H.E.S.S. discovery of very high energy gamma-ray emission from PKS 0625-354

TL;DR

This paper reports the first detection of very high energy gamma-ray emission from PKS 0625-354 by H.E.S.S., analyzing multi-wavelength data to determine the emission mechanisms and nature of the source.

Contribution

First H.E.S.S. detection of VHE gamma rays from PKS 0625-354, with multi-wavelength analysis favoring a leptonic multi-zone emission model.

Findings

Detected VHE gamma-ray emission at 6.1σ significance

Observed variability in Fermi-LAT and Swift-XRT bands

Leptonic multi-zone model best explains the data

Abstract

PKS 0625-354 (z=0.055) was observed with the four H.E.S.S. telescopes in 2012 during 5.5 hours. The source was detected above an energy threshold of 200 GeV at a significance level of 6.1$\sigma$. No significant variability is found in these observations. The source is well described with a power-law spectrum with photon index $\Gamma =2.84 \pm 0.50_{stat} \pm 0.10_{syst}$ and normalization (at $E_0$=1.0 TeV) $N_0(E_0)=(0.58 \pm 0.22_{stat} \pm 0.12_{syst})\times10^{-12}$ TeV$^{-1}$cm$^{-2}$s$^{-1}$. Multi-wavelength data collected with Fermi-LAT, Swift-XRT, Swift-UVOT, ATOM and WISE are also analysed. Significant variability is observed only in the Fermi-LAT $\gamma$-ray and Swift-XRT X-ray energy bands. Having a good multi-wavelength coverage from radio to very high energy, we performed a broadband modelling from two types of emission scenarios. The results from a one zone lepto-hadronic, and a multi-zone leptonic models are compared and discussed. On the grounds of energetics, our analysis favours a leptonic multi-zone model. Models associated to the X-ray variability constraint supports previous results suggesting a BL Lac nature of PKS 0625-354, with, however, a large-scale jet structure typical of a radio galaxy.