Broad-line region configuration of the supermassive binary black hole candidate PG1302-102 in the relativistic Doppler boosting scenario

TL;DR

This study investigates the broad-line region of the candidate supermassive binary black hole PG1302-102 under the relativistic Doppler boosting scenario, revealing a face-on orientation and misalignment with the orbital plane, with no observed periodic line variation.

Contribution

It provides the first detailed analysis of the broad-line region properties of PG1302-102 in the context of the Doppler boosting model, using archival UV spectra and MCMC fitting.

Findings

Broad emission lines show Gaussian profiles with no periodic variation.

The BLR is oriented at about 33 degrees, nearly face-on.

The BLR is misaligned by about 51 degrees with the BBH orbital plane.

Abstract

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.