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

Abstract

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. Aim: 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 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 NuSTAR. 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 cannot 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, 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.

11 pages, 7 figures, Accepted in A&A

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• Long-term study of Mkn 421 with the HAGAR Array of Telescopes
• Violent Hard X-ray Variability of Mrk 421 Observed by NuSTAR in 2013 April
• Dissecting the long-term emission behaviour of the BL Lac object Mrk 421
• Broadband Spectral Fitting of Blazars using XSPEC
• X-ray Flux and Spectral Variability of Six TeV Blazars with \textit{NuSTAR}
• Inverse Compton X-ray Emissions from TeV blazar Mrk421 during a Historical Low-Flux State Observed with NuSTAR
• The relentless variability of Mrk 421 from the TeV to the radio
• INTEGRAL view of AGN
• A time dependent approach to model X-ray and $\gamma$--ray light curves of Mrk 421 observed during the flare in February 2010
• Understanding the X-ray spectral curvature of Mkn 421 using broadband AstroSat observations
• Unfolding the X-ray Spectral Curvature of Mkn 421 for Further Clues
• Unravelling the unusually curved X-ray spectrum of RGB J0710+591 using AstroSat observations
• Do radiative losses determine the characteristic emission of the blazar Mkn 421?
• Correlations between X-ray spectral parameters of Mkn 421 using long-term Swift-XRT data
• X-ray flares in the long-term light curve of low-luminosity Active Galactic Nucleus M81*