Context : In April 2013 , the nearby ( z=0.031 ) TeV blazar , Mkn 421 , showed one of the largest flares in X-rays since the past decade .

Aims : To study all multiwavelength data available during MJD 56392 to 56403 , with special emphasis on X-ray data , and understand the underlying particle energy distribution .

Methods : We study the correlations between the UV and gamma bands with the X-ray band using the z-transformed discrete correlation function .
We model the underlying particle energy spectrum with a single population of electrons emitting synchrotron radiation , and do a statistical fitting of the simultaneous , time-resolved data from the Swift -XRT and the Nu STAR .

Results : There was rapid flux variability in the X-ray band , with a minimum doubling timescale of 1.69 \pm 0.13 hrs .
There were no corresponding flares in UV and gamma bands .
The variability in UV and gamma rays are relatively modest with \sim 8 \% and \sim 16 \% respectively , and no significant correlation was found with the X-ray light curve .
The observed X-ray spectrum shows clear curvature which can be fit by a log parabolic spectral form .
This is best explained to originate from a log parabolic electron spectrum .
However , a broken power law or a power law with an exponentially falling electron distribution can not be ruled out either .
Moreover , the excellent broadband spectrum from 0.3 - 79 keV allows us to make predictions of the UV flux .
We find that this prediction is compatible with the observed flux during the low state in X-rays .
However , during the X-ray flares , depending on the adopted model , the predicted flux is a factor of 2 - 50 smaller than the observed one .
This suggests that the X-ray flares are plausibly caused by a separate population which does not contribute significantly to the radiation at lower energies .
Alternatively , the underlying particle spectrum can be much more complex than the ones explored in this work .

Conclusions :