Multi-wavelength Variation Phenomena of PKS 0735+178 on Diverse Timescale

TL;DR

This study analyzes multi-wavelength variability of PKS 0735+178, revealing complex color behaviors, time lags, and correlations across optical and gamma-ray bands, emphasizing activity-dependent emission mechanisms.

Contribution

It provides a comprehensive long-term multi-wavelength analysis of PKS 0735+178, highlighting activity-dependent color behaviors and correlations with implications for jet emission models.

Findings

Detected intraday variability on one night.

Found a 22-day lag between R and gamma-ray bands.

Observed a harder-when-brighter trend in gamma-ray.

Abstract

BL Lac object PKS 0735+178 showed some complex multi-wavelength variation phenomena in the previous studies, especially for its color behavior. Bluer-when-brighter, redder-when-brighter and achromatic behavior were all found to be possible long-term trends of PKS 0735+178. In this work, we collected the long-term multi-wavelength data of PKS 0735+178, and also performed a multi-color optical monitoring on intraday timescale. The intraday variability was detected on one night. On the long timescale, a possible 22-day time lag was found between the $R$ and $\gamma$-ray bands. The results of the cross-correlation analysis exhibited strong correlations between various optical bands on both intraday and long timescale. However, only a mild correlation was found between the long-term $\gamma$-ray and $R$-band light curves, which could be interpreted by the different emission mechanisms of the $\gamma$-ray and optical emissions. PKS 0735+178 showed a significant harder-when-brighter in the $\gamma$-ray band, which is consistent with the observed optical bluer-when-brighter trend on both long-term and intraday timescales. We found that the HWB and BWB trends will be enhanced during the active states, especially for the historical low state. Such phenomenon indicates a special activity-dependent color behavior of PKS 0735+178, and it could be well interpreted by the jet emission model.