New Multiwavelength Observations of PKS 2155-304 and Implications for the Coordinated Variability Patterns of Blazars

TL;DR

This study presents multiwavelength observations of the blazar PKS 2155-304, revealing how variability correlations and lag times depend on brightness, supporting a model of a relativistic shock in the jet during high states.

Contribution

It provides the first detailed comparison of multiwavelength variability patterns across different flux states of PKS 2155-304, highlighting the relationship between brightness and lag times.

Findings

Stronger X-ray and UV/optical correlation during high brightness states

Lag times increase from hours to days as the source dims

Evidence for a linear relationship between brightness and lag time

Abstract

The TeV blazar PKS 2155--304 was the subject of an intensive 2 week optical and near-infrared observing campaign in 2004 August with the CTIO 0.9m telescope. During this time, simultaneous X-ray data from RXTE were also obtained. We compare the results of our observations to the results from two previous simultaneous multiwavelength campaigns on PKS 2155-304. We conclude that the correlation between the X-ray and UV/optical variability is strongest and the time lag is shortest (only a few hours) when the object is brightest. As the object becomes fainter, the correlations are weaker and the lags longer, increasing to a few days. Based on the results of four campaigns, we find evidence for a linear relationship between the mean optical brightness and lag time of X-ray and UV/optical events. Furthermore, we assert that this behavior, along with the different multiwavelength flare lag times across different flux states is consistent with a highly relativistic shock propagating down the jet producing the flares observed during a high state. In a quiescent state, the variability is likely to be due to a number of factors including both the jet and contributions outside of the jet, such as the accretion disk.