tokens: and the observation ID are given in each plot. The soft and hard LCs (left panel), HR plot (middle panel), and the DCF plot between the soft and hard bands (right panel) of 1ES 0347−121 are shown in Figure 3 . No significant spectral change is seen from the HR plot. We checked for variations quantitatively using a standard χ 2 test,where x i is the HR value, σ i is its corresponding error, and is the mean HR value. We considered a variation in the HR to be significant only if χ 2 > χ 2 99, ν , where ν is the number of degrees of freedom (DoF) and the significance level is 0.99. These results are given for all these blazars in Table 4 , where we see that for each source , so no significant spectral variations were detected. Figure 3. Soft (3–10 keV, denoted by red filled circles) and hard (10–79 keV, denoted by black filled circles) LCs (left panels), hardness ratios (middle panels), and the discrete correlation functions between soft and hard LCs (right panels) of the blazars 1ES 0347−121, 1ES 0414+009, RGB J0710+591,1ES 1101−232, 1ES 1218+304, and H 2356−309. The source names and observation IDs are given in the left panels. Table 4 Test of Temporal Variation of HR The DCF plot is flat, which, in the presence of significant variations, would indicate no correlation between the two energy bands, but since no variations are detectable this type of DCF is expected. 4.2. 1ES 0414+009 The TeV blazar 1ES 0414+009 ( α 2000 = 04 h 16 m 530; δ 2000 = +01°05′20′′) is an HBL at z = 0.287 (Halpern et al. 1991 ). It was first detected in X-rays with the High Energy Astronomy Observatory ( HEAO 1 A-1) (Ulmer et al. 1980 ) and was classified as a BL Lac object by Ulmer et al. ( 1983 ). It was observed above 200 GeV by VERITAS with source flux equal to (5.2 ± 1.1 stat ± 2.6 sys ) × 10 −12 photons cm −2 s −1 (Aliu et al. 2012 ). 1ES 0414+009 was observed with NuSTAR on 2015 November 25 with a good exposure time of 34.16 ks. The 3–79 keV LC of 1ES 0414+009 is shown in Figure 1 . The data are both noisy and sparse, resulting in no significant IDV detection, which is consistent with the value of fractional variance given in Table 2 . Consequently, the ACF plot, given in Figure 2 , does not show any hint of variability timescale. The soft and hard LCs of 1ES 0414+009 are plotted in the left panel of Figure 3 . The HR plot in the middle panel of that figure reveals no significant spectral variations (also see Table 4 ) and the DCF plot shown in the right panel of Figure 3 again is flat. 4.3. RGB J0710+591 The high-frequency-peaked BL Lacertae object RGB J0710+591 ( α 2000 = 07 h 10 m 264; δ 2000 = +59°09′00′′) is located at a redshift of z = 0.125 (Giommi et al. 1991 ) and also was first detected by HEAO A-1 (Wood et al. 1984 ). It was detected with VERITAS with the integral flux (above 300 GeV) recorded to be (3.9 ± 0.8) × 10 −12 cm −2 s −1 (Acciari et al. 2010 ). NuSTAR observed RGB J0710+591 on 2015 September 1 for 26.48 ks. As seen from the LC in Figure 1 the
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