Abstract
Here we report the results of the first ever contemporaneous multi-wavelength observation campaign on the BL Lac object PKS 2155−304 involvingSwift,NuSTAR,Fermi-LAT, and H.E.S.S. The use of these instruments allows us to cover a broad energy range, which is important for disentangling the different radiative mechanisms. The source, observed from June 2013 to October 2013, was found in a low flux state with respect to previous observations but exhibited highly significant flux variability in the X-rays. The high-energy end of the synchrotron spectrum can be traced up to 40 keV without significant contamination by high-energy emission. A one-zone synchrotron self-Compton model was used to reproduce the broadband flux of the source for all the observations presented here but failed for previous observations made in April 2013. A lepto-hadronic solution was then explored to explain these earlier observational results.
Highlights
Blazars are active galactic nuclei (AGNs) with an ultrarelativistic jet pointing towards the Earth
The results presented in this work do not provide evidence for spectral variability in the UV energy range
The only noticeable difference is at lower energies with the observed optical flux measured by Swift-Ultraviolet/Optical Telescope (UVOT): at epoch 0, the flux was higher than that measured in all the other epochs
Summary
Blazars are active galactic nuclei (AGNs) with an ultrarelativistic jet pointing towards the Earth. To distinguish between the different models, accurate and contemporaneous observations over a wide energy range are of utmost importance These are possible with the Nuclear Spectroscopic Telescope Array (NuSTAR) launched in 2012, which permits more sensitive studies above 10 keV than previous X-ray missions. PKS 2155−304 was subsequently the target of a multiwavelength campaign from June to October 2013 by NuSTAR, H.E.S.S., the Neil Gehrels Swift Observatory, and Fermi-LAT These instruments observed PKS 2155−304 to provide contemporaneous data for the first time in a very broad energy range, extending from ultra-violet up to TeV γ-rays and yielding a more complete coverage in the X-ray and γ-ray ranges than the previous campaign held in 2008 (Aharonian et al 2009).
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