Optical Variability of Blazars in the Tomo-e Gozen Northern Sky Transient Survey

TL;DR

This study analyzes optical variability in a large sample of blazars using the Tomo-e Gozen survey, revealing differences linked to gamma-ray properties and suggesting accretion disk dominance in certain FSRQs.

Contribution

It provides new insights into the optical variability characteristics of blazars, especially FSRQs with high gamma-ray photon index, using extensive multi-epoch survey data.

Findings

FSRQs with gamma-ray photon index > 2.6 show very low optical variability.

The structure function amplitude for FSRQs is higher than previously measured and than BL Lacs.

High gamma-ray photon index FSRQs have a characteristic variability timescale over 10 days.

Abstract

We studied the optical variability of 241 BL Lacs and 83 flat-spectrum radio quasars (FSRQ) from the 4LAC catalog using data from the Tomo-e Gozen Northern Sky Transient Survey, with $\sim$ 50 epochs per blazar on average. We excluded blazars whose optical variability may be underestimated due to the influence of their host galaxy, based on their optical luminosity ($L_O$). FSRQs with $\gamma$-ray photon index greater than 2.6 exhibit very low optical variability, and their distribution of standard deviation of repeated photometry is significantly different from that of the other FSRQs (KS test P value equal to $5 \times 10^{-6}$ ). Among a sample of blazars at any particular cosmological epoch, those with lower $\gamma$-ray luminosity ($L_\gamma$) tend to have lower optical variability, and those FSRQs with $\gamma$-ray photon index greater than 2.6 tend to have low $L_\gamma$. We also measured the structure function of optical variability and found that the amplitude of the structure function for FSRQs is higher than previously measured and higher than that of BL Lacs at multiple time lags. Additionally, the amplitude of the structure function of FSRQs with high $\gamma$-ray photon index is significantly lower than that of FSRQs with low $\gamma$-ray photon index. The structure function of FSRQs of high $\gamma$-ray photon index shows a characteristic timescale of more than 10 days, which may be the variability timescale of the accretion disk. In summary, we infer that the optical component of FSRQs with high $\gamma$-ray photon index may be dominated by the accretion disk.