Spikey: self-lensing flares from eccentric SMBH binaries

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 disfavour the Doppler-boost hypothesis due to discrepancies in the measured versus 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 identified in Kepler’s high-quality, high-cadence photometric data. For this source, we find a model, consisting of a combination of Doppler modulation and a narrow symmetric lensing spike, consistent with an eccentric SMBHB with a mass of $M_{\text{tot}} = 3\times 10^{7} {\, \mathrm{M}_{\odot }}$, rest-frame orbital period T = 418 d, eccentricity e = 0.5, and seen at an inclination of 8○ from edge-on. This interpretation can be tested by monitoring Spikey for periodic behaviour 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.

A self-lensing supermassive binary black hole at radio frequencies: the story of Spikey continues

“…The observed period in the optical light curve of Spikey is T obs = (1 + z sp )T = 805 d, which was recognised as the observed orbital period of the SMBH binary (Hu et al 2020). The 15-GHz radio flux density curve (Fig.…”

“…Thus the inner section of the jet is characterised by more components at 5 GHz, while the outer section is only seen at 1.7 GHz, where the array is more sensitive to the weaker, extended, steep-spectrum features. We are in a unique situation because some of the key parameters of the Spikey SMBHB system are accurately known from Hu et al (2020). Therefore we adopt the (restframe) orbital period T = 1.14 yr, the orbital inclination I = 1.43 rad, the mass of the primary SMBH m 1 = 2.5 × 10 7 M ⊙ , the mass of the secondary SMBH m 2 = 5.0 × 10 6 M ⊙ , and the orbital eccentricity e = 0.52.…”

“…In this paper, we investigate whether the available radio data are consistent with the behaviour of the object, the gravitational self-lensing model, and in particular the parameters derived for Spikey by Hu et al (2020). Based on archival data from 2008, we present sensitive and detailed VLBI images of J1918+4937 obtained at 1.7 and 5 GHz for the first time, and show the OVRO flux density curve (Sect.…”

“…Recently Hu et al (2020) interpreted a narrow spike in the densely-sampled Kepler optical light curve of the quasar J1918+4937 (also known as KIC 11606854, dubbed as Spikey by Hu et al 2020) as a result of gravitational selflensing in a supermassive black hole binary (SMBHB) system. The quasar has a spectroscopic redshift of z sp = 0.926 (Healey et al 2008).…”

“…In this scenario, the orbital plane of the binary lies sufficiently close to the line of sight so that when one of the companions -the black hole with the larger mass -passes in front of the other, the optical emission of the latter active galactic nucleus (AGN) is significantly enhanced. Taking two relativistic effects, the binary self-lensing and the orbital Doppler boosting into account, Hu et al (2020) modeled the Kepler light curve containing the spike. They found that the system is composed of two black holes (BHs), 15 GHz are being monitored at the Owens Valley Radio Observatory (OVRO, Richards et al 2011).…”