The high-energy $\gamma$-ray emission of AP Librae

Abstract

The $\gamma$-ray spectrum of the low-frequency-peaked BL Lac (LBL) object AP Librae is studied, following the discovery of very-high-energy (VHE; $E>100\mathrm{GeV}$) $\gamma$-ray emission up to the TeV range by the H.E.S.S. experiment. This makes AP Librae one of the few VHE emitters of the LBL type. The measured spectrum yields a flux of $(8.8\pm 1.5_{\mathrm{stat}}\pm 1.8_{\mathrm{sys}})\times 10^{-12}{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}$ above 130 GeV and a spectral index of $\Gamma =2.65\pm 0.19_{\mathrm{stat}}\pm 0.20_{\mathrm{sys}}$. This study also makes use of \textit{Fermi}-LAT, observations in the high energy (HE, E$>$100 MeV) range, providing the longest continuous light curve (5 years) ever published on this source. The source underwent a flaring event between MJD 56306-56376 in the HE range, with a flux increase of a factor 3.5 in the 14-day bin light curve and no significant variation in spectral shape with respect to the low-flux state. While the H.E.S.S., and (low state) \textit{Fermi}-LAT fluxes are in good agreement where they overlap, a spectral curvature between the steep VHE spectrum and the \textit{Fermi}-LAT, spectrum is observed. The maximum of the $\gamma$-ray emission in the spectral energy distribution is located below the GeV energy range.

arXiv comment15 pages, 6 figures

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