Optical variability power spectrum analysis of blazar sources on intranight timescales

TL;DR

This study systematically analyzes optical intranight variability in 14 blazars, revealing a wide range of PSD slopes and indicating a cutoff in variability spectrum around a few days, with no significant difference between BL Lacs and FSRQs.

Contribution

First detailed PSD analysis of optical intranight blazar variability using densely sampled light curves, revealing variability characteristics and potential spectral cutoff.

Findings

PSD slopes range from 1.4 to 4.0, indicating red and black noise processes

Average PSD slope is 2.9±0.3, steeper than longer timescale variability

No significant difference in PSD slopes between BL Lacs and FSRQs

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 integration time), high photometric accuracy ($\lesssim$0.2--0.5\%) R-band intranight light curves, covering timescales ranging from several hours to $\sim$15\,minutes. Our sample consists of 14 optically bright blazars, including nine BL Lacertae objects (BL Lacs) 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 uncertainty using the `power spectral response' method. Our main results are the following: (1) on 19 out of 29 monitoring sessions, the intranight PSDs show an acceptable fit to simple power-laws at the rejection confidence $\leq$ 90\%; (2) for these 19 instances, the PSD slopes show a large range from 1.4 to 4.0, consistent with statistical characters of red (slope$\sim$2) and black (slope$\geq$3) noise stochastic processes; (3) the average PSD slopes for the BL Lacs and FSRQs are indistinguishable from one another; (4) the normalization of intranight PSDs for individual blazar sources which were monitored on more than one occasion turns out to be consistent with one another with a few exceptions. The average PSD slope, 2.9$\pm$0.3 (1$\sigma$ uncertainty) is steeper than the red-noise type character of 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.