Testing two-component models on very high-energy gamma-ray-emitting BL Lac objects^⋆

¹ Inst. de Astrofísica de Canarias, E-38200 La Laguna, and Universidad de La Laguna, Dpto. Astrofísica, 38206 La Laguna, Tenerife, Spain
² Università di Udine, and INFN Trieste, 33100 Udine, Italy
³ Japanese MAGIC Consortium: ICRR, The University of Tokyo, 277-8582 Chiba, Japan; Department of Physics, Kyoto University, 606-8502 Kyoto, Japan; Tokai University, 259-1292 Kanagawa, Japan; RIKEN, 351-0198 Saitama, Japan
⁴ National Institute for Astrophysics (INAF), 00136 Rome, Italy
⁵ ETH Zurich, 8093 Zurich, Switzerland
⁶ Technische Universität Dortmund, 44221 Dortmund, Germany
⁷ Croatian Consortium: University of Rijeka, Department of Physics, 51000 Rijeka; University of Split – FESB, 21000 Split; University of Zagreb – FER, 10000 Zagreb;University of Osijek, 31000 Osijek; Rudjer Boskovic Institute, 10000 Zagreb, Croatia
⁸ Saha Institute of Nuclear Physics, HBNI, 1/AF Bidhannagar, Salt Lake, Sector-1, Kolkata 700064, India
⁹ Centro Brasileiro de Pesquisas Físicas (CBPF), 22290-180 URCA Rio de Janeiro (RJ), Brasil
¹⁰ IPARCOS Institute and EMFTEL Department, Universidad Complutense de Madrid, 28040 Madrid, Spain
¹¹ University of Lodz, Faculty of Physics and Applied Informatics, Department of Astrophysics, 90-236 Lodz, Poland
¹² Università di Siena and INFN Pisa, 53100 Siena, Italy
¹³ Deutsches Elektronen-Synchrotron (DESY), 15738 Zeuthen, Germany
¹⁴ Università di Padova and INFN, 35131 Padova, Italy
¹⁵ Istituto Nazionale Fisica Nucleare (INFN), 00044 Frascati, (Roma), Italy
¹⁶ Max-Planck-Institut für Physik, 80805 München, Germany
¹⁷ Institut de Física d’Altes Energies (IFAE), The Barcelona Institute of Science and Technology (BIST), 08193 Bellaterra, (Barcelona), Spain
¹⁸ Università di Pisa, and INFN Pisa, 56126 Pisa, Italy
¹⁹ Universitat de Barcelona, ICCUB, IEEC-UB, 08028 Barcelona, Spain
²⁰ The Armenian Consortium: ICRANet-Armenia at NAS RA, A. Alikhanyan National Laboratory, Yerevan, Armenia
²¹ Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas, 28040 Madrid, Spain
²² Universität Würzburg, 97074 Würzburg, Germany
²³ Finnish MAGIC Consortium: Finnish Centre of Astronomy with ESO (FINCA), University of Turku, -20014 Turku, Finland; Astronomy Research Unit, University of Oulu, 90014 Oulu, Finland
²⁴ Departament de Física, and CERES-IEEC, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain
²⁵ Inst. for Nucl. Research and Nucl. Energy, Bulgarian Academy of Sciences, 1784 Sofia, Bulgaria
²⁶ Now at University of Innsbruck, Innsbruck, Austria
²⁷ Also at Port d’Informació Científica (PIC), 08193 Bellaterra (Barcelona), Spain
²⁸ Also at Dipartimento di Fisica, Università di Trieste, 34127 Trieste, Italy
²⁹ Also at INAF-Trieste and Dept. of Physics & Astronomy, University of Bologna, Bologna, Italy
³⁰ Centre for Space Research, North-West University, Potchefstroom 2520, South Africa
³¹ Also at INAF-Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
³² Istituto di RadioAstronomia, 40129 Bologna, Italy
³³ Istituto Nazionale di Fisica Nucleare, Sezione di Torino, 10125 Torino, Italy
³⁴ Owens Valley Radio Observatory, California Institute of Technology, Pasadena, CA 91125, USA
³⁵ Finnish Center for Astronomy with ESO (FINCA), University of Turku, 20014 Turku, Finland
³⁶ Aalto University Metsähovi Radio Observatory, Metsähovintie 114, 02540 Kylmälä, Finland
³⁷ Institute of Astrophysics, Foundation for Research and Technology-Hellas, 71110 Heraklion, Greece
³⁸ Department of Physics, Univ. of Crete, 70013 Heraklion, Greece
³⁹ Departamento de Astronomía, Universidad de Chile, Camino El Observatorio 1515, Las Condes, Santiago, Chile
⁴⁰ Departamento de Astronomía, Universidad de Concepción, Concepción, Chile
⁴¹ Department of Physics and Astronomy, University of Turku, 20014 Turku, Finland
⁴² Hankasalmi Observatory, Murtoistentie 116-124, 41500 Hankasalmi, Finland
⁴³ Institute of Astronomy and NAO, Bulgarian Academy of Sciences, 1784 Sofia, Bulgaria
⁴⁴ Harlingten New Mexico Observatory, Mayhill, NM, USA

Received: 25 February 2020
Accepted: 8 June 2020

Abstract

Context. It has become evident that one-zone synchrotron self-Compton models are not always adequate for very high-energy (VHE) gamma-ray-emitting blazars. While two-component models perform better, they are difficult to constrain due to the large number of free parameters.

Aims. In this work, we make a first attempt at taking into account the observational constraints from very long baseline interferometry (VLBI) data, long-term light curves (radio, optical, and X-rays), and optical polarisation to limit the parameter space for a two-component model and test whether or not it can still reproduce the observed spectral energy distribution (SED) of the blazars.

Methods. We selected five TeV BL Lac objects based on the availability of VHE gamma-ray and optical polarisation data. We collected constraints for the jet parameters from VLBI observations. We evaluated the contributions of the two components to the optical flux by means of decomposition of long-term radio and optical light curves as well as modelling of the optical polarisation variability of the objects. We selected eight epochs for these five objects based on the variability observed at VHE gamma rays, for which we constructed the SEDs that we then modelled with a two-component model.

Results. We found parameter sets which can reproduce the broadband SED of the sources in the framework of two-component models considering all available observational constraints from VLBI observations. Moreover, the constraints obtained from the long-term behaviour of the sources in the lower energy bands could be used to determine the region where the emission in each band originates. Finally, we attempt to use optical polarisation data to shed new light on the behaviour of the two components in the optical band. Our observationally constrained two-component model allows explanation of the entire SED from radio to VHE with two co-located emission regions.