An eclipsing binary black hole candidate system in the blazar Mrk 421
  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 gamma-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 \pm 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 \pm 20 )%, suggesting an
eclipse resulting in a gravitational lensing brightening. Consisting with this
interpretation, the eclipse occurs for only ( 9.7 \pm 0.2)% of the orbital
period.
