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Ansoldi, S.; Antonelli, L. A.; Arbet Engels, A.; Babić, A.; Bednarek, W.; Bernardini, E.; Bernardos, M.; Carosi, R.; Cerruti, M.; Chilingarian, A.; Cikota, S.; Delfino, M.; Depaoli, D.; Di Venere, L.; Font, L.; Fukami, S.; Gasparyan, S.; Giordano, F.; Green, J. G.; Hadasch, D.; Heckmann, L.; Hoang, J.; Hrupec, D.; Inada, T.; Inoue, S.; Ishio, K.; Iwamura, Y.; Kushida, J.; Leone, F.; Longo, F.; López-Coto, R.; Loporchio, S.; Machado de Oliveira Fraga, B.; Majumdar, P.; Mićanović, S.; Minev, M.; Neustroev, V.; Nigro, C.; Nishijima, K.; Nozaki, S.; Ohtani, Y.; Paoletti, R.; Pavletić, L.; Prandini, E.; Rugliancich, A.; Saha, L.; Sakurai, S.; Saturni, F. G.; Šnidarić, I.; Sobczynska, D.; Spolon, A.; Strom, D.; Strzys, M.; Tavecchio, F.; Temnikov, P.; Terzić, T.; Tosti, L.; Vanzo, G.; Vitale, V.; Baack, D.; Eisenberger, L.; Elsaesser, D.; Hildebrand, D.; Iotov, R.; Mannheim, K.; Noethe, M.; Sliusar, V.; Madejski, G. M.; Perri, M.; Leto, C.; Lähteenmäki, A.; Tornikoski, M.; Järvelä, E.; Agudo, I.; Casadio, C.; Bozhilov, V.; Valcheva, A.; Larionov, V. M.; Hagen-Thorn, V. A.; Kopatskaya, E. N.; Larionova, E. G.; Larionova, L. V.; Morozova, D. A.; Samal, M.; Lin, H. C.; Jorstad, S. G.; Marscher, A. P.; Feige, M.; Kania, J.; Kunkel, L.; Scherbantin, A.; Carnerero, M. I.; Kurtanidze, S. O.; Chigladze, R. A.; Ivanidze, R. Z.; Kimeridze, G. N.; Joner, M. D.; Spencer, M.; Righini, S.; Rizzi, N.
Astronomy & astrophysics, 11/2021, Letnik: 655Journal Article
Aims. We present a detailed characterisation and theoretical interpretation of the broadband emission of the paradigmatic TeV blazar Mrk 421, with a special focus on the multi-band flux correlations. Methods. The dataset has been collected through an extensive multi-wavelength campaign organised between 2016 December and 2017 June. The instruments involved are MAGIC, FACT, Fermi -LAT, Swift , GASP-WEBT, OVRO, Medicina, and Metsähovi. Additionally, four deep exposures (several hours long) with simultaneous MAGIC and NuSTAR observations allowed a precise measurement of the falling segments of the two spectral components. Results. The very-high-energy (VHE; E > 100 GeV) gamma rays and X-rays are positively correlated at zero time lag, but the strength and characteristics of the correlation change substantially across the various energy bands probed. The VHE versus X-ray fluxes follow different patterns, partly due to substantial changes in the Compton dominance for a few days without a simultaneous increase in the X-ray flux (i.e., orphan gamma-ray activity). Studying the broadband spectral energy distribution (SED) during the days including NuSTAR observations, we show that these changes can be explained within a one-zone leptonic model with a blob that increases its size over time. The peak frequency of the synchrotron bump varies by two orders of magnitude throughout the campaign. Our multi-band correlation study also hints at an anti-correlation between UV-optical and X-ray at a significance higher than 3 σ . A VHE flare observed on MJD 57788 (2017 February 4) shows gamma-ray variability on multi-hour timescales, with a factor ten increase in the TeV flux but only a moderate increase in the keV flux. The related broadband SED is better described by a two-zone leptonic scenario rather than by a one-zone scenario. We find that the flare can be produced by the appearance of a compact second blob populated by high energetic electrons spanning a narrow range of Lorentz factors, from γ ′ min =2×10 4 to γ ′ max =6×10 5 .
