Free Access
Volume 522, November 2010
Article Number A96
Number of page(s) 25
Section Stellar structure and evolution
Published online 05 November 2010

Online material

Appendix A: Examples of pile-up removal

thumbnail Fig. A.1

Measure of pile-up in the EPIC pn timing mode using the SAS task epatplot for the bright source GX 349 + 2. Source events were extracted from a 64′′ (16 columns) wide box centred on the source position (left panel). Middle-left, middle-right, and right panels show the same region as in the left panel but after exclusion of the neighbouring 4, 6, and 8 columns from the centre of the box.

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Figures A.1 and A.2 show the effects of pile-up in the epatplot in the bright source GX 349 + 2 and the dim source 4U 1728 − 34, respectively. For GX 349 + 2 we observe a significant energy-dependent deviation between the data and the model  ≳ 4 keV when the full PSF region is used (Fig. A.1, upper left). As we remove the inner columns from the PSF the effects of pile-up are mitigated. After removal of 6 columns, there is still a small deviation above 9 keV. After removal of 8 columns, there is not an energy-dependent deviation anymore (Fig. A.1, right panel). For 4U 1728 − 34 we do not observe any energy-dependent deviation between the data and the model when the full PSF region is used (Fig. A.2, left panel). Similarly, when we excise the core of the PSF, the pattern ratios do not change with respect to the case where the core of the PSF is included. This indicates that pile-up is not present in this source.

thumbnail Fig. A.2

Same as Fig. A.1 but for the dim source 4U 1728 − 34.

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Appendix B: Comparison with previous results

Several sources analysed in this paper have been previously published by other authors (e.g. Bhattacharyya & Strohmayer 2007; Iaria et al. 2009; D’Aì et al. 2009). In what follows, we compare the properties of the Fe Kα line found in previous analyses with the ones inferred in this paper. For most of the sources we obtained significantly different parameters for the Fe line when fitted with a laor component, compared to previous analyses (we discuss the discrepancies in detail below).

Therefore, to obtain a quantitative measurement of the difference between the Fe line obtained by other authors and in this work we performed fits to the spectra with the best-fit model shown in Sect. 3.2 but fixing all the parameters of the laor component to the values obtained in previous analyses except the normalisation, which was left free. The of these fits is shown in Table B.1. For comparison, we show in the same table the obtained when the line is fitted both with a Gaussian and a laor component and leaving all the parameters free, as in Tables 3 − 5. We compared the results in this work with the results obtained by Cackett et al. (2010) only in Table B.1 and not in the sections that follow, since their work is being refereed at the moment of acceptance of this paper, and the parameters may still change in their final version.

We found that the of the fits to the lines with the parameters of the literature yielded higher values of for eight of the observations analysed in this work (see Table B.1). For the three observations of 4U 1636 − 536, the difference in is not statistically significant. Indeed the lines fitted in this work have a width σ near 1 keV, similar to previous analyses. However, two of the three lines are below the level of 3σ detectability significance in this work, and the third one is only slightly above such limit (see Sect. 4). Also in the case of 4U 1636 − 536 Obs 0500350401, we obtained a lower, though not significant, when using the parameters of the line from previous analyses. This is because we did not allow any inclination higher than 70° in our analysis, since the source shows neither dips nor eclipses. In contrast, the value of the inclination in the laor component in previous analyses had a higher value of  > 81° (e.g., Pandel et al. 2008; Cackett et al. 2010). For the observations of Ser X − 1, SAX J1808.4 − 3658, and GX 340 + 0, the increase in the when we use the parameters of the lines in the literature is not statistically significant, since the is already well below 1 in the fits in this work. However, since the is already below 1 for fits with the simple Gaussian model, there is no reason to consider a laor component for these fits.

Table B.1

of spectral fits for sources for which asymmetric Fe lines showing relativistic effects reported in the literature.

B.1. Ser X − 1

Bhattacharyya & Strohmayer (2007) first report a skewed Fe Kα line with a moderately extended red wing in an NS LMXB based on the three XMM-Newton observations of Ser X − 1 that we re-analysed in this paper. They fitted the line with a laor component and find EWs for the lines between 86 and 105 eV, inner radii between 4.04 and 16.19 rg, and an energy centroid of 6.4 keV. On average we found smaller EWs, between 57  ±  18 and 63  ±  12 eV, and larger energy centroids, between 6.60  ±  0.1 and 6.79 keV, when fitting the line with the same component and EWs between 50 and 52 and an average energy centroid of 6.60  ±  0.04 keV when fitting the line with a Gaussian component. We also inferred a smaller average inclination for the source from the laor fit: between  < 25 and 28  ±  9°, compared to between 39.7 and 50.2° found by Bhattacharyya & Strohmayer (2007). The inner radius was not well constrained when fitting the line with a laor component, with a value of 13.3 rg for the best-constrained observation (Obs 0084020401). In our analysis the fit with the laor component was not statistically preferred to the one with the Gaussian component.

We explain the discrepancies in the results as mainly due to two differences in the analysis. First, while Bhattacharyya & Strohmayer (2007) consider pile-up to be insignificant for these observations, we found that the epatplot shows a significant degree of pile-up before we remove the central 4 columns of the PSF. Figure B.1 shows the difference in the spectra extracted when including the core of the PSF, as done by Bhattacharyya & Strohmayer (2007), and excluding the 4 central columns to remove pile-up effects, as in this paper. The pile-up effects show up as an excess of photons from as low as 3 keV and becomes especially prominent above 5 keV in the red spectrum. This, combined with a improper continuum modelling (see below), may create an excess of photons below 6 keV, which could be misinterpreted as a red wing of the Fe line.

The second difference in the analysis is precisely that we used a different continuum model to fit the spectrum. As shown in Table 6 we got a significantly worse using a combination of diskbb and comptt components to fit the continuum, as done by Bhattacharyya & Strohmayer (2007), when compared to diskbb and bbody components used in this paper.

Therefore, for this particular source, the main contribution to the broad Fe line detected in the first analysis by Bhattacharyya & Strohmayer (2007) is likely related to residuals from the fit to the continuum, showing that modelling the spectra with different continua may significantly modify the parameters of the Fe line.

B.2. GX 340 + 0

D’Aì et al. (2009) report a broad asymmetric emission line in the XMM-Newton spectrum of GX 340 + 0. Owing to the variability of the source as revealed by the light curve (see Fig. 2), they divided the observation into five segments and analysed each segment separately. They modelled the spectral continuum with diskbb and bbody components and find a significant improvement in the fit when using the diskline component to model the Fe Kα emission compared to a Gaussian component. The line had an average energy of 6.69  ±  0.02 keV, inner radius of 13 ± 3 rg, and EW of 41  ±  3 eV. In spite of using the same continuum to model the spectrum, we obtained equally good fits when modelling the Fe line with a Gaussian or with a laor component. The line had an average energy of 6.82 keV, an inner radius  > 16 rg, and a smaller EW of 15  ±  5 eV. We inferred an average inclination for the source of  < 34°, compared to 35 ± 1° found by D’Aì et al. (2009). When fitted with a Gaussian profile, the line had an energy of 6.76  ±  0.02 keV and width of 0.24  ±  0.02 keV (D’Aì et al. 2009), consistent with our values of 6.72  ±  0.06 keV and width of 0.17 keV.

The main difference bewteen our analysis and theirs is again the treatment of pile-up. While they only considered the initial  ~ 7 ks of the observation (interval 5) to be affected by pile-up and excised the 2 central columns to remove its effects, we found that the full observation was strongly affected by pile-up so removed the central 8 columns. This is consistent with the high count rate of the source, with peaks up to  ≳ 1100 counts s-1and a minimum of 650 counts s-1 along the observation, as well as the hardness of the spectrum (see Sect. 2.1). The light curve in this paper (see Fig. 2) shows a count rate twice as high as the one shown in Fig. 1 from D’Aì et al. (2009). A potential explanation of this difference in the light curves could be that D’Aì et al. (2009) averaged intervals of real data with those for which no data were recorded, because of telemetry saturation.

thumbnail Fig. B.1

Ratio of the EPIC pn spectrum of Ser X − 1 (Obs 0084020501) to its best-fit continuum model for the spectrum free of pile-up used in this paper (black). The red points show the ratio of the piled-up spectrum of the same observation, obtained when the full PSF is used, to the best-fit continuum model of the spectrum free of pile-up. The pile-up shows up mainly as a significant hardening of the spectrum at energies  ≳ 5 keV. This adds an “artificial” red-wing to the Fe line because of the different curvature of the spectrum.

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B.3. GX 349 + 2

Iaria et al. (2009) report several emission features, consistent with the transitions of L-shell Fe xxii-Fe xxiii, S xvi, Ar xviii, Ca xix, and highly ionised Fe in the XMM-Newton spectrum of GX 349 + 2. While the first four features can be fitted equally well with Gaussian features or the relativistic diskline component, they find that the Fe Kα feature is better fitted using the diskline component at 6.76 keV or two diskline components at 6.7 and 6.97 keV. The line had an energy of 6.80 ± 0.02 keV, width (σ) of 0.28 keV, and EW of 49 eV when modelled with a Gaussian component. The same line had an energy of 6.76 ± 0.02 keV, inner radius 6.2, and EW of 61 ± 9 eV when modelled with a diskline component.

In spite of using the same continuum to model the spectrum, we obtained equally good fits when modelling the Fe line with a Gaussian or with a laor component and a significantly larger EW in any of the fits, compared to Iaria et al. (2009). In this paper, the line has an energy of 6.72 ± 0.06 keV, width (σ) of 0.32 keV and EW of 82 eV when modelled with a Gaussian component and energy of 6.94 keV, inner radius of 11, and EW of 96 eV when modelled with a laor component. We obtained a significantly different inclination for the source, 17  ±  9°, compared to 41.4° (Iaria et al. 2009). Apart from the inclination and the EW, the line parameters were not significantly different within the errors. We attribute the differences in some properties of the line to the PSF extraction regions used. While Iaria et al. (2009) exclude only the 2 central pixels of the PSF before spectral extraction, their Fig. 4 shows an excess of double events and a deficit of single events above  ~ 7.5 keV compared to the predicted model. This is a clear indication of residual pile-up in their spectra (see XMM-Newton Users Handbook). In contrast, we found that rejection of the inner 8 columns of the PSF was necessary in order to completely remove the pile-up effects.

We found a lower temperature for the diskbb and bbody components, 0.92  ±  0.03 and 1.57  ±  0.02 keV, compared to 1.05 and 1.792 keV (Iaria et al. 2009). Thus the spectrum extracted in this paper is significantly softer compared to the analysis by Iaria et al. (2009). Again, this is expected if pile-up effects have not been completely removed from the spectra in their analysis.

B.4. 4U 1705 − 44

4U 1705 − 44 was observed twice by XMM-Newton with the EPIC pn camera in timing mode. The first observation had a relatively low flux, 2.5 × 10-10erg cm-2 s-1, and was at least one order of magnitude below the typical count rate where pile-up effects become important. In 2008 XMM-Newton observed 4U 1705 − 44 at a significantly higher flux, 642 counts s-1 compared to 28 counts s-1 in 2006 (see Table 1). Di Salvo et al. (2009) report four emission features and one absorption edge in the spectral residuals after modelling the continuum with a bbody and a comptt components, which were identified with emission of S xvi, Ar xviii, Ca xix, and Fe xxv, and (redshifted) absorption from Fe xxv. The lines were broad with σ between 120 and 260 eV. They find a significant improvement in the fit when the Fe xxv feature is modelled with a diskline component compared to a Gaussian component and reported values of 6.66  ±  0.01 keV, 14  ±  2 rg, 39  ±  1 and 56  ±  2 eV for the energy, inner radius, inclination, and EW for the fit with a diskline component. The absorption edge appeared to be smeared (width of 0.7 keV) and redshifted with an energy of 8.3  ±  0.1 keV with respect to the rest frame energy of 8.83 keV. We found values of 6.45  ±  0.05 keV,  < 9 rg,  < 34°, and 135 eV for the energy, inner radius, inclination and EW for the fit with a laor component, i.e. significantly different to those reported by Di Salvo et al. (2009). When fitted with a Gaussian component, the centroid was 6.56  ±  0.05 keV and the EW 92 eV.

The difference between both analyses is again related to both the pile-up treatment and the modelling of the continuum. We found that it was necessary to excise the 7 central columns of the PSF to completely remove pile-up effects, while Di Salvo et al. (2009) did not remove any column. In addition, Di Salvo et al. (2009) used a continuum of bbody and comptt to fit the spectrum, while we used bbody and diskbb.

B.5. SAX J1808.4 − 3658

SAX J1808.4 − 3658 was observed by XMM-Newton in 2008 and its spectrum modelled with a combination of diskbb, blackbody, and power law components (Papitto et al. 2009; Patruno et al. 2009; Cackett et al. 2009). The residual emission at the Fe Kα band is modelled with a diskline profile in all the analyses, although Papitto et al. (2009) obtained only a slight improvement in the fit when using a diskline profile compared to a Gaussian profile. The parameters of the diskline component were in general consistent in the three analyses, with some exceptions such as the inclination derived for the source, probably because of small differences in the analyses such as including the RGS in the simultaneous fit or not or considering slightly different energy bands. For example, Papitto et al. (2009) (Cackett et al. 2009) used the 1.4 − 11 keV (1.2 − 11 keV) band for spectral fitting, since they found large residuals below 1.4 (1.2) keV, respectively.

As explained in Sect. 3.2 we extracted two different spectra for this observation. Spectrum 1 was extracted including the core of the PSF, as done by the authors above. Despite using the same extraction region, we found a significantly smaller line, with an EW of 51 eV when fitting the line with a laor component and 23 eV in the fits with a Gaussian component. In contrast, in the previous analyses with a laor component, the EW was found to be as large as 121 eV (Papitto et al. 2009), 118  ±  10 (Cackett et al. 2009), and 97.7  ±  31.4 eV (Patruno et al. 2009).

This may be related to the different continuum used to fit the spectrum. While all the previous analyses of the XMM-Newton observation of SAX J1808.4 − 3658 required 3 continuum components to achieve an acceptable fit, we already obtain a of 0.98 (223) with a continuum consisting of just blackbody and power-law components.

B.6. 4U 1636 − 536

Pandel et al. (2008) find evidence for relativistic lines from Fe in different ionisation states in the three XMM-Newton observations of 4U 1636 − 536 presented here. They report EWs of 215, 98, and 140 eV for the Fe Kα line present in Obs 0303250201, 0500350301, and 0500350401, respectively, which they modelled with a diskline component. In contrast, we found smaller EWs of 130  ±  14, 36, and 6  ±  3 eV when fitting the line with a laor component and of 210  ±  129, 28  ±  19, and 59 eV when using a Gaussian component instead. When fitting the line with a Gaussian component, the lines from Obs 0303250201 and 0500350301 are below the 3σ significance level so should not be considered as detections. Pandel et al. (2008) regard the high inclination,  > 81°, obtained for Obs 0303250201 and 0500350401 as unrealistic (given the values of the inclination of 36 − 74° determined for this source Casares et al. (2006)) and interpreted it as an indication that the excess at the Fe Kα band was a blend of at least two lines. In contrast, we found only a high inclination for Obs 0303250201 and an upper value of 70° (as used for all the fits in this sample) already gave an acceptable fit with a of 0.76 (216) for this observation. We obtained equally good fits when modelling the Fe line with a Gaussian or with a laor component.

We attribute the differences in some properties of the line to the PSF extraction regions used. Pandel et al. (2008) consider that none of the three XMM-Newton observations suffer from pile-up effects based on the count rate limit for the pn timing mode. In contrast we found that rejection of the inner 1(3) columns of the PSF was necessary in order to remove the pile-up effects completely for Obs 0500350301 (0500350401). The residual pile-up effects in their spectra are most likely responsible for the harder spectrum. They report a difference of their spectra with respect to the simultaneous RXTE PCA spectra both of flux and slope. The PCA spectra show a flux excess of  ~30% at 3 keV and  ~10 − 15% at 10 keV with respect to their EPIC pn spectra. Qualitatively, this is what we expect if the XMM-Newton spectra are affected by pile-up: photons will be lost so that in average the flux is lower and in addition soft photons will be counted as hard photons, so that in average the PCA will show a softer spectrum.

Appendix C: Spectra

The best fits of each individual spectrum, together with the residuals of the fits, are shown in Fig. C.1. Figure C.2 shows the unfolded spectra for each observation.

thumbnail Fig. C.1

Upper panels: 0.7 − 10 keV EPIC pn (black) spectra fit with Models 1a–1d. For each source the best fit is shown (see Tables 3 − 4 and text). Lower panels: residuals in units of standard deviation from the corresponding model.

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thumbnail Fig. C.2

0.7–10 keV EPIC pn (black) spectra fit with their best-fit model (see Tables 34) shown in flux units.

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© ESO, 2010

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