# H.E.S.S. Monitoring of PKS 2155-304 in 2015 and 2016

## TL;DR

This study presents two years of multiwavelength monitoring of blazar PKS 2155-304, revealing complex variability, correlated flares in 2015, and the first observation of orphan optical/UV outbursts in 2016.

## Contribution

It provides the first report of orphan optical/UV outbursts in PKS 2155-304 and characterizes multiwavelength variability over two years.

## Key findings

- 2015 flare observed at all wavelengths with correlated broadband emission
- 2016 orphan optical/UV outburst observed without X-ray or gamma-ray counterparts
- Complex multiwavelength variability with distinct outburst properties

## Abstract

PKS 2155-304 is one of the brightest blazar located in Southern Hemisphere, monitored with H.E.S.S. since the first light of the experiment. Here we report multiwavelength monitoring observations collected during the period of 2015-2016 with H.E.S.S.,Fermi-LAT, Swift-XRT, Swift-UVOT, and ATOM. Two years of multiwavelength data with very good temporal coverage allowed to characterize broadband emission observed from the region of PKS 2155-304 and study potential multifrequency correlations. During the period of monitoring, PKS 2155-304 revealed complex multiwavelength variability with two outbursts characterized by completely different multiband properties. The 2015 activity of the blazar is characterized by a flare observed at all wavelengths studied. The broadband emission observed during the outburst is well correlated without any time lags. Contrary to 2015, in 2016, only orphan outburst in the optical and ultraviolet wavelengths was observed. Such an orphan activity is reported for the first time for the blazar PKS 2155-304.