Detection of the blazar S4 0954+65 at very-high-energy with the MAGIC telescopes during an exceptionally high optical state

¹ ETH Zurich, 8093 Zurich, Switzerland
² Università di Udine, and INFN Trieste, 33100 Udine, Italy
³ National Institute for Astrophysics (INAF), 00136 Rome, Italy
⁴ Università di Padova and INFN, 35131 Padova, Italy
⁵ Technische Universität Dortmund, 44221 Dortmund, Germany
⁶ Croatian MAGIC Consortium: University of Rijeka, 51000 Rijeka, University of Split – FESB, 21000 Split, University of Zagreb – FER, 10000 Zagreb, University of Osijek, 31000 Osijek and Rudjer Boskovic Institute, 10000 Zagreb, Croatia
⁷ Saha Institute of Nuclear Physics, HBNI, 1/AF Bidhannagar, Salt Lake, Sector-1, Kolkata 700064, India
⁸ Max–Planck–Institut für Physik, 80805 München, Germany
⁹ now at Centro Brasileiro de Pesquisas Físicas (CBPF), 22290-180 URCA, Rio de Janeiro (RJ), Brasil
¹⁰ Unidad de Partículas y Cosmología (UPARCOS), Universidad Complutense, 28040 Madrid, Spain
¹¹ University of Łódź, Department of Astrophysics, 90236 Łódź, Poland
¹² Deutsches Elektronen-Synchrotron (DESY), 15738 Zeuthen, Germany
¹³ Institut de Física d’Altes Energies (IFAE), The Barcelona Institute of Science and Technology (BIST), 08193 Bellaterra (Barcelona), Spain
¹⁴ Università di Siena and INFN Pisa, 53100 Siena, Italy
¹⁵ Instituto de Astrofísica de Canarias, 38200 La Laguna, and Universidad de La Laguna, Departamento Astrofísica, 38206 La Laguna, Tenerife, Spain
¹⁶ Universität Würzburg, 97074 Würzburg, Germany
¹⁷ Finnish MAGIC Consortium: Tuorla Observatory and Finnish Centre of Astronomy with ESO (FINCA), University of Turku, Vaisalantie 20, 21500 Piikkiö, Astronomy Division, University of Oulu, 90014 University of Oulu, Finland
¹⁸ Departament de Física, and CERES-IEEC, Universitat Autónoma de Barcelona, 08193 Bellaterra, Spain
¹⁹ Universitat de Barcelona, ICC, IEEC-UB, 08028 Barcelona, Spain
²⁰ 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; The University of Tokushima, 770-8502 Tokushima, Japan
²¹ Institute for Nuclear Research and Nuclear Energy, Bulgarian Academy of Sciences, 1784 Sofia, Bulgaria
²² Università di Pisa, and INFN Pisa, 56126 Pisa, Italy
²³ Humboldt University of Berlin, Institut für Physik 12489 Berlin, Germany
²⁴ Dipartimento di Fisica, Università di Trieste, 34127 Trieste, Italy
²⁵ Port d’Informació Científica (PIC) 08193 Bellaterra (Barcelona), Spain
²⁶ INAF-Trieste and Deptartment of Physics and Astronomy, University of Bologna, Italy
²⁷ NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA and Department of Physics and Department of Astronomy, University of Maryland, College Park, MD 20742, USA
²⁸ Hiroshima Astrophysical Science Center, Hiroshima, Japan
²⁹ University of Maryland, Baltimore County, USA
³⁰ The Catholic University of America, Washington DC, USA
³¹ Space Science Division, NRL, Washington DC, USA
³² Aalto University Metsahovi Radio Observatory, Finland
³³ Aalto University Department of Electronics and Nanoengineering, Finland
³⁴ Tartu Observatory, Estonia
³⁵ Tuorla Observatory, University of Turku, Väisäläntie 20, 21500 Piikkiö, Finland
³⁶ Astronomical Institute, St. Petersburg State University, Russia
³⁷ Institute for Astrophysical Research, Boston University, USA
³⁸ Pulkovo Observatory, St. Petersburg, Russia
³⁹ Crimean Astrophysical Observatory, P/O Nauchny, 298409, Crimea
⁴⁰ Instituto de Astrofísica de Andalucía (CSIC), Apartado 3004, 18080 Granada, Spain
⁴¹ Max–Planck–Institut für Radioastronomie, Auf dem Hügel, 69, 53121, Bonn, Germany
⁴² Harvard-Smithsonian Center for Astrophysics, Cambridge, MA USA
⁴³ INAF, Osservatorio Astrofisico di Torino, via Osservatorio 20, 10025, Pino Torinese, Italy
⁴⁴ Departamento de Astrofisica, Universidad de La Laguna, La Laguna, 38205 Tenerife, Spain

Received: 12 January 2018
Accepted: 19 March 2018

Abstract

Aims. The very high energy (VHE ≳100 GeV) γ-ray MAGIC observations of the blazar S4 0954+65, were triggered by an exceptionally high flux state of emission in the optical. This blazar has a disputed redshift of z = 0.368 or z ≥ 0.45 and an uncertain classification among blazar subclasses. The exceptional source state described here makes for an excellent opportunity to understand physical processes in the jet of S4 0954+65 and thus contribute to its classification.

Methods. We investigated the multiwavelength (MWL) light curve and spectral energy distribution (SED) of the S4 0954+65 blazar during an enhanced state in February 2015 and have put it in context with possible emission scenarios. We collected photometric data in radio, optical, X-ray, and γ-ray. We studied both the optical polarization and the inner parsec-scale jet behavior with 43 GHz data.

Results. Observations with the MAGIC telescopes led to the first detection of S4 0954+65 at VHE. Simultaneous data with Fermi-LAT at high energy γ-ray (HE, 100 MeV < E < 100 GeV) also show a period of increased activity. Imaging at 43 GHz reveals the emergence of a new feature in the radio jet in coincidence with the VHE flare. Simultaneous monitoring of the optical polarization angle reveals a rotation of approximately 100°.

Conclusions. The high emission state during the flare allows us to compile the simultaneous broadband SED and to characterize it in the scope of blazar jet emission models. The broadband spectrum can be modeled with an emission mechanism commonly invoked for flat spectrum radio quasars (FSRQs), that is, inverse Compton scattering on an external soft photon fieldfrom the dust torus, also known as external Compton. The light curve and SED phenomenology is consistent with an interpretation of a blob propagating through a helical structured magnetic field and eventually crossing a standing shock in the jet, a scenario typically applied to FSRQs and low-frequency peaked BL Lac objects (LBL).