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tokens: of XMM - Newton and NuSTAR simultaneous data. As a first attempt, we simply adopted this absorbed PLCUT model as a pure phenomenological and statistically acceptable description of the continuum emission. The results from fitting the eight time-resolved 3–78 keV spectra in this way are reported in Table 2 . Table 2 Time-resolved Spectroscopy (3–78 keV), Fitting Only NuSTAR Data of the Flare Peaks Note. a Fluxes (corrected for the absorption) and luminosities are in the energy range 1–100 keV. In some of these spectra, negative residuals appeared at energies between 15 and 40 keV. To account for these absorption features, we modified the PLCUT continuum multiplying it with a cyclotron absorption model ( cyclabs in xspec package). At first, we calculated the line significance by increasing the Δ χ 2 value until the confidence region boundary of the depth (Depth cycl in Table 3 ) of the cyclotron line crosses 0. The significance of these lines was greater than 2 σ in five (of eight) spectra (flare numbers 1, 2, 3, 4, and 6), so we have reported absorption line parameters only for these spectra in Table 3 for clarity. The absorption lines were measured with a significance ranging from σ = 2.3 to σ = 4.4 (with cyclabs model), with the best value obtained during the first flare observed by NuSTAR . The centroid energy was different: in spectra 1, 4, and 6 the feature was in a narrow range around 17–18 keV, while during spectrum 3 it was at ∼33 keV. In spectrum 3, the line at ∼17 keV was undetected: we could place an upper limit (Depth cycl ≲ 0.18) to the depth of the cyclabs line with the centroid energy fixed at 17 keV (and line width of 2 keV). Table 3 Same as Table 2 ( NuSTAR Only), Considering Only Flare Peaks 1, 2, 3, 4, and 6, Where the Inclusion of an Absorption Feature Resulted in a Line Significance Larger than 2 σ Notes. a cyclabs model in xspec . b This line significance was calculated by increasing the Δ χ 2 value until the confidence region boundary of the depth (Depth cycl ) of the cyclotron line crosses 0. In order to test the significance of the absorption line, since the F-test could bring unreliable results (Protassov et al. 2002 ), we used Monte Carlo simulations to confirm the presence of the cyclotron line in spectrum 1 (our best case, Figure 6 ), following the approach described by several authors (Barrière et al. 2015 ; Bhalerao et al. 2015 ; Lotti et al. 2016 ). We used the simftest routine in xspec with 100,000 simulations, 6 6 simftest is a script that tests for the presence of an additional component simulating N fake spectra based on the model without such component. It then fits the spectra with both the models with and without the additional component registering the Δ χ 2 and comparing it with the Δ χ 2 obtained with the real data to obtain the probability of a false detection. fixing the N H to the values reported in Table 3 to speed up the process, and found that the probability of such a line to be required by