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

TL;DR

This paper reports the detection and analysis of very-high-energy gamma-ray emission from AP Librae, a low-frequency-peaked BL Lac object, including spectral measurements, variability, and spectral curvature insights.

Contribution

It provides the first detailed VHE spectrum of AP Librae and combines multi-year Fermi-LAT data to analyze its gamma-ray variability and spectral features.

Findings

Detection of VHE gamma-ray emission up to TeV energies

Observation of a gamma-ray flare with a 3.5-fold flux increase

Identification of spectral curvature between Fermi-LAT and H.E.S.S. spectra

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; $E>100\,{\rm GeV}$) $\gamma$-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 $(8.8 \pm 1.5_{\rm stat} \pm 1.8_{\rm sys}) \times 10^{-12}\ {\rm cm}^{-2} {\rm s}^{-1}$ above 130 GeV and a spectral index of $\Gamma = 2.65\pm0.19_{\rm stat}\pm0.20_{\rm sys}$. 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.