Blazar Microvariability: a Case Study of AO 0235+164

Summary

Introduction

The existence of microvariability at optical wavelengths has been clearly demonstrated for BL Lacertae objects by a number of groups during the past several years (Miller & Carini 1991, Carini & Miller 1992, Wagner et al. 1991). However, in no instance has the nature of the microvariability been investigated when a blazar was near a minimum in brightness in its long-term variability. Thus, the blazar AO 0235+164 was selected to be monitored with the goal of determining whether or not rapid variations are present when the object is near its minimum brightness level, based on its known historical variability.

Observations

The observations of AO 0235+164 reported here were obtained with the 42-inch telescope at Lowell Observatory equipped with a direct CCD camera. The observations were made through an R filter with an RCA CCD. Repeated exposures of typically 300 seconds were obtained for the star field containing AO 0235+164 and several standard stars. These standard stars, located on the same CCD frame as A0 0235+164, provided comparisons for use in the data reduction process. The observations were reduced using the method of Howell & Jacoby (1986). Each exposure is processed through an aperture photometry routine which reduces the data as if it were produced by a multi-star photometer. Differential magnitudes can then be computed for any pair of stars on the frame.