Compelling Evidence for a Harmonic in the Light Curve of the Supermassive Black Hole Binary Candidate PKS J1309+1154

TL;DR

This paper presents strong evidence for a fundamental and harmonic periodicity in the light curve of the SMBHB candidate PKS J1309+1154, supporting its classification as a supermassive black hole binary and highlighting the potential for future discoveries.

Contribution

It provides the first watertight evidence of both fundamental and harmonic periodicities in a blazar light curve, confirming PKS J1309+1154 as a SMBHB candidate.

Findings

Detected a 17.9-year periodicity in the light curve.

Identified a 9-year harmonic periodicity.

Supports the existence of SMBHBs in blazars.

Abstract

We recently discovered a supermassive black hole binary (SMBHB) candidate, PKS J1309+1154, in the combined 46-yr University of Michigan Radio Astronomy Observatory (UMRAO) plus Owens Valley Radio Observatory (OVRO) blazar monitoring programs at 14.5/15 GHz. The light curve of PKS 1309+1154 exhibits a 17.9 year periodicity. We also reported a hint of a first harmonic with a 9 year periodicity in this object. Further analysis of the PKS J1309+1154 light curve provides compelling evidence that both the fundamental and the harmonic are real, confirming the existence of real periodicities in blazar light curves. This is the first case, to our knowledge, of watertight evidence for a fundamental and a harmonic periodicity in a blazar light curve. It makes PKS J1309+1154 a \textit{strong\/} supermassive black hole binary (SMBHB) candidate, and thus the third such candidate to be revealed through long-term radio monitoring, the other two being PKS J0805--0111 and PKS 2131--021, both discovered through the OVRO 40 m Telescope monitoring program. It is argued that hundreds of SMBHB candidates will be discovered by the Vera Rubin and Simons Observatories. Coherent searches for gravitational waves from a network of SMBHB candidates, starting immediately, are strongly motivated.