Detection of microvariability for BL Lacertae objects

Abstract

Large-amplitude, rapid optical variability is a well-known identifying characteristic for BL Lacertae objects ('blazars'). Although large-amplitude variations on timescales ranging from days to decades have been well documented¹, considerable controversy surrounds the nature of microvariability, that is, optical variations on timescales significantly shorter than a day. Here we report observations of BL Lacertae in which rapid changes were detected in the total optical flux from this object. These variations occurred on timescales as short as 1.5 hours. Although their structure is complex, the minimum timescale for the variations may be used to place constraints on the size of the emitting region. Assuming that these variations are produced in the vicinity of a supermassive black hole, the timescales imply a black-hole mass of 1.42×10⁹ M_⊙. Relativistic beaming need not be invoked to account for the present luminosity of this object.