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

Alicja Wierzcholska,Felix Jankowsky,For For The H.E.S.S Collaboration,Stefan Wagner,Michael Zacharias

doi:10.3390/galaxies7010021

Alicja Wierzcholska, Felix Jankowsky + Show 3 more

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https://doi.org/10.3390/galaxies7010021

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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.

Highlights

BL Lacertae (BL Lac) type objects and flat spectrum radio quasars (FSRQs) constitute a class of blazars
Blazars are sources that are known for their temporal variability, which can be observed at different wavelengths and on various timescales
From top to bottom panels present: optical fluxes in B and R bands as observed with ATOM, optical and ultraviolet observations taken with Swift-UVOT, Swift-XRT flux and photon index, Fermi-LAT flux and photon index, and flux above 200 GeV observed with H.E.S.S

Summary

Introduction

BL Lacertae (BL Lac) type objects and flat spectrum radio quasars (FSRQs) constitute a class of blazars. According to the unified scheme of active galactic nuclei (AGNs), blazars are AGNs that are characterized by jets pointing at small angles with respect to the line of sight [1]. The radiative energy output from these sources is highly dominated with Doppler-boosted nonthermal emission (e.g., [2]). Blazars are sources that are known for their temporal variability, which can be observed at different wavelengths and on various timescales (see e.g., [3,4,5,6]). Of blazars, in the ν-νFν representation, is characterized by a double-humped structure. The low-energy bump in the SED is usually attributed to the synchrotron radiation of ultra-relativistic electrons accelerated within the jets. The high-energy bump is generally discussed in the context of different scenarios. In the most popular leptonic explanation, the second bump is interpreted as the inverse

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