Spikey: Self-Lensing Flares from Eccentric SMBH Binaries

TL;DR

This paper models light curves of quasars to identify signatures of eccentric supermassive black hole binaries, distinguishing between lensing and Doppler effects, and presents a candidate with specific orbital parameters.

Contribution

It extends previous models to include eccentricity in analyzing quasar light curves and identifies a candidate SMBHB with detailed orbital characteristics.

Findings

The quasar Ark 120 shows a prominent spike but is unlikely caused by lensing.

The second source, Spikey, is consistent with an eccentric SMBHB model.

Spikey's SMBHB has an estimated mass of 30 million solar masses and a 418-day orbital period.

Abstract

We examine the light curves of two quasars, motivated by recent suggestions that a supermassive black hole binary (SMBHB) can exhibit sharp lensing spikes. We model the variability of each light curve as due to a combination of two relativistic effects: the orbital relativistic Doppler boost and gravitational binary self-lensing. In order to model each system we extend previous Doppler plus self-lensing models to include eccentricity. The first quasar is identified in optical data as a binary candidate with a 20-yr period (Ark 120), and shows a prominent spike. For this source, we rule out the lensing hypothesis and disfavor the Doppler-boost hypothesis due to discrepancies in the measured vs. recovered values of the binary mass and optical spectral slope. The second source, which we nickname Spikey, is the rare case of an active galactic nucleus (AGN) identified in Kepler's high-quality, high-cadence photometric data. For this source, we find a model, consisting of a combination of a Doppler modulation and a narrow symmetric lensing spike, consistent with an eccentric SMBHB with a total mass of approximately 30 million solar masses, rest-frame orbital period T=418 days, eccentricity e=0.5, and seen at an inclination of 8 degrees from edge-on. This interpretation can be tested by monitoring Spikey for periodic behavior and recurring flares in the next few years. In preparation for such monitoring we present the first X-ray observations of this object taken by the Neil Gehrels Swift observatory.