4 Spectral variability

Figure 6: Illustrative selection of 6 PCA-PCU2 spectra of SAX J1711.6-3808 in 3 to 20 keV. The number labels to each curve represent the observation day number in 2001.

Figure 6 shows 6 RXTE-PCA spectra between 3 and 20 keV for only PCU2 that illustrate the spectral changes over the course of the outburst. As already pointed out, there is considerable contamination by 712. Only spectral features that can be attributed to SAX J1711.6-3808 with certainty are useful for the present discussion.This implies that it is not prudent to discuss the broad-band continuum from these data and we refrain from doing so. There are two attributable spectral features. The first is a temporary soft excess that is strongest in the PCA observations of days 92 and 99. This is about the time that the decay of the outburst sets in. The association of this feature with SAX J1711.6-3808 could be confirmed from an independent imaging measurement with the WFC on days 93-94, see Fig. 7. This spectrum shows the same soft excess, while the other 5 WFC spectra do not. The PCA-measured excess may be modeled by a disk black body (Mitsuda et al. 1984), with a temperature at the inner edge of the accretion disk of $kT=0.80\pm0.04$ keV and $0.79\pm0.02$ keV for days 92 and 99, and inner radii of $\sqrt{{\rm cos}(i)}R_{\rm in}/D_{\rm 10~kpc}=16.5\pm1.2$ and $11.6\pm0.8$ km, respectively. A single-temperature black body is ruled out: for the observation on day 99 a disk black body yields a best-fit with $\chi^2_\nu=1.03$ while a single-temperature black body yields $\chi^2_\nu=3.5$ (101 d.o.f.). For all other PCA spectra, any black body component is much fainter and of relatively low significance. The soft excess in the one relevant WFC spectrum has a disk black-body temperature of $0.61\pm0.07$ keV.

Figure 7: WFC-measured spectrum of SAX J1711.6-3808 between 2 and 10 keV on days 46 (solid) and 93 (dashed).

The other spectral feature that can be attributed to SAX J1711.6-3808 is the broad emission feature at 7 keV. None of the four spectra available for 712 show such a feature. These four spectra were obtained with the PCA on August 26, 1999, with the NFI on August 27, 1999, and with the PCA on June 17 and July 3, 2001. The last two observations were aimed at SAX J1711.6-3808, but that source was off at that time (see Fig. 4). In Fig. 8 we show the residuals of the NFI spectrum with respect to a Comptonized spectrum. The $3\sigma$ upper limit to the flux of a broad line at 6.5 keV in this spectrum is $5.2\times 10^{-4}$ phot s^-1 cm^-2.

Figure 8: Residuals between 1 and 10 keV of BeppoSAX NFI spectrum of 712 with respect to a best-fit Comptonized spectrum with $kT_{\rm plasma}=52$ keV, an optical depth of 0.5 for a disk geometry, kT=0.04 keV for the seed photons, $N_{\rm H}=2.08\times 10^{22}$ cm-2, and $\chi ^2_\nu =1.01$ for $\nu =110$ (the data are described in detail in Cocchi et al., in prep.). The fit was accomplished on 0.5-100 keV data, this figure only shows part of the data. Compare with Fig. 5.

Figure 9: Residual of PCA-PCU2 spectrum of SAX J1711.6-3808 on day 92 with respect to an absorbed power law. The shape of the emission feature at 6.5 keV is consistent with a Gaussian with a width of 2.6 keV (FWHM).

We modeled the continuum of SAX J1711.6-3808 with an absorbed power law, with fixed $N_{\rm H}=2.8\times 10^{22}$ cm^-2 and free power law index, plus a disk black body and included a fixed model for the Galactic ridge. In each spectrum, the Gaussian emission line energy and flux were left free. The width was fixed at 2.6 keV, in accordance with the NFI spectrum. All fits are acceptable, but we note that due to the contamination the disk black body very probably has a contribution from 712, as exemplified by the measurement on day 47 which is inconsistent with that determined from the simultaneous and uncontaminated NFI measurement. Figure 9 shows the emission feature in the PCA data for the observation with the highest soft excess. It is clearly present. In Fig. 10 we plot the line energy and flux as a function of time for all sixteen PCA observations during which SAX J1711.6-3808 was active, as well as the disk black body parameters. The weighted mean of the line energy is $6.51\pm0.04$ keV. The data are consistent with it being constant: a fit with a constant yields $\chi^2_\nu=1.11$($\nu=15$). The line flux decays but not exactly in tandem with the continuum flux. The equivalent width is higher on days 85, 92 and 99, with a maximum of 0.8 keV on day 92. This increase coincides with the appearance of the soft excess which is modeled through the disk black body component.

Figure 10: Results of spectral fitting to the 16 PCA spectra. The upper panel shows the Gaussian centroid energy, the second panel the line flux (dashed curve) and the continuum 3-20 keV flux (solid curve). The third panel shows the temperature at the inner edge of the accretion disk (dashed curve) and and the radius of that edge (solid curve). The disk black body data do not continue till the end of the outburst because they are not constrained till then. Also, this component is likely to be contaminated outside days 92 and 99. The bottom panel shows the resulting reduced $\chi ^2$ value per spectrum (lowest panel). The width of the Gaussian line was fixed at 2.6 keV (FWHM), in accordance with the NFI spectrum and the first few PCA spectra, as well as $N_{\rm H}$ at $2.8\times 10^{22}$ cm-2. All errors are $\chi ^2_{\rm min}+1.0$ ranges.