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Astron. Astrophys. 361, 85-91 (2000)

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2. WFC observations

The WFCs (Jager et al. 1997) on the BeppoSAX satellite (Boella et al. 1997a) carry out a monitoring program where they point at arbitrary positions in the sky. This program is the `secondary mode' program whereby the WFC pointings are dictated by those of the Narrow Field Instruments (NFI) on the same satellite and the constraints of the satellite attitude with respect to the direction of the sun. The secondary mode observations entail roughly 90% of all observations.

There are two identical WFCs. They each have [FORMULA] square degrees field of views (full width to zero response) with [FORMULA]´ angular resolution. The bandpass is 2 to 26 keV. The WFCs have been active continuously since mid-1996, except for a 1-month period in May 1998 when unit 1 was turned off, and a 3 week period in early 1999 when both cameras were turned off. During mid-May to mid-August 1997, the satellite was in a standby mode, with both cameras turned on but without sufficient stabilization for sensitive imaging.

During 103.4 d on-source time with a net exposure time of 27.8 d, the secondary observations covered the field around the supernova remnant Cas A. This exposure is roughly uniformly distributed in time. The latest data that we report here were obtained on Dec. 15, 1999. Except for Cas A, there are no bright X-ray sources present in this field. Compared to the background levels in the WFCs, Cas A is not so bright. The sensitivity is, therefore, close to optimum in the field. During two instances, a relatively faint transient turned up in the field, at an angular distance of 6.0o from Cas A. They were on March 4, 1997, and May 8, 1999. We note that sometimes Cyg X-2 is in the same field of view as this transient, but at an angular distance of 24.5o which implies it does not degrade the signal of the transient. The celestial position of the WFC-detected transient is [FORMULA], [FORMULA] = +61o16´47" with a 99% confidence error radius of [FORMULA].

The detection in March, 1997, was the strongest WFC detection obtained so far. In Fig. 1, we present a light curve with a resolution of about a day. The period over which we detected activity is about 1 week. The variation in brightness during this event seems to suggest that it was not active for much longer.

[FIGURE] Fig. 1. Light curve of the first WFC-detected outburst of SAX J2239.3+6116 in March, 1997, for 2 to 26 keV with a time resolution set by the duration of the measurements (these are of order 1 d). The arrows indicate [FORMULA] upper limits. These limits vary considerably because of varying exposure times and varying exposed detector areas. March 6.877, 1997, is MJD 50513.877.

A light curve of the second WFC-detected outburst is presented in Fig. 2. The measured peak flux of this outburst is about a factor of 3 lower than that of the first outburst but this could very well be a sampling effect. The source was above the detection threshold for one week but due to an unfavorable sampling it could just as well have been active for 50 days.

[FIGURE] Fig. 2. Light curve of the second WFC-detected outburst of SAX J2239.3+6116 for 2 to 26 keV. May 8.241, 1999, is MJD 51306.241.

We have extracted the spectrum from the observation at the peak of the first outburst and modeled it with a simple power law plus absorption due to interstellar gas of cosmic abundances (according to Morrison & McCammon 1983). We kept the value for the hydrogen column density [FORMULA] fixed at [FORMULA] cm-2, as found from HI maps (Dickey & Lockman 1990). The fit was satisfactory, with [FORMULA] (26 dof). The photon index is quite hard at [FORMULA]. The spectrum is shown in Fig. 3. The 2-10 keV flux is [FORMULA] erg cm-2 s-1 which is 0.016 times the flux of the Crab in the same bandpass. In the energy range 2 to 26 keV the flux is [FORMULA] erg cm-2 s-1 or 0.033 times that of the Crab.

[FIGURE] Fig. 3. Top panel: WFC-measured spectrum of SAX J2239.3+6116 (crosses) and the fitted power law model (histogram). Bottom panel: residuals between data and model in units of [FORMULA].

In Fig. 4, we present a light curve with a relatively high time resolution when the flux was at maximum during the first outburst. The source appears to be variable but no coherent signal could be found in this data set. The 90%-confidence upper limit on the amplitude of a periodic sinusoidal variation is 10% for periods between 1 and 103 s.

[FIGURE] Fig. 4. 2 to 26 keV light curve of SAX J2239.3+6116 on March 6, 1997. The time resolution is 75 s.

We have analyzed WFC data taken on December 13-15, 1999. This is 2 weeks after the optical measurements described in Sect. 6. No X-ray detection was found. The upper limit on the X-ray flux is 6[FORMULA] erg cm-2 s-1 (2-10 keV).

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© European Southern Observatory (ESO) 2000

Online publication: September 5, 2000