The intra-day Optical Monitoring of BL Lacerate Object 1ES 1218+304 at Its Highest X-ray Flux Level

TL;DR

This study presents optical monitoring of the BL Lac object 1ES 1218+304 during its highest recorded X-ray flux, revealing intra-day variability, chromatic relationships, and potential quasi-periodic oscillations, enhancing understanding of jet physics.

Contribution

First simultaneous optical monitoring of 1ES 1218+304 during peak X-ray activity, identifying intra-day variability and chromatic behavior with potential oscillations.

Findings

Intra-day optical variability detected in 7 of 8 nights.

Bluer-when-brighter chromatic trend observed in 5 nights.

Possible quasi-periodic oscillation on the first night.

Abstract

We here report a monitor of the BL Lac object 1ES 1218+304 in both B- and R-bands by the GWAC-F60A telescope in eight nights, when it was triggerd to be at its highest X-ray flux in history by the VERITAS Observatory and Swift follow-ups. Both ANOVA and $\chi^2$-test enable us to clearly reveal an intra-day variability in optical wavelengths in seven out of the eight nights. A bluer-when-brighter chromatic relationship has been clearly identified in five out of the eight nights, which can be well explained by the shock-in-jet model. In addtion, a quasi-periodic oscilation phenomenon in both bands could be tentatively identified in the first night. A positive delay between the two bands has been revealed in three out of the eight nights, and a negative one in the other nights. The identfied minimum time delay enables us to estimate the $M_{\mathrm{BH}}=2.8\times10^7 \rm M_{\odot}$that is invalid.