Optical Variability of HBLs on Diverse Timescales

Since their discovery almost 60 years ago, BL Lac objects have been defined by their strong optical variability and their classification in the spectral energy distribution scheme. High-synchrotron-peaked BL Lacs (HBLs) are those whose synchrotron component peaks at frequencies higher than UV/X-rays. Historically, optical variability studies have focused mostly on their counterparts, low-synchrotron-peaked BL Lacs (LBLs), since HBLs have shown weaker optical variability. However, a population-wide study of HBL optical variability is still lacking, and it remains unclear whether HBLs are intrinsically less optically variable as a class or whether this reflects observational biases. Only a handful of HBL sources have been studied extensively due to their strong variability and reported periodicity. These sources have motivated several theoretical models, often conflicting, to explain the optical variability when present. Nevertheless, understanding the connection between the apparent weaker optical variability and the emission processes of HBLs remains a challenge. In this work, we review the current state of knowledge on this topic, with the expectation that upcoming optical monitoring observatories, such as the Vera C. Rubin, will provide new insights into the optical emission (and variability) mechanisms in HBLs.