Optical Monitoring of PKS 1510-089: A Binary Black Hole System?
  Three deep flux minima were observed with nearly the same time-scales and intervals for the blazar PKS 1510-089 in the past few years. A binary black hole system was proposed to be at the nucleus of this object, and a new minimum was predicted to occur in 2002 March. We monitored this source with a 60/90 cm Schmidt telescope from 2002 February to April. In combination with the data obtained by Xie et al. (2004) in the same period, we presented for the 2002 minimum a nearly symmetric light curve, which would be required by an eclipsing model of a binary black hole system. We also constrained the time-scale of the minimum to be 35 min, which is more consistent with the time-scales ~42 min of the three previous minima than the 89 min time-scale given by the same authors. The wiggling miniarcsecond radio jet observed in this object is taken as a further evidence for the binary black hole system. The `coupling' of the periodicity in light curve and the helicity in radio jet is discussed in the framework of a binary black hole system. 
J. Wu, X. Zhou, B. Peng, J. Ma, Z. Jiang, J. Chen