摘要
分析了１９９３－１９９５年ＢＬＬａｃ、ＯＪ２８７和ＰＫＳ０７３５＋１７８三颗ＢＬＬａｃ天体光变的监测结果，并结合文献上他人发表的资料，发现一些ＢＬＬａｃ天体处于高态时活动性较强，短时标光变频繁发生，而处于低态时活动性较弱，短时标光变很少发生．文中提出活动星系核的相对论性喷流方向会改变，从而导致长期光变；高态是喷流方向与观测者视线夹角较小的表现，而低态则是夹角较大的表现。利用该模型能够解释ＢＬＬａｃ天体短时标光变行为在高态和低态下的差别，并能很好地解释Ｔａｋａｌｏ等人监测ＯＪ２８７时发现的新现象．
n this paper, the data of optical photometric (BVRI) monitoring for three BL Lacobjects: BL Lac, OJ 287, PKS 0735+178 were analysed. Combining data from other authors,we found that for some BL Lac objects, stronger activity and more small flares and flickerstake place in the high state than in the low state. We suggest that the orientation of therelativistic jet of a BL Lac object is Changing with the time, causing variability on longtimescale. The high state of a BL Lac object is when the jet is close to line of sight,and the low state, is when away from the line of sight. With this scenario we explain thedifference in the short timescale variability between the high and low states, as well as thenew phenomena in OJ 287 discovered by Takalo et al.
