tokens: power-law template used by Wik et al. ( 2014b ) to actually fit the harder high-mass binary dominated sources) gives essentially the same results. Figure 2. NuSTAR spectrum (black) within a circular region of 30′′ centered on the X-ray peak joint-fitted with the Chandra ICM (blue) and LMXB (red) spectra. The dashed lines are the individual components discussed in the text. The vertical dashed blue lines below 3 keV are the Gaussian models, to take into account the line emission. Beyond ∼5 keV, the X-ray emission is dominated by the AGN-related activities (core, knots, and jet) and is completely dominated by these activities beyond ∼15 keV. Error bars are at 1 σ . The 3–40 keV NuSTAR spectrum was joint-fitted with the Chandra ICM and LMXB spectra by including these two component models as well as a single power-law model to represent the combined emission from the core, knots, and the jet. The same Galactic absorption model was also used for all the components. We tied all the parameters for the ICM and LMXB models, except for the ICM component. The ratio of the two APEC normalizations is tied to that of the Chandra model but not the overall normalizations to allow for the cross-calibration uncertainty of NuSTAR and Chandra and the difference in response due to the much larger PSF of NuSTAR . The difference in best-fit normalizations turns out to be small, with less than a 1% difference when constraining the overall hard X-ray emission from the AGN activities (this section) and about 6% when decomposing the core emission from the knots/jet (Section 4 ). The best-fit spectra with all the model components are shown in Figure 2 . The best-fit power-law index of the overall AGN-related emission is and the associated 20–40 keV flux, which completely dominates the spectrum, is erg cm −2 s −1 . The spectrum is well characterized by a single power law with no evidence of cut-off in energy below ∼100 keV (3 σ ). 11 11 The lower limit of the cut-off energy is determined by the fitting of a cut-off power-law model ( cutoffpl). The 20–40 keV flux measured is an order of magnitude lower than that determined by de Jong et al. ( 2015 ) with the Suzaku observation taken in 2006. It is believed that the X-ray emission above 20 keV detected near the end of 2006 was due to the HST -1 flare (de Jong et al. 2015 ; Prieto et al. 2016 ). The Chandra observations in 2017 show that the soft emission (≲10 keV) from HST -1 has become fainter by a factor of ∼40 since 2006 November. By simple scaling, the contribution from HST -1 in 20–40 keV should be a factor of a few lower than that from the core. This is indeed the case where HST -1 is a factor of six fainter than the core in 2017 measured in this band (Section 4 ). Therefore, for the first time, we have detected hard X-ray emission above ∼10 keV dominated by the core, which also gives the upper limit of its quiescent emission in hard X-rays. 4. Decomposing the Hard X-Ray Emission of the Core,
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