Results from the 2013 Multi-wavelength Campaign on Mkn 421

Mkn 421 is a nearby BL Lacertae object of the high-synchrotron peaked type (HSP). Its very broad spectral energy distribution (SED) peaks in the X-ray and gamma-ray bands and can be explained as a non-thermal continuum arising from the relativistic jet seen at a small angle to the line of sight. Emission near the two peaks of the SED is dominated by cooling of high-energy electrons in the jet. To study the electron distribution and its evolution, we carried out a large coordinated multi-wavelength campaign between December 2012 and May 2013. The observations were made using space observatories (NuSTAR, Swift and Fermi), ground-based Cherenkov-array telescopes (MAGIC and VERITAS), as well as several optical and radio observatories. The range of X-ray flux observed spans nearly two orders of magnitude and includes the faintest states ever observed in Mkn 421 with sufficient spectral and temporal resolution to resolve short-timescale spectral variability. We find that at low flux the hard X-ray spectrum assumes a very soft/steep power-law shape and gradually becomes harder as the flux increases. The flux variations in the X-ray and the very high energy (VHE) gamma-ray domains are positively correlated and consistent with the expectations of the one-zone synchrotron self-Compton scenario with the up-scattering occurring in the suppressed Klein-Nishina regime. For the first time, it was observed that, for very low X-ray and VHE activity, both the low- and high-energy SED bumps shifted to substantially lower energies with respect to the typical SED of Mkn 421. The unprecedented data quality and the extension of high sensitivity above 10 keV enabled by NuSTAR have proven to be instrumental for deepening our understanding of the energy distribution of particles in relativistic jets.