NuSTAR Observations of Blazar Mkn 421

We present NuSTAR observations of the famous blazar Mkn 421. The object was observed during the NuSTAR's calibration phase in July 2012, contemporaneous with increased activity seen with Fermi. Since January 2013, it has been a subject of a five-month multi-wavelength campaign consisting of three pointings per month, strictly simultaneous with observations with the Veritas and MAGIC TeV telescopes, aimed at characterization of any correlated variability. Mkn 421 was clearly detected up to highest NuSTAR energies with a spectrum that is a gradually steepening power law, with no evidence for hardening at high energies. Significant variability spanning almost an order of magnitude in flux was seen between the observations, and flux changes by a factor of 3 have been observed within a single 12-hour period. The flux variability was correlated with spectral variations, such that the spectrum was softer when the object was fainter. This behaviour is consistent with previous soft X-ray observations in same flux range. The unprecedented data quality for the faint non-flaring state and the extension of high sensitivity above 10 keV allow us to study the distribution of radiating particles in the context of standard leptonic models.