If the period of 14.2 years is really the true period of variation, taking the first relative maximum of the light curve (JD 2418346) as the first outburst, we can obtain the expected following outbursts and indicate them in vertical light line in Fig. 1 (click here). It seems that the expected outbursts are well consistent with the observation. Also, we can say two outbursts may have not been observed since there are no available data during the periods of JD 2400000+22375 and 25645, and JD 2400000+36227 and 41279. Webb et al. (1988) and Smith et al. (1987) constructed the light curve of PKS 0735+178 (JD 2400000+40896 and 46520), analysed its period and found periods of 4.89 and 1.2 years. From Fig. 3 (click here) it can be seen that there is a very weak period of 4.85 years (Vm2 = 0.80, f = 0.25) and there is no period of 1.2 years. When we consider the data during the period of 1970 and 1996, we found that the period of 4.89 years is only a possible period (Vm2 = 0.70, f = 0.428). Since the data in the considered 90 years are not so well distributed as there are no data during period of JD 2400000+22375 and 25645, and JD 2400000+36227 and 41279 and there are fewer data in the early years than are there in the recent years, the period of 4.89 years can not be ruled out and should be confirmed with more observations.
In this paper, we have shown the light curve of PKS 0735+178 with some new data presented. It is a violent variable object with a large variation of about 4.6 magnitudes and rapid variation of 0.68 magnitude on a time scale of 24 min. The variation is periodic and a period of 14.2 years have been found using Jurkevich method. This kind of long-term period variation may be from the reasons: binary black hole (Sillanpaa et al. 1988) and the thermal and viscous instability in slim accretion disk (Meyer & Meyer-Hofmeister 1984; Horiuchi & Kato 1990). In 1991, Valtaoja et al. tried to discuss the variation mechanism in optical band by investigating the correlation of variations in the radio and optical bands. They found that there is no definite correlation of variations in the two bands and proposed that the variation mechanism in the optical band is an unclear mechanism differing from that in the radio band. The period of 4.89 years (Smith et al. 1987a; Webb et al. 1988) can not be ruled out and might be confirmed with more observations, but the period of 1.2 years proposed by them has no sign in our analysis and can be ruled out.
Acknowledgements
We thank an anonymous referee for his valuable comments. This work is supported by the National Natural Science foundation of China and the Natural Science foundation of Guangdong province. We thank Dr. Teresa for her help with the English.