A possible close supermassive black-hole binary in a quasar with optical periodicity

TL;DR

This paper reports the detection of a strong, smooth optical periodicity in a quasar, suggesting the possible presence of a close supermassive black-hole binary, which has implications for galaxy evolution models.

Contribution

It presents the first systematic search for periodic signals in a large quasar dataset, identifying a candidate binary black-hole system based on optical variability.

Findings

Detected a 1,884-day periodicity in quasar PG 1302-102

Analyzed light curves of 247,000 quasars over 9 years

Proposed binary black-hole system as the most plausible explanation

Abstract

Quasars have long been known to be variable sources at all wavelengths. Their optical variability is stochastic, can be due to a variety of physical mechanisms, and is well-described statistically in terms of a damped random walk model. The recent availability of large collections of astronomical time series of flux measurements (light curves) offers new data sets for a systematic exploration of quasar variability. Here we report on the detection of a strong, smooth periodic signal in the optical variability of the quasar PG 1302-102 with a mean observed period of 1,884 $\pm$ 88 days. It was identified in a search for periodic variability in a data set of light curves for 247,000 known, spectroscopically confirmed quasars with a temporal baseline of $\sim9$ years. While the interpretation of this phenomenon is still uncertain, the most plausible mechanisms involve a binary system of two supermassive black holes with a subparsec separation. Such systems are an expected consequence of galaxy mergers and can provide important constraints on models of galaxy formation and evolution.