Study of the variable broadband emission of Markarian 501 during the most extreme Swift X-ray activity

Research output: Contribution to journalArticleScientificpeer-review


Research units

  • Instituto de Astrofísica de Canarias
  • University of La Laguna
  • Università di Udine, and INFN Trieste
  • INAF National Institute for Astrophysics
  • Croatian MAGIC Consortium
  • Saha Institute of Nuclear Physics
  • Centro Brasileiro de Pesquisas Físicas
  • Universidad Complutense
  • University of Łódź
  • Deutsches Elektronen-Synchrotron
  • Istituto Nazionale di Fisica Nucleare
  • Max-Planck-Institut für Physik
  • Barcelona Institute of Science and Technology (BIST)
  • Università di Siena, and INFN Pisa
  • Università di Padova and INFN
  • Università di Pisa
  • INFN Pisa
  • Finnish MAGIC Consortium
  • Technische Universität Dortmund
  • Universitat Autonoma de Barcelona
  • Universitat de Barcelona
  • International Center for Relativistic Astrophysics Network (ICRANet)
  • Japanese MAGIC Consortium
  • Bulgarian Academy of Sciences
  • Humboldt University of Berlin
  • University of Trieste
  • INAF, Osservatorio Astronomico di Trieste
  • University of Bologna
  • ETH Zurich
  • University of Geneva
  • University of Würzburg
  • RWTH Aachen University
  • Center for Research and Exploration in Space Science and Technology
  • NASA Goddard Space Flight Center
  • University of Maryland
  • ASI Science Data Center
  • INAF, Osservatorio Astronomico di Roma


Mrk501 is a very high-energy (VHE) gamma-ray blazar located at z=0.034. During a period of two weeks in July 2014, the highest X-ray activity of Mrk501 was observed in ~14 years of operation of the Neil Gehrels Swift Gamma-ray Burst Observatory. We characterize the broadband variability of Mrk501 from radio to VHE gamma rays, and evaluate whether it can be interpreted within theoretical scenarios widely used to explain the broadband emission from blazars. The temporal evolution of the most prominent and variable segments of the SED is described with a one-zone synchrotron self-Compton model with variations in the break energy of the electron energy distribution (EED), and with some adjustments in the magnetic field strength and spectral shape of the EED. A narrow feature at ~3 TeV was observed in the VHE spectrum measured on 2014 July 19 (MJD 56857.98), which is the day with the highest X-ray flux (>0.3 keV) measured during the entire Swift mission. This feature is inconsistent with the classical analytic functions to describe the measured VHE spectra (power law, log-parabola, and log-parabola with exponential cutoff) at more than 3σ. A fit with a log-parabola plus a narrow component is preferred over the fit with a single log-parabola at more than 4σ, and a dedicated Monte Carlo simulation estimated the significance of this extra component to be larger than 3σ. Under the assumption that this VHE spectral feature is real, we show that it can be reproduced with three distinct theoretical scenarios: a) a pileup in the EED due to stochastic acceleration; b) a structured jet with two-SSC emitting regions, with one region dominated by an extremely narrow EED; and c) an emission from an IC pair cascade.


Original languageUndefined/Unknown
JournalAstronomy & Astrophysics
Publication statusAccepted/In press - 1 Jan 2020
MoE publication typeA1 Journal article-refereed

ID: 40660143