An eclipsing binary black hole candidate system in the blazar Mrk 421

Abstract

Removing outbursts from multiwavelength light curves of the blazar Mrk 421, we construct outburstless time series for this system. A model-independent power spectrum light curve analysis in the optical, hard X-ray and γ-rays for this outburstless state and also the full light-curves, show clear evidence for a periodicity of ≈ 310 days across all wavelengths studied. A subsequent full maximum likelihood analysis fitting an eclipse model confirms a periodicity of 310 ± 1 days. The power spectrum of the signal in the outburstless state of the source does not follow a flicker noise behaviour and so, the system producing it is not self-organised. This and the fact that the periodicity is better defined in the outburstless state, strongly suggests that it is not produced by any internal physical processes associated to the central engine. The simplest physical mechanism to which this periodicity could be ascribed is a dynamical effect produced by an orbiting supermassive black hole companion eclipsing the central engine. Interestingly, the optimal eclipse model infers a brightness enhancement of (136.4 ± 20) %, suggesting an eclipse resulting in a gravitational lensing brightening. Consisting with this interpretation, the eclipse occurs for only (9.7 ± 0.2) % of the orbital period.

Characterizing Long-term Optical Variability Properties of $γ$-ray Bright Blazars

“…The more significant γ-ray QPO at 285 d in the source (see BD20), however, was not detected in the V-band light curves. Similarly, 310 d optical QPO seen after removing flares in the optical observations, including AAVSO data partly, reported by Benitez et al (2015) and the QPO with a characteristic timescale of ∼ 480 day in the rest frame, equivalent to observed ∼ 490 days, as reproted by Nilsson et al (2018) were not observed in the LSP analysis…”