Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Inverse Compton X-ray Emissions from TeV blazar Mrk421 during a
  Historical Low-Flux State Observed with NuSTAR   We report on the detection of excess hard X-ray emission from the TeV BL Lac
object Mrk421 during the historical low-flux state of the source in January
2013. NuSTAR observations were conducted four times between MJD56294 and
MJD56312 with a total exposure of 80.9 ksec. The source flux in the 3-40 keV
range was nearly constant except for MJD56307, when the average flux level
increased by a factor of three. Throughout the exposure, the X-ray spectra of
Mrk421 were well represented by a steep power-law model with a photon index of
3.1, although a significant excess was noted above 20 keV in the MJD56302 data
when the source was in its faintest state. Moreover, Mrk421 was detected at
more than the 4-sigma level in the 40-79 keV count maps for both MJD56307 and
MJD56302 but not during the remaining two observations. The detected excess
hard X-ray emissions connect smoothly with the extrapolation of the high-energy
gamma-ray continuum of the blazar constrained by Fermi-LAT during the source
quiescence. These findings indicate that, while the overall X-ray spectrum of
Mrk421 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 a
week) is related to the inverse Compton emission component. We discuss the
resulting constraints on the variability and spectral properties of the
low-energy segment of the electron energy distribution in the source.

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The categories of this paper are: math
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The categories of this paper are: math-ph
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The categories of this paper are: hep-ex
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The categories of this paper are: acc-phys
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The categories of this paper are: comp-gas
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The categories of this paper are: econ
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The categories of this paper are: alg-geom
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The categories of this paper are: q-bio
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The categories of this paper are: adap-org
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The categories of this paper are: nlin
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The categories of this paper are: hep-lat
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The categories of this paper are: atom-ph
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The categories of this paper are: mtrl-th
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The categories of this paper are: q-fin
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The categories of this paper are: hep-th
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The categories of this paper are: funct-an