We present coordinated multiwavelength observations of the bright, nearby BL
Lac 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. 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 power law with a photon index of approximately 3,
with no evidence for an exponential cutoff or additional hard components up to
about 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 which 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.
