Luminous and high-frequency peaked blazars: the origin of the γ-ray emission from PKS 1424+240

¹ Sorbonne Universités, UPMC, Université Paris Diderot, Sorbonne Paris Cité, CNRS,LPNHE, 4 place Jussieu, 75252 Paris, France
e-mail: matteo.cerruti@lpnhe.in2p3.fr
² Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
³ Institute of Physics and Astronomy, University of Potsdam, 14476 Potsdam-Golm, Germany
⁴ DESY, Platanenallee 6, 15738 Zeuthen, Germany
⁵ Oskar Klein Centre and Dept. of Physics, Stockholm University, 10691 Stockholm, Sweden
⁶ School of Physics and Astronomy, University of Minnesota, Minneapolis, MN 55455, USA
⁷ Minnesota Institute for Astrophysics (MIfA), University of Minnesota, Minneapolis, MN 55455, USA

Received: 16 March 2017
Accepted: 8 July 2017

Abstract

Context. The current generation of ground-based Cherenkov telescopes, together with the LAT instrument on-board the Fermi satellite, have greatly increased our knowledge of γ-ray blazars. Among them, the high-frequency-peaked BL Lacertae object (HBL) PKS 1424+240 (z ≃ 0.6) is the farthest persistent emitter of very-high-energy (VHE; E ≥ 100 GeV) γ-ray photons. Current emission models can satisfactorily reproduce typical blazar emission assuming that the dominant emission process is synchrotron-self-Compton (SSC) in HBLs; and external-inverse-Compton (EIC) in low-frequency-peaked BL Lacertae objects and flat-spectrum-radio-quasars. Alternatively, hadronic models are also able to correctly reproduce the γ-ray emission from blazars, although they are in general disfavored for bright quasars and rapid flares.

Aims. The blazar PKS 1424+240 is a rare example of a luminous HBL, and we aim to determine which is the emission process most likely responsible for its γ-ray emission. This will impact more generally our comprehension of blazar emission models, and how they are related to the luminosity of the source and the peak frequency of the spectral energy distribution.

Methods. We have investigated different blazar emission models applied to the spectral energy distribution of PKS 1424+240. Among leptonic models, we study a one-zone SSC model (including a systematic study of the parameter space), a two-zone SSC model, and an EIC model. We then investigated a blazar hadronic model, and finally a scenario in which the γ-ray emission is associated with cascades in the line-of-sight produced by cosmic rays from the source.

Results. After a systematic study of the parameter space of the one-zone SSC model, we conclude that this scenario is not compatible with γ-ray observations of PKS 1424+240. A two-zone SSC scenario can alleviate this issue, as well as an EIC solution. For the latter, the external photon field is assumed to be the infra-red radiation from the dusty torus, otherwise the VHE γ-ray emission would have been significantly absorbed. Alternatively, hadronic models can satisfactorily reproduce the γ-ray emission from PKS 1424+240, both as in-source emission and as cascade emission.