ner_tags: O O O O O O O O O O O O O O O O O O O O O O O O O B-Telescope O O O B-URL O O O O O O O O O O O O O O O O O B-Wavelength I-Wavelength O B-URL O O O O O O O O O O O O O B-Telescope O O O B-URL O O O O O O O O O O O O O O O O O O O O O O O O O B-URL O O O O O O O O O O O B-Wavelength I-Wavelength O O O O O O O O O O O B-Wavelength I-Wavelength O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Formula O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Telescope O O O O O O O O O O O O O O O O O O O O O O O O
tokens: by photon pile-up (corresponding to a nominal count rate threshold of about 150 counts s −1 ), which is known to induce distortion of the XRT spectral response (see http://www.swift.ac.uk/analysis/xrt/pileup.php ). High photoelectric absorption can also cause distortion of the spectral shape at low energies (i.e. below 1 keV, see http://www.swift.ac.uk/analysis/xrt/digest_cal.php ). For these reasons, we treated our data as follows. First, following the XRT reduction threads ( http://www.swift.ac.uk/analysis/xrt/#abs ), we extracted only grade 0 events, which helps minimize the effects of pile-up when the source is bright (>150 counts s −1 , see http://www.swift.ac.uk/xrt_curves/cppdocs.php ) and reduces the spectral distortion encountered in WT mode below 1.0 keV when the spectra are highly absorbed. Furthermore, we ignored data below 0.6 keV as at such low energies, the spectra can be dominated by strong redistribution effects associated with the WT readout process, as well as trailing charge released from deep charge traps in the CCD on time-scales comparable to the WT readout time, which results in additional low-energy events. We extracted events in circular regions centred at the source position, with variable inner radius to mitigate pile-up, and outer radius fixed at 10, 20 or 30 pixels to adapt the photons extraction to the source properties. In order to reduce the effects of pile-up to the minimum, we varied the inner radius of the extraction region until the spectral shape was no longer varying as a function of the inner radius and the count rate was lower than ∼150 counts s −1 . The outer radius had been determined based on the brightness of the source and depending on the presence of a dust scattering halo around it, which could contaminate the source emission (see Appendix A2 ). In general, we used a 20 pixels outer radius extraction region. However, when the source was very bright, we extracted the source photons in a annular region with outer radius 30 pixels (see http://www.swift.ac.uk/analysis/xrt/spectra.php ). Instead, in the cases when the presence of a dust scattering halo was likely contaminating the source (see Appendix A2 for details), we shrank the outer radius of the extraction region to 10 pixels, in order to minimize the halo contamination. In Table 1 we reported, for each observation/snapshot, the outer radius of the extraction region. We divided each event file thus obtained into time segments of variable duration. Then, we determined the duration of each segment by requiring that each segment contained about 1600 counts after mitigating the pile-up. We set the minimum exposure to 16 s, while the maximum exposure we obtained was of 224 s. These requirements ensure that all the spectra have roughly constant overall S/N and that they will be well described by the XRT response calibration files. This choice has been made in order to obtain a good compromise between S/N and sensitivity to the spectral variations –