6. Optical observations
We looked for the optical counterpart of SAX J2239.3+6116 by obtaining spectra of the three brightest stars in the NFI error circle on 1999 Dec. 2 and 3 UT, using the KPNO 2.1 m telescope and Goldcam spectrograph. The date is 50 d before a predicted outburst (the last indicated in Fig. 6a). The spectra covered the wavelength range 3900-7500 Å at 5 Å resolution. The second brightest star within the error circle (Fig. 9) is the only one that has broad emission lines and other spectral features that are characteristic of a reddened Be star. Be stars are often hard X-ray sources because of emission from an unseen compact companion such as a neutron star. On this basis we identify the Be star with SAX J2239.3+6116. This star is listed in the USNO-A2.0 catalog (Monet et al. 1996, Monet 1998), at position , (uncertainty ), which is from the NFI centroid and well within the error circle. The USNO-A2.0 catalog gives approximate magnitudes of and .
The summed spectrum from 3000 s of exposure is shown in Fig. 10. We measure approximate magnitudes , , and from the spectrum. The estimated uncertainty in these numbers is 0.2 mag. The magnitudes are consistent with the USNO magnitudes. The H emission line has an equivalent width (EW) of 6.7 Å, a flux of ergs cm-2 s-1, and a full-width at half maximum (FWHM) of 400 km s-1 after correcting for the instrumental resolution. Emission lines of He I 5876, He I 6678, and He I 7064 appear to be double-peaked, with the peaks separated by 300 km s-1. Double-peaked emission lines are commonly seen in Be star spectra, where they are attributed to a circumstellar disk.
The reddening to the star may be estimated from many diffuse interstellar band (DIBs) that are apparent in the spectrum (Fig. 10). We measure equivalent widths of EW = 3.4 Å, 3.2 Å, and 2.3 Å for 4430, Na I D, and the 5780,5797 blend, respectively. From the calibrations of Herbig (1975) and Tüg & Schmidt-Kaler (1981) we estimate . This is less than the value for the total Galactic extinction as derived from the hydrogen column density measurements through HI maps (Dickey & Lockman 1990). The integrated H I column density is cm-2 in this direction. From the standard conversions cm-2 mag-1 (Predehl & Schmitt 1995) and , one obtains . Also, the reddening is less than determined from IRAS m dust maps (Schlegel et al. 1998) which give .
Assuming that , the dereddened magnitudes from the spectrum become , and . Since these are somewhat redder than the colors of an early B star, we suspect that there may be some additional circumstellar extinction, and that dereddened is a realistic estimate. Also, if we assume an average mag kpc-1, then the distance to SAX J2239.3+6116 can be estimated as 4.4 kpc. The absolute visual magnitude would then be -3.5, as expected for a star in the range B0 V to B2 III.
In a 16 square-degree area around the NFI position, there are 3 cataloged Be stars brighter than . None of these are coincident with the NFI error box.
© European Southern Observatory (ESO) 2000
Online publication: September 5, 2000