Extreme HBL behavior of Markarian 501 during 2012

Research output: Contribution to journalArticleScientificpeer-review

Researchers

  • MAGIC Collaboration
  • VERITAS Collaboration

Research units

  • Swiss Federal Institute of Technology Zurich
  • Università di Udine, and INFN Trieste
  • INAF National Institute for Astrophysics
  • Università di Padova and INFN
  • Croatian MAGIC Consortium
  • Saha Institute of Nuclear Physics
  • Max-Planck-Institut für Physik
  • Universidad Complutense
  • University of La Laguna
  • Deutsches Elektronen-Synchrotron
  • University of Łódź
  • Barcelona Institute of Science and Technology (BIST)
  • Università di Siena, and INFN Pisa
  • Technische Universität Dortmund
  • University of Würzburg
  • Finnish MAGIC Consortium
  • Autonomous University of Barcelona
  • University of Barcelona
  • Japanese MAGIC Consortium
  • Bulgarian Academy of Sciences
  • Università di Pisa
  • Centro Brasileiro de Pesquisas Físicas (CBPF\MCTI)
  • Humboldt University of Berlin
  • University of Trieste
  • University of Bologna
  • University of Geneva
  • RWTH Aachen University
  • Washington University
  • Harvard-Smithsonian Center for Astrophysics
  • University of California
  • University of Potsdam
  • California Polytechnic State University
  • Purdue University
  • Tsinghua University
  • Pennsylvania State University
  • McGill University
  • National University of Ireland, Galway
  • University of Delaware
  • Columbia University
  • University of Iowa
  • University of Utah
  • DePauw University
  • Iowa State University
  • University College Dublin
  • Georgia Institute of Technology
  • University of Chicago
  • Instituto de Astronomia y Fisica del Espacio
  • Cork Institute of Technology
  • University of Minnesota
  • University of Alabama
  • ASI Science Data Center
  • INAF, Osservatorio Astronomico di Roma
  • INAF, Osservatorio Astrofisico di Torino
  • St. Petersburg State University
  • Pulkovo Observatory
  • University of Crete
  • Foundation for Research and Technology-Hellas
  • Abastumani Observatory
  • Kazan Federal University
  • Chinese Academy of Sciences
  • University of Colorado Denver
  • Brigham Young University
  • National Central University
  • European Southern Observatory
  • Liverpool John Moores University
  • Universidad Nacional Autonoma de Mexico
  • University of Arizona
  • Universidad de Chile
  • California Institute of Technology
  • University of Turku
  • University of Maryland, Baltimore County
  • NASA Goddard Space Flight Center
  • Rice University
  • Boston University

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.

Details

Original languageEnglish
Article numberA181
JournalAstronomy and Astrophysics
Volume620
Publication statusPublished - 1 Dec 2018
MoE publication typeA1 Journal article-refereed

    Research areas

  • Acceleration of particles, Astroparticle physics, BL Lacertae objects: general, BL Lacertae objects: individual: Mrk501, Radiation mechanisms: non-thermal

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