Multiband variability studies and novel broadband SED modeling of Mrk 501 in 2009

¹ ETH Zurich, 8093 Zurich, Switzerland
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, Univ. Rijeka, Univ. Split and Univ. of Zagreb, Croatia
⁶ Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Salt Lake, Sector-1, Kolkata 700064, India
⁷ Max-Planck-Institut für Physik, 80805 München, Germany
⁸ Universidad Complutense, 28040 Madrid, Spain
⁹ Inst. de Astrofísica de Canarias, 38200 La Laguna, Tenerife, Universidad de La Laguna, Dpto. Astrofísica, 38206 La Laguna, Tenerife, Spain
¹⁰ University of Łódź, 90236 Lodz, Poland
¹¹ Deutsches Elektronen-Synchrotron (DESY), 15738 Zeuthen, Germany
¹² Institut de Fisica d’Altes Energies (IFAE), The Barcelona Institute of Science and Technology, Campus UAB, 08193 Bellaterra ( Barcelona), Spain
¹³ Universität Würzburg, 97074 Würzburg, Germany
¹⁴ Università di Padova and INFN, 35131 Padova, Italy
¹⁵ Institute for Space Sciences (CSIC/IEEC), 08193 Barcelona, Spain
¹⁶ Technische Universität Dortmund, 44221 Dortmund, Germany
¹⁷ Finnish MAGIC Consortium, Tuorla Observatory, University of Turku and Astronomy Division, University of Oulu, Finland
¹⁸ Unitat de Física de les Radiacions, Departament de Física, and CERES-IEEC, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain
¹⁹ Universitat de Barcelona, ICC, IEEC-UB, 08028 Barcelona, Spain
²⁰ Japanese MAGIC Consortium, ICRR, The University of Tokyo, Department of Physics and Hakubi Center, Kyoto University, Tokai University, The University of Tokushima, KEK, Japan
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 for Space Sciences (CSIC/IEEC), 08193 Barcelona, Spain
²⁴ Now at Centro Brasileiro de Pesquisas Físicas (CBPF/MCTI), R. Dr. Xavier Sigaud, 150 – Urca, Rio de Janeiro – RJ 22290-180, Brazil
²⁵ NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
²⁶ Humboldt University of Berlin, Institut für Physik Newtonstr. 15, 12489 Berlin, Germany
²⁷ Also at University of Trieste, 34127 Trieste, Italy
²⁸ Now at École Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland
²⁹ Now at Max-Planck-Institut fur Kernphysik, PO Box 103980, 69029 Heidelberg, Germany
³⁰ Also at Japanese MAGIC Consortium, ICRR, The University of Tokyo, Japan
³¹ Now at Finnish Centre for Astronomy with ESO (FINCA), Turku, Finland
³² Also at INAF – Trieste and Dept. of Physics & Astronomy, University of Bologna, 40126 Bologna, Italy
³³ Also at ISDC – Science Data Center for Astrophysics, 1290 Versoix, Geneva, Switzerland
³⁴ Also at Department of Physics and Department of Astronomy, University of Maryland, College Park, MD 20742, USA
³⁵ Department of Physics and Astronomy, University of Utah, Salt Lake City, UT 84112, USA
³⁶ Physics Department, McGill University, Montreal, QC H3A 2T8, Canada
³⁷ Department of Physics, Washington University, St. Louis, MO 63130, USA
³⁸ Fred Lawrence Whipple Observatory, Harvard-Smithsonian Center for Astrophysics, Amado, AZ 85645, USA
³⁹ School of Physics, University College Dublin, Belfield, Dublin 4, Ireland
⁴⁰ Department of Physics and Astronomy, University of California, Los Angeles, CA 90095, USA
⁴¹ School of Physics, National University of Ireland Galway, University Road, Galway, Ireland
⁴² Department of Physics and Astronomy, Purdue University, West Lafayette, IN 47907, USA
⁴³ Department of Physics and Center for Astrophysics, Tsinghua University, Beijing 100084, PR China
⁴⁴ Department of Physics and Astronomy, Iowa State University, Ames, IA 50011, USA
⁴⁵ Department of Astronomy and Astrophysics, 525 Davey Lab., Pennsylvania State University, University Park, PA 16802, USA
⁴⁶ School of Physics and Astronomy, University of Minnesota, Minneapolis, MN 55455, USA
⁴⁷ Astronomy Department, Adler Planetarium and Astronomy Museum, Chicago, IL 60605, USA
⁴⁸ Department of Physics and Astronomy and the Bartol Research Institute, University of Delaware, Newark, DE 19716, USA
⁴⁹ Physics Department, Columbia University, New York, NY 10027, USA
⁵⁰ Department of Physics and Astronomy, University of Iowa, Van Allen Hall, Iowa City, IA 52242, USA
⁵¹ Department of Physics and Astronomy, DePauw University, Greencastle, IN 46135-0037, USA
⁵² Department of Physics and Astronomy, Barnard College, Columbia University, NY 10027, USA
⁵³ Enrico Fermi Institute, University of Chicago, Chicago, IL 60637, USA
⁵⁴ Instituto de Astronomia y Fisica del Espacio, Casilla de Correo 67 – Sucursal 28, (C1428ZAA) Ciudad Automa de Buenos Aires, Argentina
⁵⁵ Institute of Physics and Astronomy, University of Potsdam, 14476 Potsdam-Golm, Germany
⁵⁶ Department of Physical Sciences, Cork Institute of Technology, Bishopstown, Cork, Ireland
⁵⁷ School of Physics and Center for Relativistic Astrophysics, Georgia Institute ofTechnology, 837 State Street NW, Atlanta, GA 30332-0430, USA
⁵⁸ Santa Cruz Institute for Particle Physics and Department of Physics, University of California, Santa Cruz, CA 95064, USA
⁵⁹ Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, IL 60439, USA
⁶⁰ Department of Physics, University of Johannesburg, PO Box 524, Auckland Park 2006, South Africa
⁶¹ INAF–Osservatorio Astrofisico di Torino, 10025 Pino Torinese (TO), Italy
⁶² Department of Astronomy, University of Michigan, Ann Arbor, MI 48109-1042, USA
⁶³ Astronomical Institute, St. Petersburg State University, Universitetskij Pr. 28, Petrodvorets, 198504 St. Petersburg, Russia
⁶⁴ Pulkovo Observatory, St. Petersburg, Russia
⁶⁵ Department of Physics and Institute for Plasma Physics, University of Crete, 71003 Heraklion, Greece
⁶⁶ Foundation for Research and Technology – Hellas, IESL, Voutes, 71110 Heraklion, Greece
⁶⁷ Circolo Astrofili Talmassons, 33030 Campoformido (UD), Italy
⁶⁸ Osservatorio Astrofisico della Regione Autonoma Valle d’Aosta, Italy
⁶⁹ Abastumani Observatory, Mt. Kanobili, 0301 Abastumani, Georgia
⁷⁰ Engelhard Astronomical Observatory, Kazan Federal University, Tatarstan, Russia
⁷¹ Center for Astrophysics, Guangzhou University, Guangzhou 510006, PR China
⁷² Graduate Institute of Astronomy, National Central University, 300 Zhongda Road, Zhongli 32001, Taiwan
⁷³ Department of Physics and Mathematics, College of Science and 952 Engineering, Aoyama Gakuin University, 5-10-1 Fuchinobe, Chuoku, Sagamihara-shi Kanagawa 252-5258, Japan
⁷⁴ Department of Physics, University of Colorado Denver, Denver, Colorado, CO 80217-3364, USA
⁷⁵ Department of Physics and Astronomy, Brigham Young University, Provo, Utah 84602, USA
⁷⁶ Department of Physics, Tokyo Institute of Technology, Meguro City, Tokyo 152-8551, Japan
⁷⁷ Instituto Nacional de Astrofísica, Óptica y Electrónica, Tonantzintla, Puebla 72840, Mexico
⁷⁸ Instututo de Astronomia y Meteorologia, Dpto. de Fisica, CUCEI, Universidad de Guadalajara, Mexico
⁷⁹ Department of Physics and Astronomy, University of Denver, Denver, CO 80208, USA
⁸⁰ Steward Observatory, University of Arizona, Tucson, AZ 85721, USA
⁸¹ Space Science Institute, Boulder, CO 80301, USA
⁸² Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA
⁸³ Department of Physics and Astronomy, Pomona College, Claremont, CA 91711-6312, USA
⁸⁴ Department of Physics and Astronomy, University of Leicester, Leicester, LE1 7RH, UK
⁸⁵ ASI Science Data Center, via del Politecnico snc, 00133 Roma, Italy
⁸⁶ INAF–Osservatorio Astronomico di Roma, via di Frascati 33, 00040 Monteporzio, Italy
⁸⁷ Department of Physics, University of Trento, 38050 Povo, Trento, Italy
⁸⁸ Center for Research and Exploration in Space Science and Technology (CRESST), USA
⁸⁹ Universities Space Research Association (USRA), Columbia, MD 21044, USA
⁹⁰ Astro Space Center of Lebedev Physical Institute, Profsoyuznaya 84/32, 117997 Moscow, Russia
⁹¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁹² Observatori Astrono mic, Universitat de València, Parc Científic, C. Catedrático José Beltrán 2, 46980 Paterna, València, Spain
⁹³ Departament d’Astronomia i Astrofísica, Universitat de València, C. Dr. Moliner 50, 46100 Burjassot, València, Spain
⁹⁴ Crimean Astrophysical Observatory, 98409 Nauchny, Crimea
⁹⁵ Sternberg Astronomical Institute, M.V. Lomonosov Moscow State University, Universiteskij prosp. 13, Moscow 119991, Russia
⁹⁶ Institute of Astronomy, Astrophysics, Space Applications and Remote Sensing, National Observatory of Athens, Vas. Pavlou & I. Metaxa, 15 236 Penteli, Greece
⁹⁷ Department of Physics and Astronomy, Whittier College, Whittier, CA, USA
⁹⁸ Aalto University Metsähovi Radio Observatory, 02540 Kylmälä, Finland
⁹⁹ INAF Istituto di Radioastronomia, Sezione di Noto, Contrada Renna Bassa, 96017 Noto (SR), Italy
¹⁰⁰ INAF Istituto di Radioastronomia, Stazione Radioastronomica di Medicina, 40059 Medicina (Bologna), Italy
¹⁰¹ INAF Osservatorio Astrofisico di Catania, via S. Sofia 78, 95123 Catania, Italy
¹⁰² Cahill Center for Astronomy and Astrophysics, California Institute of Technology, Pasadena, CA 91125, USA

^⋆ Corresponding author: e-mail: marlene.doert@tu-dortmund.de, dpaneque@mppmu.mpg.de

Received: 17 August 2016
Accepted: 7 December 2016

Abstract

Context. We present an extensive study of the BL Lac object Mrk 501 based on a data set collected during the multi-instrument campaign spanning from 2009 March 15 to 2009 August 1, which includes, among other instruments, MAGIC, VERITAS, Whipple 10 m, and Fermi-LAT to cover the γ-ray range from 0.1 GeV to 20 TeV; RXTE and Swift to cover wavelengths from UV tohard X-rays; and GASP-WEBT, which provides coverage of radio and optical wavelengths. Optical polarization measurements were provided for a fraction of the campaign by the Steward and St. Petersburg observatories. We evaluate the variability of the source and interband correlations, the γ-ray flaring activity occurring in May 2009, and interpret the results within two synchrotron self-Compton (SSC) scenarios. Aims. The multiband variability observed during the full campaign is addressed in terms of the fractional variability, and the possible correlations are studied by calculating the discrete correlation function for each pair of energy bands where the significance was evaluated with dedicated Monte Carlo simulations. The space of SSC model parameters is probed following a dedicated grid-scan strategy, allowing for a wide range of models to be tested and offering a study of the degeneracy of model-to-data agreement in the individual model parameters, hence providing a less biased interpretation than the “single-curve SSC model adjustment” typically reported in the literature. Methods. We find an increase in the fractional variability with energy, while no significant interband correlations of flux changes are found on the basis of the acquired data set. The SSC model grid-scan shows that the flaring activity around May 22 cannot be modeled adequately with a one-zone SSC scenario (using an electron energy distribution with two breaks), while it can be suitably described within a two (independent) zone SSC scenario. Here, one zone is responsible for the quiescent emission from the averaged 4.5-month observing period, while the other one, which is spatially separated from the first, dominates the flaring emission occurring at X-rays and very-high-energy (>100 GeV, VHE) γ rays. The flaring activity from May 1, which coincides with a rotation of the electric vector polarization angle (EVPA), cannot be satisfactorily reproduced by either a one-zone or a two-independent-zone SSC model, yet this is partially affected by the lack of strictly simultaneous observations and the presence of large flux changes on sub-hour timescales (detected at VHE γ rays). Results. The higher variability in the VHE emission and lack of correlation with the X-ray emission indicate that, at least during the 4.5-month observing campaign in 2009, the highest energy (and most variable) electrons that are responsible for the VHE γ rays do not make a dominant contribution to the ~1 keV emission. Alternatively, there could be a very variable component contributing to the VHE γ-ray emission in addition to that coming from the SSC scenario. The studies with our dedicated SSC grid-scan show that there is some degeneracy in both the one-zone and the two-zone SSC scenarios probed, with several combinations of model parameters yielding a similar model-to-data agreement, and some parameters better constrained than others. The observed γ-ray flaring activity, with the EVPA rotation coincident with the first γ-ray flare, resembles those reported previously for low frequency peaked blazars, hence suggesting that there are many similarities in the flaring mechanisms of blazars with different jet properties.