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Context: In April 2013, the nearby (z=0.031) TeV blazar, Mkn 421, showed oneof the largest flares in X-rays since the past decade. Aim: To study allmultiwavelength data available during MJD 56392 to 56403, with special emphasison X-ray data, and understand the underlying particle energy distribution.Methods: We study the correlations between the UV and gamma bands with theX-ray band using the z-transformed discrete correlation function. We model theunderlying particle spectrum with a single population of electrons emittingsynchrotron radiation, and do a statistical fitting of the simultaneous,time-resolved data from the Swift-XRT and the NuSTAR. Results: There was rapidflux 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. Thevariability in UV and gamma rays are relatively modest with $ \sim 8 \% $ and$\sim 16 \% $ respectively, and no significant correlation was found with theX-ray light curve. The observed X-ray spectrum shows clear curvature which canbe fit by a log parabolic spectral form. This is best explained to originatefrom a log parabolic electron spectrum. However, a broken power law or a powerlaw with an exponentially falling electron distribution cannot be ruled outeither. Moreover, the excellent broadband spectrum from $0.3-79$ keV allows usto make predictions of the UV flux. We find that this prediction is compatiblewith the observed flux during the low state in X-rays. However, during theX-ray flares, the predicted flux is a factor of $2-50$ smaller than theobserved one. This suggests that the X-ray flares are plausibly caused by aseparate population which does not contribute significantly to the radiation atlower energies. Alternatively, the underlying particle spectrum can be muchmore complex than the ones explored in this work.

Study of underlying particle spectrum during huge X-ray flare of Mkn 421 in April 2013