Multiwavelength observations of Mrk 501 in 2008^⋆

¹ IFAE, Edifici Cn., 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 Lodz, 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 Technische Universität Dortmund, 44221 Dortmund, Germany
¹⁵ Inst. de Astrofísica de Andalucía (CSIC), 18080 Granada, Spain
¹⁶ Università di Padova and INFN, 35131 Padova, Italy
¹⁷ Università dell’Insubria, Como, 22100 Como, Italy
¹⁸ Unitat de Física de les Radiacions, Departament de Física, and CERES-IEEC, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain
¹⁹ Institut de Ciències de l’Espai (IEEC-CSIC), 08193 Bellaterra, Spain
²⁰ Finnish MAGIC Consortium, Tuorla Observatory, University of Turku and Department of Physics, University of Oulu, 900147 Oulu, Finland
²¹ Japanese MAGIC Consortium, Division of Physics and Astronomy, Kyoto University, 606-8501 Kyoto, Japan
²² Inst. for Nucl. Research and Nucl. Energy, 1784 Sofia, Bulgaria
²³ Universitat de Barcelona (ICC/IEEC), 08028 Barcelona, Spain
²⁴ Università di Pisa, and INFN Pisa, 56126 Pisa, Italy
²⁵ Now at École polytechnique fédérale de Lausanne (EPFL), Lausanne, Switzerland
²⁶ Now at Department of Physics & Astronomy, UC Riverside, CA 92521, USA
²⁷ Now atFinnish Centre for Astronomy with ESO (FINCA), Turku, Finland
²⁸ Also at INAF-Trieste  
²⁹ Also at Instituto de Fisica Teorica, UAM/CSIC, 28049 Madrid, Spain
³⁰ Now at Stockholms universitet, Oskar Klein Centre for Cosmoparticle Physics, Sweden
³¹ Now at GRAPPA Institute, University of Amsterdam, 1098 XH Amsterdam, The Netherlands
³² Department of Physics, Washington University, St. Louis, MO 63130, USA
³³ Fred Lawrence Whipple Observatory, Harvard-Smithsonian Center for Astrophysics, Amado, AZ 85645, USA
³⁴ Department of Physics and Astronomy and the Bartol Research Institute, University of Delaware, Newark, DE 19716, USA
³⁵ School of Physics, University College Dublin, Belfield, Dublin 4, Ireland
³⁶ Santa Cruz Institute for Particle Physics and Department of Physics, University of California, Santa Cruz, CA 95064, USA
³⁷ Institute of Physics and Astronomy, University of Potsdam, 14476 Potsdam-Golm, Germany
³⁸ Astronomy Department, Adler Planetarium and Astronomy Museum, Chicago, IL 60605, USA
³⁹ Department of Physics, Purdue University, West Lafayette, IN 47907, USA
⁴⁰ Department of Physics, Grinnell College, Grinnell, IA 50112-1690, USA
⁴¹ School of Physics and Astronomy, University of Minnesota, Minneapolis, MN 55455, USA
⁴² Department of Astronomy and Astrophysics, 525 Davey Lab, Pennsylvania State University, University Park, PA 16802, USA
⁴³ School of Physics, National University of Ireland Galway, University Road, Galway, Ireland
⁴⁴ Physics Department, McGill University, Montreal, QC H3A 2T8, Canada
⁴⁵ Department of Physics and Astronomy, University of Iowa, Van Allen Hall, Iowa City, IA 52242, USA
⁴⁶ Department of Physics and Astronomy, DePauw University, Greencastle, IN 46135-0037, USA
⁴⁷ Department of Physics and Astronomy, University of Utah, Salt Lake City, UT 84112, USA
⁴⁸ Department of Physics and Astronomy, Iowa State University, Ames, IA 50011, USA
⁴⁹ Department of Physics and Astronomy, University of California, Los Angeles, CA 90095, USA
⁵⁰ Saha Institute of Nuclear Physics, 700064 Kolkata, India
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 Life and Physical Sciences, Galway-Mayo Institute of Technology, Dublin Road, Galway, Ireland
⁵³ Department of Physics and Astronomy, Barnard College, Columbia University, NY 10027, USA
⁵⁴ Physics Department, Columbia University, New York, NY 10027, USA
⁵⁵ Instituto de Astronomia y Fisica del Espacio, Casilla de Correo 67 – Sucursal 28, (C1428ZAA) Ciudad Autnoma 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 Enrico Fermi Institute, University of Chicago, Chicago, IL 60637, USA
⁵⁹ Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, IL 60439, USA
⁶⁰ INAF, Osservatorio Astronomico di Torino, 10025 Pino Torinese (TO), Italy
⁶¹ Space Sciences Laboratory, 7 Gauss Way, University of California, Berkeley, CA 94720-7450, USA
⁶² ASI-Science Data Center, via del Politecnico, 00133 Rome, Italy
⁶³ Department of Astronomy, University of Michigan, Ann Arbor, MI 48109-1042, USA
⁶⁴ Astron. Inst., St.-Petersburg State Univ., 198504 St. Petersburg, Russia
⁶⁵ Pulkovo Observatory, 196140 St. Petersburg, Russia
⁶⁶ Isaac Newton Institute of Chile, St.-Petersburg Branch, Russia
⁶⁷ Graduate Institute of Astronomy, National Central University, 300 Jhongda Rd., 32001 Jhongli, Taiwan
⁶⁸ Moscow M.V. Lomonosov State University, Sternberg Astronomical Institute, Russia
⁶⁹ Abastumani Observatory, Mt. Kanobili, 0301 Abastumani, Georgia
⁷⁰ Landessternwarte, Zentrum für Astronomie der Universität Heidelberg, Königstuhl 12, 69117 Heidelberg, Germany
⁷¹ School of Cosmic Physics, Dublin Institute For Advanced Studies, Ireland
⁷² INAF–Osservatorio Astrofisico di Catania, Italy
⁷³ Aalto University Metsähovi Radio Observatory Metsähovintie 114, 02540 Kylmälä, Finland
⁷⁴ Finnish Centre for Astronomy with ESO (FINCA) University of Turku Väisäläntie 20, 21500 Piikkiö, Finland
⁷⁵ INAF Istituto di Radioastronomia, 40129 Bologna, Italy
⁷⁶ University of Trento, Department of Physics, 38050 Povo, Trento, Italy
⁷⁷ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁷⁸ Astro Space Center of the Lebedev Physical Institute, 117997 Moscow, Russia
⁷⁹ Engelhardt Astronomical Observatory, Kazan Federal University Tatarstan Russia
⁸⁰ Aalto University Department of Radio Science and Engineering, PO Box 13000, 00076 Aalto, Finland

Received: 24 October 2013
Accepted: 15 July 2014

Abstract

Context. Blazars are variable sources on various timescales over a broad energy range spanning from radio to very high energy (>100 GeV, hereafter VHE). Mrk 501 is one of the brightest blazars at TeV energies and has been extensively studied since its first VHE detection in 1996. However, most of the γ-ray studies performed on Mrk 501 during the past years relate to flaring activity, when the source detection and characterization with the available γ-ray instrumentation was easier toperform.

Aims. Our goal is to characterize the source γ-ray emission in detail, together with the radio-to-X-ray emission, during the non-flaring (low) activity, which is less often studied than the occasional flaring (high) activity.

Methods. We organized a multiwavelength (MW) campaign on Mrk 501 between March and May 2008. This multi-instrument effort included the most sensitive VHE γ-ray instruments in the northern hemisphere, namely the imaging atmospheric Cherenkov telescopes MAGIC and VERITAS, as well as Swift, RXTE, the F-GAMMA, GASP-WEBT, and other collaborations and instruments. This provided extensive energy and temporal coverage of Mrk 501 throughout the entire campaign.

Results. Mrk 501 was found to be in a low state of activity during the campaign, with a VHE flux in the range of 10%–20% of the Crab nebula flux. Nevertheless, significant flux variations were detected with various instruments, with a trend of increasing variability with energy and a tentative correlation between the X-ray and VHE fluxes. The broadband spectral energy distribution during the two different emission states of the campaign can be adequately described within the homogeneous one-zone synchrotron self-Compton model, with the (slightly) higher state described by an increase in the electron number density.

Conclusions. The one-zone SSC model can adequately describe the broadband spectral energy distribution of the source during the two months covered by the MW campaign. This agrees with previous studies of the broadband emission of this source during flaring and non-flaring states. We report for the first time a tentative X-ray-to-VHE correlation during such a low VHE activity. Although marginally significant, this positive correlation between X-ray and VHE, which has been reported many times during flaring activity, suggests that the mechanisms that dominate the X-ray/VHE emission during non-flaring-activity are not substantially different from those that are responsible for the emission during flaring activity.