John C. Noble

The timescales of the optical variability of blazars. III - OJ 287 and BL Lacertae

The BL Lacertae objects OJ 287 and BL Lac have been photometrically monitored in an effort to study the nature of optical variations which may occur on timescales ranging from years to less than a day. The results of ten years of photometric monitoring of these two objects show variations which are consistent with those reported by other authors. No strong dependence of color with source brightness was detected, although both sources exhibited a weak tendency to be bluer when brighter. Microvariability was observed for both objects; variations as large as 0.1 mag/hr were observed for BL Lac and changes as large as 0.08 mag/hr were observed for OJ 287. No evidence for a periodicity was found in the observed variations of either object.

Optical Microvariability in Radio-quiet Quasars

We report the results of a study of optical microvariability in radio-quiet and radio-intermediate quasars. Observations were obtained on a total of seven objects: five radio-quiet quasars and two radio-intermediate quasars. No microvariability was detected in either the radio-quiet or radio-intermediate objects in our sample, despite intensive monitoring for several consecutive nights. In one object, PG 1257+346, evidence for interday variability was detected. We examined a sample of 117 radio-quiet objects found in the literature that have been studied for microvariability. This sample is discussed in terms of classification, redshift distribution, R (the ratio of the radio [5 GHz] flux to the optical [4400 A] flux), optical magnitude, luminosity, and observing strategy. Objects with 10 > R > 1 were found to show a higher instance of microvariability than those with R < 1. This suggests that R = 1 is the appropriate cutoff between radio-loud and radio-quiet objects. A preference for low-redshift objects to display microvariability more often than higher redshift objects is seen; however, this is explainable via selection effects. We find a dependence in detection probability on observation length similar to that seen in radio-loud objects.

The Timescales of the Optical Variability of Blazars. V. 3C 371

Optical photometric observations of the blazar 3C 371 are presented as part of an ongoing study of variations in blazars occurring on timescales of minutes to hours, which has been termed microvariability. The observations presented here provide further evidence of the classwide nature of this phenomenon. Timescales as short as these provide important clues about the nature of blazars, and a discussion of the types of possible mechanisms responsible for producing these variations is given.