Unprecedented study of the broadband emission of Mrk 421 during flaring activity in March 2010^⋆,⋆⋆

¹ IFAE, Campus UAB, 08193 Bellaterra, Spain
² Università di Udine, and INFN Trieste, 33100 Udine, Italy
³ INAF National Institute for Astrophysics, 00136 Rome, Italy
⁴ Università di Siena, and INFN Pisa, 53100 Siena, Italy
⁵ Croatian MAGIC Consortium, Rudjer Boskovic Institute, University of Rijeka and University of Split, 10000 Zagreb, Croatia
⁶ Max-Planck-Institut für Physik, 80805 München, Germany
⁷ Universidad Complutense, 28040 Madrid, Spain
⁸ Inst. de Astrofísica de Canarias, 38200 La Laguna, Tenerife, Spain
⁹ University of Łódź, 90236 Lodz, Poland
¹⁰ Deutsches Elektronen-Synchrotron (DESY), 15738 Zeuthen, Germany
¹¹ ETH Zurich, 8093 Zurich, Switzerland
¹² Universität Würzburg, 97074 Würzburg, Germany
¹³ Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas, 28040 Madrid, Spain
¹⁴ Institute of Space Sciences, 08193 Barcelona, Spain
¹⁵ Università di Padova and INFN, 35131 Padova, Italy
¹⁶ Technische Universität Dortmund, 44221 Dortmund, Germany
¹⁷ Unitat de Física de les Radiacions, Departament de Física, and CERES-IEEC, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain
¹⁸ Universitat de Barcelona, ICC, IEEC-UB, 08028 Barcelona, Spain
¹⁹ Japanese MAGIC Consortium, Division of Physics and Astronomy, Kyoto University, 277-8582 Chiba, Kashiwa, Japan
²⁰ Finnish MAGIC Consortium, Tuorla Observatory, University of Turku and Department of Physics, University of Oulu, 90014 Oulu, Finland
²¹ Inst. for Nucl. Research and Nucl. Energy, 1784 Sofia, Bulgaria
²² Università di Pisa, and INFN Pisa, 56126 Pisa, Italy
²³ ICREA and Institute of Space Sciences, 08193 Barcelona, Spain
²⁴ Università dell’Insubria and INFN Milano Bicocca, Como, 22100 Como, Italy
²⁵ Now at: Centro Brasileiro de Pesquisas Físicas (CBPF/MCTI), R. Dr. Xavier Sigaud, 150 − Urca, 22290-180 Rio de Janeiro – RJ, Brazil
²⁶ NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
²⁷ Department of Physics and Department of Astronomy, University of Maryland, College Park, MD 20742, USA
²⁸ Now at: École polytechnique fédérale de Lausanne (EPFL), 1015 Lausanne, Switzerland
²⁹ Now at: Institut für Astro- und Teilchenphysik, Leopold-Franzens- Universität Innsbruck, 6020 Innsbruck, Austria
³⁰ Now at: Finnish Centre for Astronomy with ESO (FINCA), 20100 Turku, Finland
³¹ Now at: Astrophysics Science Division, Bhabha Atomic Research Centre, 400085 Mumbai, India
³² Now at: School of Chemistry & Physics, University of Adelaide, 5005 Adelaide, Australia
³³ Physics Department, McGill University, Montreal, QC H3A 2T8, Canada
³⁴ Department of Physics, Washington University, St. Louis, MO 63130, USA
³⁵ Fred Lawrence Whipple Observatory, Harvard-Smithsonian Center for Astrophysics, Amado, AZ 85645, USA
³⁶ Department of Physics and Astronomy and the Bartol Research Institute, University of Delaware, Newark, DE 19716, USA
³⁷ School of Physics, University College Dublin, Belfield, Dublin 4, Ireland
³⁸ Santa Cruz Institute for Particle Physics and Department of Physics, University of California, Santa Cruz, CA 95064, USA
³⁹ Institute of Physics and Astronomy, University of Potsdam, 14476 Potsdam-Golm, Germany
⁴⁰ Astronomy Department, Adler Planetarium and Astronomy Museum, Chicago, IL 60605, USA
⁴¹ Department of Physics and Astronomy, Purdue University, West Lafayette, IN 47907, USA
⁴² Department of Physics and Astronomy, Iowa State University, Ames, IA 50011, USA
⁴³ Department of Astronomy and Astrophysics, 525 Davey Lab, Pennsylvania State University, University Park, PA 16802, USA
⁴⁴ School of Physics and Astronomy, University of Minnesota, Minneapolis, MN 55455, USA
⁴⁵ School of Physics, National University of Ireland Galway, University Road, Galway, Ireland
⁴⁶ Department of Physics and Astronomy, University of Iowa, Van Allen Hall, Iowa City, IA 52242, USA
⁴⁷ Department of Physics and Astronomy, University of Utah, Salt Lake City, UT 84112, USA
⁴⁸ Department of Physics and Astronomy, DePauw University, Greencastle, IN 46135-0037, USA
⁴⁹ Enrico Fermi Institute, University of Chicago, Chicago, IL 60637, USA
⁵⁰ Kavli Institute for Cosmological Physics, University of Chicago, Chicago, IL 60637, USA
⁵¹ School of Physics and Center for Relativistic Astrophysics, Georgia Institute of Technology, 837 State Street NW, Atlanta, GA 30332-0430, USA
⁵² Department of Physics, Anderson University, 1100 East 5th Street, Anderson, IN 46012, USA
⁵³ Department of Life and Physical Sciences, Galway-Mayo Institute of Technology, Dublin Road, Galway, Ireland
⁵⁴ Department of Physics and Astronomy, University of California, Los Angeles, CA 90095, USA
⁵⁵ Instituto de Astronomia y Fisica del Espacio, Casilla de Correo 67 − Sucursal 28, (C1428ZAA) Ciudad Autònoma de Buenos Aires, Argentina
⁵⁶ Physics Department, California Polytechnic State University, San Luis Obispo, CA 94307, USA
⁵⁷ Department of Applied Physics and Instrumentation, Cork Institute of Technology, Bishopstown, Cork, Ireland
⁵⁸ Space Science Division, Naval Research Laboratory, Washington, DC 20375-5352, USA
⁵⁹ INAF, Osservatorio Astronomico di Torino, 10025 Pino Torinese (TO), Italy
⁶⁰ Department of Astronomy, University of Michigan, Ann Arbor, MI 48109-1042, USA
⁶¹ Department of Physics, University of Trento, 38050 Povo, Trento, Italy
⁶² Graduate Institute of Astronomy, National Central University, 32054 Jhongli, Taiwan
⁶³ Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA
⁶⁴ Institute for Astrophysical Research, Boston University, 725 Commonwealth Avenue, Boston, MA 02215, USA
⁶⁵ Astronomical Institute, St. Petersburg State University, Universitetskij Pr. 28, Petrodvorets, 198504 St. Petersburg, Russia
⁶⁶ Institute of Astronomy, National Tsing Hua University, 101 Guanfu Rd., 30013 Hsinchu, Taiwan
⁶⁷ Abastumani Observatory, Mt. Kanobili, 0301 Abastumani, Georgia
⁶⁸ Landessternwarte, Zentrum für Astronomie der Universität Heidelberg, Königstuhl 12, 69117 Heidelberg, Germany
⁶⁹ Aalto University Metsähovi Radio Observatory, Metsähovintie 114, 02540 Kylmälä, Finland
⁷⁰ Aalto University Department of Radio Science and Engineering, PO Box 13000, 00076 Aalto, Finland
⁷¹ Astron. Inst., St.-Petersburg State University, 198504 St. Peetersburg, Russia
⁷² Pulkovo Observatory, St.-Petersburg, 198504 St. Peetersburg, Russia
⁷³ Isaac Newton Institute of Chile, St.-Petersburg Branch, Russia
⁷⁴ Department of Physics and Astronomy, Brigham Young University, Provo, UT 84602, USA
⁷⁵ Cahill Center for Astronomy and Astrophysics, California Institute of Technology, 1200 E California Blvd, Pasadena, CA 91125, USA
⁷⁶ ASI-Science Data Center, via del Politecnico, 00133 Rome, Italy
⁷⁷ Agrupació Astronòmica de Sabadell, 08206 Sabadell Barcelona, Spain
⁷⁸ Department of Physics, University of Colorado Denver, Denver, Colorado, Co 80217-3364, USA
⁷⁹ Department of Physics and Mathematics, College of Science and Engineering, Aoyama Gakuin University, 5-10-1 Fuchinobe, 1105 Chuoku, 252-5258 Sagamihara-shi Kanagawa, Japan
⁸⁰ Steward Observatory, University of Arizona, Tucson, AZ 85721, USA
⁸¹ Space Science Institute, Boulder, CO 80301, USA
⁸² School of Cosmic Physics, Dublin Institute for Advanced Studies, Belfield, Dublin 2, Ireland

^⋆⋆⋆ Corresponding author: David Paneque, e-mail: dpaneque@mpp.mpg.de; Shangyu Sun, e-mail: sysun@mpp.mpg.de; Hajime Takami, e-mail: takami@post.kek.jp

Received: 14 August 2014
Accepted: 5 December 2014

Abstract

Context. Because of its proximity, Mrk 421 is one of the best sources on which to study the nature of BL Lac objects. Its proximity allows us to characterize its broadband spectral energy distribution (SED).

Aims. The goal is to better understand the mechanisms responsible for the broadband emission and the temporal evolution of Mrk 421. These mechanisms may also apply to more distant blazars that cannot be studied with the same level of detail.

Methods. A flare occurring in March 2010 was observed for 13 consecutive days (from MJD 55 265 to MJD 55 277) with unprecedented wavelength coverage from radio to very high energy (VHE; E> 100 GeV) γ-rays with MAGIC, VERITAS, Whipple, Fermi-LAT, MAXI, RXTE, Swift, GASP-WEBT, and several optical and radio telescopes. We modeled the day-scale SEDs with one-zone and two-zone synchrotron self-Compton (SSC) models, investigated the physical parameters, and evaluated whether the observed broadband SED variability can be associated with variations in the relativistic particle population.

Results. The activity of Mrk 421 initially was high and then slowly decreased during the 13-day period. The flux variability was remarkable at the X-ray and VHE bands, but it was minor or not significant at the other bands. The variability in optical polarization was also minor. These observations revealed an almost linear correlation between the X-ray flux at the 2–10 keV band and the VHE γ-ray flux above 200 GeV, consistent with the γ-rays being produced by inverse-Compton scattering in the Klein-Nishina regime in the framework of SSC models. The one-zone SSC model can describe the SED of each day for the 13 consecutive days reasonably well, which once more shows the success of this standard theoretical scenario to describe the SEDs of VHE BL Lacs such as Mrk 421. This flaring activity is also very well described by a two-zone SSC model, where one zone is responsible for the quiescent emission, while the other smaller zone, which is spatially separated from the first, contributes to the daily variable emission occurring at X-rays and VHE γ-rays. The second blob is assumed to have a smaller volume and a narrow electron energy distribution with 3 × 10⁴<γ< 6 × 10⁵, where γ is the Lorentz factor of the electrons. Such a two-zone scenario would naturally lead to the correlated variability at the X-ray and VHE bands without variability at the optical/UV band, as well as to shorter timescales for the variability at the X-ray and VHE bands with respect to the variability at the other bands.

Conclusions. Both the one-zone and the two-zone SSC models can describe the daily SEDs via the variation of only four or five model parameters, under the hypothesis that the variability is associated mostly with the underlying particle population. This shows that the particle acceleration and cooling mechanism that produces the radiating particles might be the main mechanism responsible for the broadband SED variations during the flaring episodes in blazars. The two-zone SSC model provides a better agreement with the observed SED at the narrow peaks of the low- and high-energy bumps during the highest activity, although the reported one-zone SSC model could be further improved by varying the parameters related to the emitting region itself (δ, B and R), in addition to the parameters related to the particle population.