The well studied blazar Markarian 421 ( Mrk 421 , z =0.031 ) was the subject of an intensive multi-wavelength campaign when it flared in 2013 April .
The recorded X-ray and very high energy ( VHE , E > 100 GeV ) \gamma -ray fluxes are the highest ever measured from this object .
At the peak of the activity , it was monitored by the hard X-ray focusing telescope Nuclear Spectroscopic Telescope Array ( NuSTAR ) and Swift X-Ray Telescope ( XRT ) .
In this work , we present a detailed variability analysis of NuSTAR and Swift -XRT observations of Mrk 421 during this flaring episode .
We obtained the shortest flux doubling time of 14.01 \pm 5.03 minutes , which is the shortest hard X-ray ( 3 - 79 keV ) variability ever recorded from Mrk 421 and is on the order of the light crossing time of the black hole ’ s event horizon .
A pattern of extremely fast variability events superposed on slowly varying flares is found in most of the NuSTAR observations .
We suggest that these peculiar variability patterns may be explained by magnetic energy dissipation and reconnection in a fast moving compact emission region within the jet .
Based on the fast variability , we derive a lower limit on the magnetic field strength of B \geq 0.73 \delta _ { 1 } ^ { -2 / 3 } \nu _ { 19 } ^ { 1 / 3 } G , where \delta _ { 1 } is the Doppler factor in units of 10 , and \nu _ { 19 } is the characteristic X-ray synchrotron frequency in units of 10 ^ { 19 } Hz .