Multiwavelength Study of Quiescent States of Mrk 421 with Unprecedented Hard X-Ray Coverage Provided by NuSTAR in 2013

Research output: Contribution to journalArticle


  • NuSTAR Team
  • VERITAS Collaboration
  • MAGIC Collaboration

Research units

  • Stanford University
  • CNRS
  • Technical University of Denmark
  • Lawrence Livermore National Laboratory
  • INAF—Osservatorio Astronomico di Brera
  • University of Crete
  • Université de Toulouse, UPS—OMP, IRAP
  • California Institute of Technology
  • Clemson University
  • Durham University
  • Universite de Toulouse
  • University of California at Berkeley
  • Hiroshima University
  • University of California
  • Yale University
  • Space Sciences Laboratory
  • University of Oulu
  • University of Turku
  • Max-Planck-Institut für Physik
  • ASI Science Data Center
  • Columbia University
  • INAF, Osservatorio Astronomico di Roma
  • NASA Goddard Space Flight Center
  • McGill University
  • Washington University
  • Harvard-Smithsonian Center for Astrophysics
  • University College Dublin
  • Saha Institute of Nuclear Physics
  • Graduate Institute of Astronomy, National Central University


We present coordinated multiwavelength observations of the bright, nearby BL Lacertae object Mrk 421 taken in 2013 January–March, involving GASP-WEBT, Swift, NuSTAR, Fermi-LAT, MAGIC, VERITAS, and other collaborations and instruments, providing data from radio to very high energy (VHE) γ-ray bands. NuSTAR yielded previously unattainable sensitivity in the 3–79 keV range, revealing that the spectrum softens when the source is dimmer until the X-ray spectral shape saturates into a steep {{Γ }}≈ 3 power law, with no evidence for an exponential cutoff or additional hard components up to ∼80 keV. For the first time, we observed both the synchrotron and the inverse-Compton peaks of the spectral energy distribution (SED) simultaneously shifted to frequencies below the typical quiescent state by an order of magnitude. The fractional variability as a function of photon energy shows a double-bump structure that relates to the two bumps of the broadband SED. In each bump, the variability increases with energy, which, in the framework of the synchrotron self-Compton model, implies that the electrons with higher energies are more variable. The measured multi band variability, the significant X-ray-to-VHE correlation down to some of the lowest fluxes ever observed in both bands, the lack of correlation between optical/UV and X-ray flux, the low degree of polarization and its significant (random) variations, the short estimated electron cooling time, and the significantly longer variability timescale observed in the NuSTAR light curves point toward in situ electron acceleration and suggest that there are multiple compact regions contributing to the broadband emission of Mrk 421 during low-activity states.


Original languageEnglish
Article number156
Number of pages30
JournalAstrophysical Journal
Issue number2
Publication statusPublished - Mar 2016
MoE publication typeA1 Journal article-refereed

    Research areas

  • BL Lacertae objects: individual: Markarian 421, galaxies: active, gamma rays: general, radiation mechanisms: nonthermal, X-rays: galaxies

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