Linking radio and gamma-ray emission in Ap Librae

Abstract

Ap Lib is one of the rare low-synchrotron-peaked blazars detected so far at TeV energies. This type of source is not properly modelled by standard one-zone leptonic synchrotron self-Compton (SSC) emission scenarios. The aim of this paper is to study the relevance of additional components that should naturally occur in an SSC scenario for a better understanding of the emission mechanisms, especially at very high energies (VHE). We use simultaneous data from a multi-wavelength campaign of the Planck, Swift-UVOT, and Swift-XRT telescopes carried out in February 2010, as well as quasi-simultaneous data of WISE, Fermi, and HESS taken in 2010. The multi-lambda emission of Ap Lib is modelled by a blob-in-jet SSC scenario including the contribution of the base of the VLBIextended jet, the radiative blob-jet interaction, the accretion disk, and its associated external photon field. We show that signatures of a strong parsec-scale jet and of an accretion disk emission are present in the spectral energy distribution. We can link the observational VLBI jet features from MOJAVE to parameters expected for a VHE-emitting blob accelerated near the jet base. The VHE emission appears to be dominated by the inverse-Compton effect of the blob relativistic electrons interacting with the jet synchrotron radiation. In this scenario, Ap Lib appears as an intermediate source between BL Lac objects and flat-spectrum radio quasars. Ap Lib could be a bright representative of a specific class of blazars, in which the parsec-scale jet luminosity is no more negligible compared to the blob and contributes to the high-energy emission through inverse-Compton processes.

AP Librae: The Extended Jet as the Source of VHE Emission?

“…Unlike the models of [3,4], we do not expect variability in the extended jet emission, because such a large object must be steady over time scales much longer than a few times l jet /c. Thus, the flux above a few GeV should not drop below the observed level.…”

“…The remaining multiwavelength data is taken from the following papers or publicly available data bases: Radio [6,7], and IR/optical/UV [2,3].…”

“…The failure of the leptonic one-zone model provoked [3] to propose a leptonic multi-zone model, where the moving blob is immersed in the surrounding pc-scale jet structure producing inverse Compton emission on the jet's synchrotron radiation. Hadronic modeling by [4] also gives a successful fit of the data.…”

Expected signatures from hadronic emission processes in the TeV spectra of BL Lacertae objects

“…An additional extensive jet could have been added, as is done for certain leptonic models (see e.g. Katarzyński et al 2001;Abramowski et al 2012;Hervet et al 2015) if one wants to account for electron synchrotron emission in the radio band, but its contribution to the total jet power would be much smaller than that from the "blob". The value of L j is not constrained from first principles, but can be compared to the Eddington luminosity L edd as an order-of-magnitude reference.…”

Variation of bulk Lorentz factor in AGN jets due to Compton rocket in a complex photon field

“…We assume a jet composed of spherical emitting zones called blobs moving forward (see, e.g., Katarzynski et al 2001;Boutelier et al 2008;or Hervet et al 2015, for blob-in-jet models). The emergence of a blob at the base of the jet would then correspond to a flare.…”