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We present coordinated multiwavelength observations of the bright, nearby BLLac 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) gamma-ray bands. NuSTARyielded previously unattainable sensitivity in the 3-79 keV range, revealingthat the spectrum softens when the source is dimmer until the X-ray spectralshape saturates into a steep power law with a photon index of approximately 3,with no evidence for an exponential cutoff or additional hard components up toabout 80 keV. For the first time, we observed both the synchrotron and theinverse-Compton peaks of the spectral energy distribution (SED) simultaneouslyshifted to frequencies below the typical quiescent state by an order ofmagnitude. The fractional variability as a function of photon energy shows adouble-bump structure which relates to the two bumps of the broadband SED. Ineach bump, the variability increases with energy which, in the framework of thesynchrotron self-Compton model, implies that the electrons with higher energiesare more variable. The measured multi-band variability, the significantX-ray-to-VHE correlation down to some of the lowest fluxes ever observed inboth bands, the lack of correlation between optical/UV and X-ray flux, the lowdegree of polarization and its significant (random) variations, the shortestimated electron cooling time, and the significantly longer variabilitytimescale observed in the NuSTAR light curves point toward in-situ electronacceleration, and suggest that there are multiple compact regions contributingto the broadband emission of Mrk 421 during low-activity states.

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