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Appendix A: The OSA 10 data reduction

Our spectral analysis was performed with the OSA 10 software, which at the time of writing, is the latest version of the INTEGRAL analysis software. OSA 10 is supplied with a new energy calibration of the ISGRI detector, with respect to the previous version (OSA 9). As a result, a more reliable photon energy reconstruction is ensured. The details about the new energy reconstruction are reported in Caballero et al. (2013).

To investigate the difference in energy calibration between OSA 9 and OSA 10, we extracted the data set used in this work with both the software versions. Following Klochkov et al. (2012), after the OSA 9 data reduction, we performed an additional gain correction based on the background spectral line complex of tungsten (W, nominally at 58.8297 keV). After the OSA 10 data reduction, which includes a more accurate gain correction, a small deviation in the inferred energy from the nominal Tungsten line was still noticeable. The deviation amounted up to + 0.5 keV in the Science Windows (ScWs, i.e., INTEGRAL pointings) at the beginning of each INTEGRAL revolution. According to the ISGRI instrument team (priv. comm.), the deviation can be attributed to possible orbital drifts in the detector response, which are responsible for systematic scatter when revolutions start. However, even if orbital variations are not currently corrected, they are mitigated when averaged over a number of revolutions. Nonetheless, to eliminate this deviation, we performed an additional gain correction to ensure the stability of the Tungsten reference line. After the correction, the inferred Tungsten line energy in all ScWs is found to be consistent with its nominal value within 3σ.

To illustrate the effects of the different energy calibrations, we plotted the E_cyc–flux relation for the spectra obtained with the two software versions (Fig. A.1). We note that the energy range indicated in Klochkov et al. (2012) for calculating the flux is erroneously reported as 4−80 keV, while the correct range is 13−80 keV (where the lower limit is constrained by the response matrix energy range of the ISGRI instrument). The two different sets of results clearly show a systematic shift of the

cyclotron line centroid energy, with a deviation (proportional to the measured energy) up to ~ 2 keV. This is the result of the new energy calibration implemented in OSA 10 plus a minor contribution due to the additional gain correction performed after the OSA 10 data reduction. In our scientific analysis, we used the data reduced with OSA 10 with the additional gain correction. This ensures the most accurate measurements of the cyclotron line centroid energy.

Fig. A.1 Cyclotron line centroid energy Ecyc as a function of the logarithm of flux in the 13−80 keV range. The black dots are the results of Klochkov et al. (2012), obtained with OSA 9 and corrected for the ISGRI energy gain drift. The red triangles are the results of this work, obtained with OSA 10 and then gain-corrected. The dashed and dotted lines are the results of linear fits to the Ecyc − log 10 (Flux) data for the two sets resulting from OSA 9 and OSA 10 data reduction, respectively. The error bars indicate 1σ-uncertainties (the statistical flux uncertainties are smaller than the symbol size). The top x-axis shows the corresponding isotropic source luminosity assuming a distance of 2.4 kpc.

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