Relativistic boost as the cause of periodicity in a massive black-hole binary candidate

TL;DR

This paper proposes that the observed periodicity in a quasar's light curve is caused by relativistic Doppler boosting from a compact black-hole binary, providing a new explanation for such signals.

Contribution

It demonstrates that relativistic Doppler boosting can explain the periodicity in PG~1302-102, supporting the existence of a relativistic black-hole binary.

Findings

The variability of PG~1302-102 can be modeled by relativistic Doppler boosting.

UV light curves should show larger amplitude variations, consistent with archival data.

Follow-up observations can confirm the binary's relativistic nature.

Abstract

Because most large galaxies contain a central black hole, and galaxies often merge, black-hole binaries are expected to be common in galactic nuclei. Although they cannot be imaged, periodicities in the light curves of quasars have been interpreted as evidence for binaries, most recently in PG~1302-102, with a short rest-frame optical period of 4 years. If the orbital period matches this value, then for the range of estimated black hole masses the components would be separated by 0.007-0.017 pc, implying relativistic orbital speeds. There has been much debate over whether black hole orbits could be smaller than 1 pc. Here we show that the amplitude and the sinusoid-like shape of the variability of PG~1302-102 can be fit by relativistic Doppler boosting of emission from a compact, steadily accreting, unequal-mass binary. We predict that brightness variations in the ultraviolet light curve track those in the optical, but with a 2-3 times larger amplitude. This prediction is relatively insensitive to the details of the emission process, and is consistent with archival UV data. Follow-up UV and optical observations in the next few years can test this prediction and confirm the existence of a binary black hole in the relativistic regime.