

Graham et al. (2015) have detected a 5.2 year periodic optical variability of the quasar PG 1302-102 at redshift $z=0.3$, which they interpret as the redshifted orbital period $(1+z)t_{\rm bin}$ 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 ($q\equiv M_2/M_1\gtrsim 0.3$). 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 fueled and produce bright emission even in this late stage of the merger. The expected fraction of binaries with the shorter $t_{\rm bin}$, among bright quasars, would be reduced by 1-2 orders of magnitude, compared to the 5.2 year period, in better agreement with the rarity of candidates reported by Graham et al. (2015). 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$\alpha$ line. The CBD model predicts additional periodic variability on timescales of $t_{\rm bin}$ and $\approx 0.5 t_{\rm bin}$, 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.