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tokens: Figure 2 reports the flux measurements in these bands, each split into three distinct bands, 3–7 keV, 7–30 keV, and 30–80 keV for 70 70 The upper edge of the NuSTAR energy range is actually 79 keV, but owing to the negligible impact on the flux values, in this paper, we will use 80 keV for simplicity. NuSTAR and 0.2–0.4 TeV, 0.4–0.8 TeV, and >0.8 TeV for MAGIC and VERITAS. The temporal coverage for the >0.8 TeV band is a about 2 hr longer than for the 0.2–0.4 TeV band because of the increasing analysis energy threshold with the increasing zenith angle of the observations. The exquisite characterization of the multiband flux variations in the X-ray and VHE gamma-ray bands reported in Figure 2 will be used in the sections that follow for the broadband variability and correlation studies. Figure 2. Light curves in various VHE and X-ray energy bands obtained with data from MAGIC, VERITAS, and NuSTAR (split in 15 minute time bins) and Swift-XRT (from several observations with an average duration of about 17 minutes). For the sake of clarity, the 0.3–3 keV fluxes have been scaled by a factor 0.5. The statistical uncertainties are, in most cases, smaller than the size of the marker used to depict the VHE and X-ray fluxes. (The data used to create this figure are available .) 4. Multiband and Multi-timescale Variability 4.1. Fractional Variability The flux variability reported in the multiband light curves can be quantified using the fractional variability parameter F var , as prescribed in Vaughan et al. ( 2003 ): denotes the average photon flux, S denotes the standard deviation of N flux measurements, and denotes the mean squared error, all determined for a given instrument and energy band. The uncertainty on F var is calculated using the prescription from Poutanen et al. ( 2008 ), as described in Aleksić et al. ( 2015a ). This formalism allows one to quantify the variability amplitude, with uncertainties dominated by the flux measurement errors and the number of measurements performed. The systematic uncertainties on the absolute flux measurements 71 71 The systematic uncertainties in the flux measurements at the radio, optical, X-ray, and GeV bands are of the order of 10%–15%, while those of the VHE bands are ∼20%–25%. do not directly add to the uncertainty in F var . The caveats in the usage of F var to quantify the variability in the flux measurements performed with different instruments are described in Aleksić et al. ( 2014 , 2015a , 2015b ). The most important caveat is that the ability to quantify the variability depends on the temporal coverage (observing sampling) and the sensitivity of the instruments used, which are somewhat different across the electromagnetic spectrum. A big advantage of the study presented here is that the temporal coverage of the three bands in X-rays (from NuSTAR) and the three bands in VHE gamma-rays (from MAGIC and VERITAS) is exactly the same, which allows us to make a more direct comparison of the variability