Multiwavelength variability and correlation studies of Mrk 421 during historically low X-ray and γ-ray activity in 2015–2016
Authors: Acciari V A , Ansoldi S , Antonelli L A , Asano K , Babić A , Banerjee B , Baquero A , de Almeida U Barres , Barrio J A , González J Becerra , Bednarek W , Bellizzi L , Bernardini E , Bernardos M , Berti A , Besenrieder J , Bhattacharyya W , Bigongiari C , Blanch O , Bonnoli G , Bošnjak Ž , Busetto G , Carosi R , Ceribella G , Cerruti M , Chai Y , Chilingarian A , Cikota S , Colak S M , Colombo E , Contreras J L , Cortina J , Covino S , D’Amico G , D’Elia V , Da Vela P , Dazzi F , De Angelis A , De Lotto B , Delfino M , Delgado J , Mendez C Delgado , Depaoli D , Girolamo T Di , Pierro F Di , Venere L Di , Espi neira E Do Souto , Prester D Dominis , Donini A , Doro M , Ramazani V Fallah , Fattorini A , Ferrara G , Foffano L , Fonseca M V , Font L , Fruck C , Fukami S , López R J García , Garczarczyk M , Gasparyan S , Gaug M , Giglietto N , Giordano F , Gliwny P , Godinović N , Green J G , Green D , Hadasch D , Hahn A , Heckmann L , Herrera J , Hoang J , Hrupec D , Hütten M , Inada T , Inoue S , Ishio K , Iwamura Y , Jormanainen J , Jouvin L , Kajiwara Y , Karjalainen M , Kerszberg D , Kobayashi Y , Kubo H , Kushida J , Lamastra A , Lelas D , Leone F , Lindfors E , Lombardi S , Longo F , López M , López-Coto R , López-Oramas A , Loporchio S , Fraga B Machado de Oliveira , Maggio C , Majumdar P , Makariev M , Mallamaci M , Maneva G , Manganaro M , Maraschi L , Mariotti M , Martínez M , Mazin D , Mender S , Mićanović S , Miceli D , Miener T , Minev M , Miranda J M , Mirzoyan R , Molina E , Moralejo A , Morcuende D , Moreno V , Moretti E , Munar-Adrover P , Neustroev V , Nigro C , Nilsson K , Ninci D , Nishijima K , Noda K , Nozaki S , Ohtani Y , Oka T , Otero-Santos J , Palatiello M , Paneque D , Paoletti R , Paredes J M , Pavletić L , Pe nil P , Perennes C , Persic M , Moroni P G Prada , Prandini E , Priyadarshi C , Puljak I , Rhode W , Ribó M , Rico J , Righi C , Rugliancich A , Saha L , Sahakyan N , Saito T , Sakurai S , Satalecka K , Schleicher B , Schmidt K , Schweizer T , Sitarek J , Šnidarić I , Sobczynska D , Spolon A , Stamerra A , Strom D , Strzys M , Suda Y , Surić T , Takahashi M , Tavecchio F , Temnikov P , Terzić T , Teshima M , Torres-Albà N , Tosti L , Truzzi S , van Scherpenberg J , Vanzo G , Acosta M Vazquez , Ventura S , Verguilov V , Vigorito C F , Vitale V , Vovk I , Will M , Zarić D , Arbet-Engels A , Baack D , Balbo M , Beck M , Biederbeck N , Biland A , Bretz T , Bruegge K , Buss J , Dorner D , Elsaesser D , Hildebrand D , Iotov R , Klinger M , Mannheim K , Neise D , Neronov A , Noethe M , Paravac A , Rhode W , Schleicher B , Sliusar V , Theissen F , Walter R , Valverde J , Horan D , Giroletti M , Perri M , Verrecchia F , Leto C , Sadun A C , Moody J W , Joner M , Marscher A P , Jorstad S G , Lähteenmäki A , Tornikoski M , Ramakrishnan V , Järvelä E , Vera R J C , Righini S , Lien A Y 
Published: December 2020

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&gt;1TeV &lt; 1.7× 10−12 ph cm−2 s−1) and in the 2-10 keV (X-ray) band (F2 − 10 keV &lt; 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&gt;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.