Multiwavelength variability and correlation studies of Mrk 421 during historically low X-ray and γ-ray activity in 2015–2016
V. A. Acciari, Stefano Ansoldi, L. A. Antonelli, Katsuaki Asano, A. Babić, B. Banerjee, Andrés Baquero, U. Barres de Almeida, J. A. Barrio, J. Becerra González, W. Bednarek, Lorenzo Bellizzi, E. Bernardini, M. Bernardos, A. Berti, Jürgen Besenrieder, W. Bhattacharyya, C. Bigongiari, O. Blanch, G. Bonnoli, Ž. Bošnjak, G. Busetto, R. Carosi, G. Ceribella, M. Cerruti, Yating Chai, A. Chilingarian, S. Cikota, S. M. Colak, E. Colombo, J. L. Contreras, J. Cortina, S. Covino, Giacomo D’Amico, V. D’Elia, P. Da Vela, F. Dazzi, A. De Angelis, B. De Lotto, M. Delfino, J. Delgado, C. Delgado Mendez, Davide Depaoli, T. Di Girolamo, F. Di Pierro, L. Di Venere, E. Do Souto Espiñeira, D. Dominis Prester, Alice Donini, M. Doro, V. Fallah Ramazanı, Alicia Fattorini, G. Ferrara, L. Foffano, M. V. Fonseca, L. Font, C. Fruck, Satoshi Fukami, R. J. Garcı́a López, M. Garczarczyk, Sargis Gasparyan, M. Gaug, N. Giglietto, F. Giordano, Paweł Gliwny, N. Godinović, Jarred Gershon Green, D. Green, D. Hadasch, A. Hahn, Lea Heckmann, J. Herrera, John Hoang, Dario Hrupec, Moritz Hütten, Tomohiro Inada, Susumu Inoue, Kazuma Ishio, Y. Iwamura, Jenni Jormanainen, L. Jouvin, Y. Kajiwara, Marie Karjalainen, D. Kerszberg, Yukiho Kobayashi, H. Kubo, J. Kushida, A. Lamastra, D Lelas, F. Leone, E. Lindfors, S. Lombardi, F. Longo, Marcos López, R. López-Coto, A. López-Oramas, S. Loporchio, B. Machado de Oliveira Fraga, C. Maggio, P. Majumdar
Abstract
Abstract We report a characterization of the multi-band flux variability and correlations of the nearby (z=0.031) blazar Markarian 421 (Mrk 421) using data from Metsähovi, Swift, Fermi-LAT, MAGIC, FACT and other collaborations and instruments from November 2014 till June 2016. Mrk 421 did not show any prominent flaring activity, but exhibited periods of historically low activity above 1 TeV (F>1TeV < 1.7× 10−12 ph cm−2 s−1) and in the 2-10 keV (X-ray) band (F2 − 10 keV < 3.6 × 10−11 erg cm−2 s−1), during which the Swift-BAT data suggests an additional spectral component beyond the regular synchrotron emission.The highest flux variability occurs in X-rays and very-high-energy (E>0.1 TeV) γ-rays, which, despite the low activity, show a significant positive correlation with no time lag. The HRkeV and HRTeV show the harder-when-brighter trend observed in many blazars, but the trend flattens at the highest fluxes, which suggests a change in the processes dominating the blazar variability. Enlarging our data set with data from years 2007 to 2014, we measured a positive correlation between the optical and the GeV emission over a range of about 60 days centered at time lag zero, and a positive correlation between the optical/GeV and the radio emission over a range of about 60 days centered at a time lag of $43^{+9}_{-6}$ days.This observation is consistent with the radio-bright zone being located about 0.2 parsec downstream from the optical/GeV emission regions of the jet. The flux distributions are better described with a LogNormal function in most of the energy bands probed, indicating that the variability in Mrk 421 is likely produced by a multiplicative process.