2 Observations and data analysis issues

Table 1 presents a log of all X-ray observations carried out on SAX J1711.6-3808 between 2001 Feb. 5 and 2001 Nov. 9. Each of the four data sets has a unique diagnostic value: the BeppoSAX-WFC data provide coverage of the early parts of the outburst, the BeppoSAX-NFI data provide simultaneous wide spectral coverage, and the RXTE-PCA data provide coverage of the decay of the outburst combined with high time resolution analysis capabilities. We note that in addition to the data specified in Table 1, there are daily data after Jan. 22 from the All-Sky Monitor (ASM) on RXTE.

2.1 BeppoSAX-WFC

WFC unit 1 (Jager et al. 1997) on BeppoSAX (Boella et al. 1997a) observed the position of SAX J1711.6-3808 seven times, at various off-axis angles and, therefore, sensitivities in the 2-28 keV bandpass. The first observation started on Feb. 5 (day 36 of 2001) and the last ended on April 23 (day 113). All observations except the last yielded detections. The discovery was made from the second observation (In 't Zand et al. 2001). The total exposure time was 320 ksec. The statistical quality of the WFC data is limited; the highest signal-to-noise ratio is 54 and applies to the third observation. In all other cases this ratio is at least a factor of 2 lower. This kind of quality only allows rough spectral constraints. Figure 1 shows the wide-field WFC image taken on day 75 of 2001. It shows another active source at only 33$\arcmin$ from SAX J1711.6-3808. This is 712, a bursting low-mass X-ray binary transient that was discovered with the WFC in 1999 (In 't Zand et al. 1999; Cocchi et al. 1999). On day 75 it had an average flux of $33\pm2$ mCrab (2-28 keV), which is similar to the peak flux in 1999. During the other WFC observations of SAX J1711.6-3808 in 2001, 712 was not detected with upper limits between 6 and 30 mCrab. However, archival WFC data show 712 sometimes slightly popping up above the WFC detection threshold (Cocchi et al., in prep.).

Figure 1: Image of 13$\fdg$6 $\times$ 13$\fdg$6 region around SAX J1711.6-3808 taken with the WFCs on day 75 in the 2-28 keV bandpass. Pixel values are in units of significance of detection.

Figure 2: RXTE/ASM light curve of SAX J1711.6-3808  from 1996 Jan. on, with a time resolution of 4 d. Standard criteria were employed to exclude bad data; most importantly data points with background levels in excess of 10 c s-1. A bias level of 0.33 c s-1 was subtracted which is the average level before -400 d.

Figure 3: RXTE/ASM light curve of 712. A bias level of 0.6 c s-1was subtracted.

   
2.2 RXTE-ASM

ASM (Levine et al. 1996) archival data was processed to generate a light curve of SAX J1711.6-3808 since the start of mission operations in January 1996, see Fig. 2. Only one clear outburst was detected from SAX J1711.6-3808, in 2001. During the outburst, up to 33 ASM dwells of 90 s each were carried out per day, with a total number of dwells of 1194 and a total exposure time of 107 ksec. The typical sensitivity in one dwell on a non source-confused region is 0.03 Crab units. The number of sources solved for per dwell is between 7 and 37. Figure 3 shows the ASM light curve of 712. The highest point here corresponds to the August 1999 outburst when the source was discovered with WFC (In 't Zand et al. 1999). Further activity is visible in 1996 and, particularly, in 2001.

2.3 BeppoSAX-NFI

SAX J1711.6-3808 was observed with the Narrow-Field Instruments (NFI) on BeppoSAX between 2001 Feb. 16.68 and 17.57 UT (day 47.68-48.57 of 2001). The NFI consist of two imaging devices that operate at photon energies below 10 keV: the Low-Energy (LECS; 0.1-10 keV bandpass; Parmar et al. 1997) and Medium-Energy Concentrator Spectrometer (MECS; 1.6-10 keV; Boella et al. 1997b). Two other - non-imaging - devices operate roughly above 10 keV: the High-Pressure Gas Scintillation Proportional Counter (HP-GSPC; 7-34 keV; Manzo et al. 1997) and the Phoswich Detector System (PDS; 15-220 keV; Frontera et al. 1997). The circular fields of view of the LECS and MECS are 37$\arcmin$ and 56$\arcmin$ in diameter respectively. The HP-GSPC and PDS fields of view are 1$\fdg$ 1 and 1$\fdg$ 4 full-width at half-maximum (FWHM) respectively. The energy resolutions at 6 keV are 8% (FWHM) for the MECS and 11% for the HP-GSPC. The net exposure times are 9.5 (LECS), 36 (MECS), 36 (HP-GSPC) and 18 ksec (PDS). The analysis of the data sets was carried out in a standard manner: accumulation radii of 8$\arcmin$ and 4$\arcmin$ were applied for the LECS and MECS images, the normalizations of the LECS and PDS data sets with respect to the MECS data were left as free parameters (while it was checked that these numbers are within acceptable limits), and spectral bins were combined up to the point where each bin contains at least 20 photons so that the $\chi ^2$ statistic applies.

The PDS consists of two pairs of detectors that each have an independent collimator which rocks on and off the source at 192 s intervals. The off source positions are 3$\fdg$5 on opposite sides from the source. For the observation on SAX J1711.6-3808, these two positions are not on blank sky: 2S 1711-339 is 53$\arcmin$ off-axis in one off-source pointing, while 4U 1708-40 is 50$\arcmin$ in the other. On top of that, 712 contaminates the signal in the on-source pointing. All three contaminating sources have been observed on-axis with the PDS before and the relevant data are publicly available from the BeppoSAX archive. 2S 1711-339 is a transient X-ray source which has a fairly high X-ray flux during "quiescence''. It was observed during quiescence on 2000 Feb. 29 (e.g., Cornelisse et al. 2002), with a 2-6 keV flux of $2.4\times 10^{-11}$ erg cm^-2 s^-1, but the source was not detected above a 3$\sigma$upper limit of 0.12 c s^-1 in the 15-100 keV band. At other times during quiescence, the source was seen at 2-6 keV flux levels between 0.3 and $5\times 10^{-11}$ erg cm^-2 s^-1 (Cornelisse et al. 2002), so one would expect a maximum PDS count rate of approximately 0.24 c s^-1. 4U 1708-40 is a persistent X-ray source with a mean 2-12 keV flux of about 10^-9 erg cm^-2 s^-1, according to publicly available measurements with the ASM on RXTE. A PDS observation on August 15, 1999, did not reveal a detection above a 3$\sigma$ upper limit of 0.12 c s^-1. Judging from the ASM data, the flux is not expected to vary by more than a factor of about 2, so also for this source we set the upper limit to 0.24 c s^-1. Taking into account these upper limits and the collimator responses, we estimate that the two sources in the off-source pointings introduce systematic flux uncertainties of less than one percent in the flux of SAX J1711.6-3808 (whose intensity is 21 c s^-1). We verified this by comparing the PDS spectra of both off-source pointings, for both pairs of detectors. They fall on top of each other, and the flux does not differ by more than 1%.

712 was observed with the PDS on August 27, 1999 (Cocchi et al., in prep.) when it had a 2-10 keV flux of $1.1\times 10^{-10}$ erg cm^-2 s^-1 and a photon count rate in the PDS of 0.76 c s^-1 (15-100 keV). During the NFI observation on Feb. 16-17, the ASM dwelled on 712 18 times. The combined ASM data do not show a detection above a $3\sigma$upper limit of approximately $3.3\times 10^{-10}$ erg cm^-2 s^-1 (2-10 keV). During the 2 days prior to the NFI observation of SAX J1711.6-3808, WFC observations show that the 2-10 keV flux of 712 was at most equal to that during the 1999 NFI observation. Scaling the 1999 PDS flux to this upper limit, we conclude that the contribution of 712 to the PDS flux of SAX J1711.6-3808 amounts to 11% at maximum (2.3 c s^-1), but more likely less than 3.5% (as inferred from the WFC observations 2 days earlier). In the HP-GSPC data, the contribution is expected to be even less. We note that it is not possible to employ the PDS data in the same way as the PCA slew data (see Sect. 2.4) because the on/off slews are too fast, lasting less than 3 s.

   
2.4 RXTE-PCA

Figure 4: 2-10 keV photon rates in the top layer of all active PCA PCUs, for SAX J1711.6-3808 (diamonds) and 712 (squares) as predicted from slew data for an on-source pointing of SAX J1711.6-3808. These data represent true counting rates and have not been corrected for collimator vignetting for either source.

RXTE observed SAX J1711.6-3808 19 times. The first 6 observations, amounting to 14.8 ksec exposure time, were carried out under an accepted AO5 TOO program (PI J. Swank), and the remainder (30.9 ksec exposure) as public TOO observations. Wijnands & Miller (2002) already reported about the first 8 public TOO observations. We here report on all PCA measurements. The PCA instrument (for a detailed description, see Jahoda et al. 1996) consists of an array of 5 co-aligned Proportional Counter Units (PCUs) that are sensitive to photons of energy 2 to 60 keV with a total collecting area of 6500 cm². The spectral resolution is 18% FWHM at 6 keV and the field of view is nearly hexagonal with a size of 1$^{\rm o}$ FWHM. During the observations the number of active PCUs varied between 1 and 5. While the first 17 observations have the same pointing (equal to the WFC-determined position of SAX J1711.6-3808), the last two observations consisted of additional pointings that are more than 1 from 712, to determine the flux of SAX J1711.6-3808 without ambiguity at the cost of some sensitivity. During analyses, we refrain from using PCU0, because this unit is missing its top propane layer and hence has higher background and a different spectral response. Furthermore, we only include PCU2 and 3, because of calibration differences with PCU1 and 4.

712 is potentially an important source of contamination in the PCA data. At an angle of 33$\arcmin$ to the pointing axis, the collimator response to 712 is 44% with the exception of the last two observations. To determine the contamination by 712 with far better accuracy than provided by the ASM data, we investigated data taken at times when the PCA is slewing to or from SAX J1711.6-3808, immediately before and after each observation. The profile of the photon count rate during these slews can be decomposed into the contributions from both sources if the slew direction has a sufficiently large component along the line connecting both sources, and if both sources are steady enough during the slews to not disturb the slew profiles in the rates. It turns out that half of all PCA observations have useful slew data. In Fig. 4 we present the count rates for both sources as determined from these slews. It is clear that 712 has a significant contribution which can never be neglected: in no case is the photon rate smaller than 0.4 times that of SAX J1711.6-3808. In the final three observations it is even the sole source of photons.

We note that the second WFC observation overlaps the first PCA observation. 712 was not detected with the WFCs and the upper limit is relatively sensitive at 6 mCrab which translates to roughly 13 c s^-1 PCU^-1 on axis or 6 c s^-1 PCU^-1 at 33$\arcmin$ off axis.

Additionally, there is contamination from the Galactic ridge. We determined the local ridge emission from a 1 ksec exposure in the one but last observation which was offset from 712 so that no emission from 712 nor SAX J1711.6-3808 was detected. The pointing is at $l^{\rm II}=$347$\fdg$ 69, $b^{\rm II}=$+0$\fdg$ 84. This position is offset by 0$\fdg$ 73 in Galactic longitude from the other pointings on SAX J1711.6-3808, but only 0$\fdg$ 05 in latitude (the spatial profile of the ridge is particularly dependent on latitude). The photon count rate derived for the local ridge emission is 2.8 c s^-1PCU^-1 (top layer, 3-20 keV; compare with the source photon rates in Fig. 4). The ridge emission also includes a narrow Fe-K emission line with flux $2.1\times 10^{-4}$ phot s^-1 cm^-2.

2.5 XMM-Newton

A public TOO was carried out with XMM-Newton from March 2.93 to 3.09 U.T. On this platform, measurements are done through 3 identical telescopes (e.g., Jansen et al. 2001). For 2 of these, 50% of the X-radiation is diverted to reflection gratings. The other 50% of the two telescopes are collected by the EPIC-MOS1 and MOS2 CCD arrays (Turner et al. 2001) while 100% of the third telescope is collected by the EPIC-pn CCD array (Strüder et al. 2001). The 3 EPIC cameras enable imaging measurements at $\sim$6$\arcsec$ resolution in a 30$\arcmin$-diameter field of view. The energy resolution intrinsic to all CCDs is $E/\delta E=20$ to 50 in a 0.1 to 15 keV bandpass. The exposure times per device are 12 ksec for EPIC-MOS1 and MOS2, and 10 ksec for EPIC-pn CCD. All cameras were operated in "full window'' data acquisition mode. A treatment of the spectral XMM-Newton CCD data, which is heavily piled up, and the (heavily absorbed) RGS data is deferred to a later paper. We will here only address the source position resulting from these data.