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We report on the detection of excess hard X-ray emission from the TeV BL Lacobject Mrk421 during the historical low-flux state of the source in January2013. NuSTAR observations were conducted four times between MJD56294 andMJD56312 with a total exposure of 80.9 ksec. The source flux in the 3-40 keVrange was nearly constant except for MJD56307, when the average flux levelincreased by a factor of three. Throughout the exposure, the X-ray spectra ofMrk421 were well represented by a steep power-law model with a photon index of3.1, although a significant excess was noted above 20 keV in the MJD56302 datawhen the source was in its faintest state. Moreover, Mrk421 was detected atmore than the 4-sigma level in the 40-79 keV count maps for both MJD56307 andMJD56302 but not during the remaining two observations. The detected excesshard X-ray emissions connect smoothly with the extrapolation of the high-energygamma-ray continuum of the blazar constrained by Fermi-LAT during the sourcequiescence. These findings indicate that, while the overall X-ray spectrum ofMrk421 is dominated by the highest-energy tail of the synchrotron continuum,the variable excess hard X-ray emission above 20 keV (on the timescale of aweek) is related to the inverse Compton emission component. We discuss theresulting constraints on the variability and spectral properties of thelow-energy segment of the electron energy distribution in the source.

Inverse Compton X-ray Emissions from TeV blazar Mrk421 during a Historical Low-Flux State Observed with NuSTAR