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

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3. NFI observations

Observations were made with the NFI on Nov. 24.46-25.04 (UT), 1998, which is 627 d after the first and 165 d before the second WFC detection. The NFI include the low-energy concentrator/spectrometer (LECS; 0.1-10 keV; Parmar et al. 1997), the medium-energy concentrator/spectrometer (MECS; 1.8-10 keV; Boella et al. 1997b), the high-pressure gas scintillator proportional counter (HP-GSPC; 4-120 keV; Manzo et al. 1997), and the phoswich detector system (PDS; 15-200 keV, Frontera et al. 1997). The first two instruments are imaging devices. The effective exposure times on SAX J2239.3+6116 are 11.9 ks (LECS), 20.1 ks (MECS) and 10.0 ks (PDS). The HP-GSPC was turned off for operational reasons.

Only the MECS data show a detection of SAX J2239.3+6116 in an otherwise empty field of view (see Fig. 5). The position of the source is [FORMULA] = [FORMULA], [FORMULA] = +61o16´20" with a 90% confidence error radius of [FORMULA]. This is [FORMULA] from the WFC centroid position and the two positions are fully consistent (see Fig. 9). The chance probability of a background point source with a flux in excess of [FORMULA] erg cm-2 s-1 in the WFC error region is 0.3% (based on the counts by Giommi et al. 1999). We reduced the spectrum of the point source from the MECS data through application of the maximum likelihood method (e.g., In 't Zand et al. 2000) and tried to model it with absorbed single-component models like a power-law function and thermal bremsstrahlung, while fixing [FORMULA] to 1[FORMULA] cm-2. Only black-body radiation gave a satisfactory fit to the data of a single-component model, with [FORMULA] for 7 dof. The color temperature is [FORMULA] keV and the emission area at 4.4 kpc (see Sect. 6) for isotropic emission is equal that of a sphere with a radius of [FORMULA] km. The 2-10 keV flux is [FORMULA] erg cm-2 s-1 which is [FORMULA] times fainter than the peak flux of the March 1997 outburst.

[FIGURE] Fig. 5. MECS image of observation on SAX J2239.3+6116. MECS units 2 and 3 data are combined.

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© European Southern Observatory (ESO) 2000

Online publication: September 5, 2000