SIMBAD references

Three deep flux minima with nearly the same time-scales and intervals have been observed for the blazar PKS 1510-089 in the past few years. It has been proposed that there is a binary black hole system 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. in the same period, for the 2002 minimum we present a nearly symmetric light curve, which would be required by an eclipsing model of a binary black hole system. We also constrain 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 is the 89 min time-scale given by Xie et al. The wiggling milliarcsecond radio jet observed in this object is taken as further evidence for the binary black hole system. The `coupling' of the periodicity in the light curve and the helicity in the radio jet is discussed for blazars in the framework of a binary black hole system.