5 Flux history

From all X-ray observations except 8 PCA observations without useful slew data, we generated a time profile of the unabsorbed 3-20 keV flux as shown in Fig. 11. We included the first PCA observation, despite the absence of useful slew data, because a simultaneous WFC observation shows 712 to be at least 10 times as faint as SAX J1711.6-3808 (see Sect. 2.4; this data point is not shown in Fig. 4). At the time of discovery, the flux appears to be on the rise to its peak of $1.8\times10^{-9}$ erg cm^-2 s^-1. However, this is difficult to assess with certainty because the onset of the outburst was missed. Data obtained with the ASM on RXTE are incomplete at the time of the onset of the outburst: there is a gap in the coverage between November 20, 2000, and January 22, 2001 (see Fig. 2). After January 22, the data show detections with fluxes that are of similar magnitude as seen with the PCA, and before November 22 there are only non-detections below a sensitivity limit of roughly $2\times10^{-10}$ erg cm^-2 s^-1 on a weekly time scale. After reaching the peak flux at about the time of the NFI observation, the source hovered at about the same flux level for one month, Subsequently, it steadily declined by about a factor of 5 in two months. Such a decay rate is typical for bright X-ray novae (e.g., Chen et al. 1997). When investigated in higher time resolution, none of the 412 ksec worth of data show bursting activity which would have diagnosed the compact object as a neutron star. Also, none of the ASM data shows signs of bursting activity, but it should be noted that the time resolution of 90 s diminishes the sensitivity considerably towards typical bursts with an e-folding decay time of 10 s.

We note that the unabsorbed 3-20 keV fluxes that we find are between 25 to 48% lower than those obtained by Wijnands & Miller (2002), due to the correction for the contribution by 712.

Figure 11: The points connected through a solid line represent the history of the unabsorbed 3-20 keV flux as measured with WFC (asterisks), NFI (square), PCA (diamonds, only for data resolved from 712). The WFC data are per pointing. To calculate fluxes, spectra were modeled with an absorbed power law plus disk black body. Subsequently, the PCA fluxes were proportioned based on the top-layer photon rates plotted in Fig. 4. Since this is not possible for the first PCA point, we assumed a rate of 20 c s-1 PCU-1 for 712 in that case, which equals the minimum rate over the other observations and is similar to the upper limit in the simultaneous WFC observation. Typical flux errors are estimated at 10%, except for the NFI point which has an error of 2%. The points connected through a dashed line represent the 1-20 keV flux, as estimated by a mild extrapolation of most spectra downward from 3 to 1 keV. The peak at day 90-100 is caused by the appearance of a soft excess.

The 3-20 keV band is not a good tracer of the bolometric flux. As spectral analyses showed, a substantial part of the bolometric flux is contained below 3 keV and above 20 keV. For instance, during the NFI observation, 13% of the unabsorbed 0.1 to 200 keV flux is contained within 0.1-3 keV and 53% within 20-200 keV. These percentages vary considerably: during the PCA observation of day 92 of 2001, 74% of the 0.1-200 keV flux is estimated to be within 0.1-3 keV. This is due to the temporary appearance of a strong soft excess. We estimated how the time profile of the bolometric flux might look like by extrapolating the spectral fits that were done between 3 and 20 keV. We estimate that the 1 to 20 keV flux is rather constant between days 45 and 70 at a level of about $2.3\times 10^{-9}$ erg cm^-2 s^-1. Thereafter, it starts to decline, but revives after 10 days to an absolute peak of $4\times 10^{-9}$ erg cm^-2 s^-1 whereafter the decline resumes. This 1-20 keV peak luminosity happens to be close to the 1-200 keV luminosity during the NFI measurement ( $5\times 10^{-9}$ erg cm^-2 s^-1), when there was no strong soft excess present.