Optical Variability Power Spectrum Analysis of Blazar Sources on Intranight Timescales

Abstract

We report the first results of a systematic investigation to characterize blazar variability power spectral densities (PSDs) at optical frequencies using densely sampled (5–15 minutes of integration time), high photometric accuracy (≲0.2%–0.5%) R -band intranight light curves, covering timescales ranging from several hours to ∼15 minutes. Our sample consists of 14 optically bright blazars—nine BL Lacertae objects and five flat-spectrum radio quasars (FSRQs)—which have shown statistically significant variability during 29 monitoring sessions. We model the intranight PSDs as simple power laws and derive the best-fit slope along with the uncertainty using the “power spectral response” method. Our main results are as follows: (1) in 19 out of 29 monitoring sessions, the intranight PSDs show an acceptable fit to simple power laws at a rejection confidence ≤90%; (2) for these 19 instances, the PSD slopes show a wide range of 1.4 to 4.0, consistent with the statistical characters of red-noise (slope ∼ 2) and black-noise (slope ≥ 3) stochastic processes; (3) the average PSD slopes for the BL Lac objects and FSRQs are indistinguishable from one another; and (4) the normalization of intranight PSDs for individual blazar sources monitored on more than one occasion turns out to be consistent with one another with a few exceptions. The average PSD slope, 2.9 ± 0.3 (1 σ uncertainty), is steeper than that of red noise–type variability found on longer timescales (many decades to days), indicative of a cutoff in the variability spectrum on timescales around a few days at the synchrotron frequencies of the emission spectrum.