The challenging SED of AP Librae

Abstract

Abstract. Matching the broad-band emission of active galaxies with the predictions of theoretical models can be used to derive constraints on the properties of the emitting region and to probe the physical processes involved. AP Librae is the third low frequency peaked BL Lac (LBL) detected at very high energy (VHE, E>100GeV) by an Atmospheric Cherenkov Telescope; most VHE BL Lacs (34 out of 39) belong to the high-frequency and intermediate-frequency BL Lac classes (HBL and IBL). LBL objects tend to have a higher luminosity with lower peak frequencies than HBLs or IBLs. The characterization of their time-averaged spectral energy distribution is challenging for emission models such as synchrotron self-Compton (SSC) models.

Linking radio and gamma-ray emission in Ap Librae

“…More recently, Raiteri et al (2010) highlighted the existence of two SSC zones in the BL Lac SED. Previous attempts to model Ap Lib with simple one-zone SSC were not successful (Tavecchio et al 2010;Sanchez et al 2012). The very broad high-energy bump of Ap Lib suggests that a more complex SSC model is needed to describe the emission scenario, as we show here.…”

“…We used the contemporaneous data recently published by Giommi et al (2012), including data from Planck (from 17-02-2010 to 23-02-2010), Swift-UVOT (20-02-2010 and 22-02-2010), Swift-XRT (20-02-2010), and Fermi-LAT (January to March 2010). ApLib was observed by HESS from 11-05-2010 to 10-07-2010 (Fortin et al 2010), with a spectral slope of Γ = 2.45 ± 0.20 (Sanchez et al 2012).…”

“…No SSC model published on this source does account for VHE data (Tavecchio et al 1998;Sanchez et al 2012), and other radiating components are probably needed to describe the Ap Lib spectrum. However, before considering more complex models, we first analyse the limits of pure SSC scenarios for AP Librae in this section.…”