Extreme HBL behavior of Markarian 501 during 2012^⋆

¹ ETH Zurich, 8093 Zurich, Switzerland
² Universitàdi Udine, and INFN Trieste, 33100 Udine, Italy
³ National Institute for Astrophysics (INAF), 00136 Rome, Italy
⁴ Università di Padova and INFN, 35131 Padova, Italy
⁵ Croatian MAGIC Consortium: University of Rijeka 51000 Rijeka; University of Split - FESB, 21000 Split; University of Zagreb - FER, 10000 Zagreb; University of Osijek, 31000 Osijek; Rudjer Boskovic Institute, 10000 Zagreb, Croatia
⁶ Saha Institute of Nuclear Physics, HBNI, 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; Universidad de La Laguna, Dpto. Astrofísica, 38206 La Laguna Tenerife, Spain
¹⁰ University of ᴌódź, Department of Astrophysics, 90236 ᴌódź, Poland
¹¹ Deutsches Elektronen-Synchrotron (DESY), 15738 Zeuthen, Germany
¹² Institut de Física d’Altes Energies (IFAE), The Barcelona Institute of Science and Technology (BIST), 08193 Bellaterra, Spain
¹³ Università di Siena, and INFN Pisa, 53100 Siena, Italy
¹⁴ Technische Universität Dortmund, 44221 Dortmund, Germany
¹⁵ Universität Würzburg, 97074 Würzburg, Germany
¹⁶ Finnish MAGIC Consortium: Tuorla Observatory and Finnish Centre of Astronomy with ESO (FINCA), University of Turku, Vaisalantie 20, FI-21500 Piikkiö; Astronomy Division, University of Oulu, 90014 Finland
¹⁷ Departament de Física, and CERES-IEEC, Universitat Autónoma de Barcelona, 08193 Bellaterra, Spain
¹⁸ Universitat de Barcelona, ICC, IEEC-UB, 08028 Barcelona, Spain
¹⁹ Japanese MAGIC Consortium: ICRR, The University of Tokyo 277-8582 Chiba; Department of Physics, Kyoto University, 606-8502 Kyoto; Tokai University, 259-1292 Kanagawa; The University of Tokushima, 770-8502 Tokushima, Japan
²⁰ Inst. for Nucl. Research and Nucl. Energy, Bulgarian Academy of Sciences, 1784 Sofia, Bulgaria
²¹ Università di Pisa, and INFN Pisa, 56126 Pisa, Italy
²² Centro Brasileiro de Pesquisas Físicas (CBPF), 22290-180 URCA Rio de Janeiro, Brasil
²³ Humboldt University of Berlin, Institut für Physik, 12489 Berlin, Germany
²⁴ Dipartimento di Fisica, Università di Trieste, 34127 Trieste, Italy
²⁵ Port d’Informació Científica (PIC), 08193 Bellaterra, Spain
²⁶ INAF-Trieste and Dept. of Physics & Astronomy, University of Bologna, Italy
²⁷ University of Geneva, ISDC Data Center for Astrophysics, Chemin dÉcogia 16, 1290 Versoix, Switzerland
²⁸ RWTH Aachen University, III. Physikalisches Institut A, Aachen, Germany
²⁹ Department of Physics, Washington University, St. Louis, MO, 63130 USA
³⁰ Fred Lawrence Whipple Observatory, Harvard-Smithsonian Center for Astrophysics, Amado, AZ, 85645 USA
³¹ Department of Physics and Astronomy, University of California, Los Angeles, CA, 90095 USA
³² Institute of Physics and Astronomy, University of Potsdam, 14476 Potsdam-Golm, Germany
³³ Physics Department, California Polytechnic State University, San Luis Obispo, CA, 94307 USA
³⁴ Department of Physics and Astronomy, Purdue University, West Lafayette, IN, 47907 USA
³⁵ Department of Physics and Center for Astrophysics, Tsinghua University, Beijing, 100084 PR China
³⁶ Department of Astronomy and Astrophysics, 525 Davey Lab, Pennsylvania State University, University Park, PA, 16802 USA
³⁷ Physics Department, McGill University, Montreal, QC, H3A 2T8 Canada
³⁸ School of Physics, National University of Ireland Galway, University Road, Galway, Ireland
³⁹ Santa Cruz Institute for Particle Physics and Department of Physics, University of California, Santa Cruz, CA, 95064 USA
⁴⁰ Department of Physics and Astronomy and the Bartol Research Institute, University of Delaware, Newark, DE, 19716 USA
⁴¹ Physics Department, Columbia University, New York, NY, 10027 USA
⁴² Department of Physics and Astronomy, University of Iowa, Van Allen Hall, Iowa City, IA, 52242 USA
⁴³ Department of Physics and Astronomy, University of Utah, Salt Lake City, UT, 84112 USA
⁴⁴ Department of Physics and Astronomy, DePauw University, Greencastle, IN, 46135-0037 USA
⁴⁵ Department of Physics and Astronomy, Iowa State University, Ames, IA, 50011 USA
⁴⁶ Department of Physics and Astronomy, Barnard College, Columbia University, NY, 10027 USA
⁴⁷ School of Physics, University College Dublin, Belfield, Dublin 4, Ireland
⁴⁸ School of Physics and Center for Relativistic Astrophysics, Georgia Institute of Technology, 837 State Street NW, Atlanta, GA, 30332-0430 USA
⁴⁹ Enrico Fermi Institute, University of Chicago, Chicago, IL, 60637 USA
⁵⁰ Instituto de Astronomía y Física del Espacio (IAFE, CONICET-UBA), CC 67 -Suc. 28 C1428ZAA, Ciudad Autónoma de Buenos Aires, Argentina
⁵¹ Department of Physical Sciences, Cork Institute of Technology, Bishopstown, Cork, Ireland
⁵² School of Physics and Astronomy, University of Minnesota, Minneapolis, MN, 55455 USA
⁵³ Department of Physics and Astronomy, University of Alabama, Tuscaloosa, AL, 35487 USA
⁵⁴ Fred Lawrence Whipple Observatory, Harvard-Smithsonian Center for Astrophysics, Amado, AZ, 85645 USA
⁵⁵ Space Science Data Center - ASI, via del Politecnico, s.n.c., 00133 Roma, Italy
⁵⁶ INAF, Osservatorio Astronomicodi Roma, via di Frascati 33, 00040 Monteporzio, Italy
⁵⁷ INAF, Osservatorio Astrofisico di Torino, 10025 Pino Torinese, Italy
⁵⁸ Astronomical Institute, St. Petersburg State University, Universitetskij Pr. 28, Petrodvorets, 198504 St. Petersburg, Russia
⁵⁹ Pulkovo Observatory, St.-Petersburg, Russia
⁶⁰ Department of Physics and Institute for Theoretical and Computational Physics (ITCP), University of Crete, 71003 Heraklion, Greece
⁶¹ Foundation for Research and Technology - Hellas, IESL, Voutes, 7110 Heraklion, Greece
⁶² Abastumani Observatory, Mt. Kanobili, 0301 Abastumani, Georgia
⁶³ Engelhardt Astronomical Observatory, Kazan Federal University, Tatarstan, Russia
⁶⁴ Xinjiang Astronomical Observatory, Chinese Academy of Sciences, Urumqi, 830011 PR China
⁶⁵ Institute of Astronomy and National Astronomical Observatory, Bulgarian Academy of Sciences, 72 Tsarigradsko shosse Blvd., 1784 Sofia, Bulgaria
⁶⁶ Department of Physics, University of Colorado Denver, Denver, Colorado, CO, 80217-3364 USA
⁶⁷ Department of Physics and Astronomy, Brigham Young University, Provo, UT, 84602 USA
⁶⁸ Graduate Institute of Astronomy, National Central University, 300 Zhongda Road, Zhongli, 32001 Taiwan
⁶⁹ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
⁷⁰ Astrophysics Research Institute, Liverpool John Moores University, IC2, 146 B rownlow Hill, Liverpool, L3 5RF UK
⁷¹ Instituto de Astronomía, Universidad Nacional Autónoma de México, Apdo. Postal 70-264, 04510 Cd. de México, Mexico
⁷² Instituto de Astronomía, Universidad Nacional Autónoma de México, Apdo.Postal 810, 22800 Ensenada, B.C., Mexico
⁷³ Steward Observatory, University of Arizona, Tucson, AZ, 85721 USA
⁷⁴ Universidad de Chile, Departamento de Astronomía, Camino El Observatorio 1515, Las Condes, Santiago, Chile
⁷⁵ Cahill Centre for Astronomy and Astrophysics, California Institute of Technology, Pasadena, CA, 91125 USA
⁷⁶ Tuorla Observatory, Department of Physics and Astronomy, University of Turku, 20014 Turku, Finland
⁷⁷ Aalto University Metsähovi Radio Observatory, Metsähovintie 114, 02540 Kylmälä, Finland
⁷⁸ Aalto University Department of Electronics and Nanoengineering, PO BOX 15500, 00076 AALTO, Finland
⁷⁹ Department of Physics, University of Maryland Baltimore County, 1000 Hilltop Circle, Baltimore, MD, 21250 USA
⁸⁰ NASA Goddard Space Flight Center, Code 663, Greenbelt, MD, 20771 USA
⁸¹ Department of Physics and Astronomy – MS 108, Rice University, 6100 Main Street, Houston, TX, 77251-1892 USA
⁸² Institute for Astrophysical Research, Boston University, 725 Commonwealth Avenue, Boston, MA, 02215 USA

Received: 23 June 2018
Accepted: 3 August 2018

Abstract

Aims. We aim to characterize the multiwavelength emission from Markarian 501 (Mrk 501), quantify the energy-dependent variability, study the potential multiband correlations, and describe the temporal evolution of the broadband emission within leptonic theoretical scenarios.

Methods. We organized a multiwavelength campaign to take place between March and July of 2012. Excellent temporal coverage was obtained with more than 25 instruments, including the MAGIC, FACT and VERITAS Cherenkov telescopes, the instruments on board the Swift and Fermi spacecraft, and the telescopes operated by the GASP-WEBT collaboration.

Results. Mrk 501 showed a very high energy (VHE) gamma-ray flux above 0.2 TeV of ∼0.5 times the Crab Nebula flux (CU) for most of the campaign. The highest activity occurred on 2012 June 9, when the VHE flux was ∼3 CU, and the peak of the high-energy spectral component was found to be at ∼2 TeV. Both the X-ray and VHE gamma-ray spectral slopes were measured to be extremely hard, with spectral indices < 2 during most of the observing campaign, regardless of the X-ray and VHE flux. This study reports the hardest Mrk 501 VHE spectra measured to date. The fractional variability was found to increase with energy, with the highest variability occurring at VHE. Using the complete data set, we found correlation between the X-ray and VHE bands; however, if the June 9 flare is excluded, the correlation disappears (significance < 3σ) despite the existence of substantial variability in the X-ray and VHE bands throughout the campaign.

Conclusions. The unprecedentedly hard X-ray and VHE spectra measured imply that their low- and high-energy components peaked above 5 keV and 0.5 TeV, respectively, during a large fraction of the observing campaign, and hence that Mrk 501 behaved like an extreme high-frequency-peaked blazar (EHBL) throughout the 2012 observing season. This suggests that being an EHBL may not be a permanent characteristic of a blazar, but rather a state which may change over time. The data set acquired shows that the broadband spectral energy distribution (SED) of Mrk 501, and its transient evolution, is very complex, requiring, within the framework of synchrotron self-Compton (SSC) models, various emission regions for a satisfactory description. Nevertheless the one-zone SSC scenario can successfully describe the segments of the SED where most energy is emitted, with a significant correlation between the electron energy density and the VHE gamma-ray activity, suggesting that most of the variability may be explained by the injection of high-energy electrons. The one-zone SSC scenario used reproduces the behavior seen between the measured X-ray and VHE gamma-ray fluxes, and predicts that the correlation becomes stronger with increasing energy of the X-rays.