A fast, very-high-energy γ-ray flare from BL Lacertae during a period of multi-wavelength activity in June 2015^⋆

¹ Inst. de Astrofísica de Canarias, and Universidad de La Laguna, Dpto. Astrofísica, 38206 La Laguna, Tenerife, Spain
² Università di Udine, and INFN Trieste, 33100 Udine, Italy
³ National Institute for Astrophysics (INAF), 00136 Rome, Italy
⁴ ETH Zurich, 8093 Zurich, Switzerland
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 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 ᴌódź, Department of Astrophysics, 90236 ᴌódź, 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 Università di Pisa, and INFN Pisa, 56126 Pisa, Italy
¹⁶ Università di Padova and INFN, 35131 Padova, Italy
¹⁷ Universität Würzburg, 97074 Würzburg, Germany
¹⁸ Finnish MAGIC Consortium: Tuorla Observatory and Finnish Centre of Astronomy with ESO (FINCA), University of Turku FIN-20014 University of Turku, Finland, Astronomy Division, University of Oulu, 90014, University of Oulu, Finland
¹⁹ Departament de Física, and CERES-IEEC, Universitat Autónoma de Barcelona, 08193 Bellaterra, Spain
²⁰ Universitat de Barcelona, ICCUB, IEEC-UB, 08028 Barcelona, Spain
²¹ 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
²² Inst. for Nucl. Research and Nucl. Energy, Bulgarian Academy of Sciences, 1784 Sofia, Bulgaria
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 Dept. of Physics & Astronomy, University of Bologna, Italy
²⁷ Dipartimento di Fisica e Astronomia, Università di Bologna, Via Gobetti 93/2, 40129 Bologna, Italy
²⁸ INAF, Instituto di Radioastronomia di Bologna, Via P. Gobetti 101, 40129 Bologna, Italy
²⁹ Mizusawa VLBI Observatory, National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
³⁰ Department of Astronomical Science, The Graduate University for Advanced Studies (SOKENDAI), 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
³¹ Institute for Astrophysical Research, Boston University, 725 Commonwealth Avenue, Boston, MA 02215, USA
³² Astronomical Institute, St. Petersburg State University, Universitetskij Pr. 28, Petrodvorets, 198504 St. Petersburg, Russia
³³ Tuorla Observatory, Department of Physics and Astronomy, University of Turku, Finland
³⁴ Pulkovo Observatory, St.-Petersburg, Russia
³⁵ Crimean Astrophysical Observatory, Crimea
³⁶ Aalto University, Metsähovi Radio Observatory, Metsähovintie 114, 02540 Kylml, Finland
³⁷ Aalto University, Department of Electronics and Nanoengineering, PO Box 15500, 00076 Aalto, Finland
³⁸ Tartu Observatory, Observatooriumi 1, 61602 Töravere, Estonia
³⁹ Instituto de Astrofísica de Andalucía-CSIC, Apdo. 3004, 18080 Granada, Spain

Received: 3 August 2018
Accepted: 30 December 2018

Abstract

The mechanisms producing fast variability of the γ-ray emission in active galactic nuclei (AGNs) are under debate. The MAGIC telescopes detected a fast, very-high-energy (VHE, E  >  100 GeV) γ-ray flare from BL Lacertae on 2015 June 15. The flare had a maximum flux of (1.5 ± 0.3) × 10⁻¹⁰ photons cm⁻² s⁻¹ and halving time of 26 ± 8 min. The MAGIC observations were triggered by a high state in the optical and high-energy (HE, E  >  100 MeV) γ-ray bands. In this paper we present the MAGIC VHE γ-ray data together with multi-wavelength data from radio, optical, X-rays, and HE γ rays from 2015 May 1 to July 31. Well-sampled multi-wavelength data allow us to study the variability in detail and compare it to the other epochs when fast, VHE γ-ray flares have been detected from this source. Interestingly, we find that the behaviour in radio, optical, X-rays, and HE γ-rays is very similar to two other observed VHE γ-ray flares. In particular, also during this flare there was an indication of rotation of the optical polarization angle and of activity at the 43 GHz core. These repeating patterns indicate a connection between the three events. We also test modelling of the spectral energy distribution based on constraints from the light curves and VLBA observations, with two different geometrical setups of two-zone inverse Compton models. In addition we model the γ-ray data with the star-jet interaction model. We find that all of the tested emission models are compatible with the fast VHE γ-ray flare, but all have some tension with the multi-wavelength observations.