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

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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 [FORMULA], [FORMULA] (uncertainty [FORMULA]), which is [FORMULA] from the NFI centroid and well within the error circle. The USNO-A2.0 catalog gives approximate magnitudes of [FORMULA] and [FORMULA].

[FIGURE] Fig. 9. Finding chart for the optical counterpart of SAX J2239.3+6116 from the Digitized Palomar Observatory Sky Survey. The field is [FORMULA]. North is up, and east is to the left. The large and small circles are the WFC and NFI error circles, respectively. The optical counterpart is indicated by tick marks. Its position is [FORMULA] = [FORMULA], [FORMULA] = [FORMULA],

The summed spectrum from 3000 s of exposure is shown in Fig. 10. We measure approximate magnitudes [FORMULA], [FORMULA], and [FORMULA] from the spectrum. The estimated uncertainty in these numbers is 0.2 mag. The magnitudes are consistent with the USNO magnitudes. The H[FORMULA] emission line has an equivalent width (EW) of 6.7 Å, a flux of [FORMULA] 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 [FORMULA]5876, He I [FORMULA]6678, and He I [FORMULA]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.

[FIGURE] Fig. 10. Optical spectrum of SAX J2239.3+6116 obtained on the KPNO 2.1m telescope on 1999 December 2 and 3. In the lower panel, the flux scale refers to the upper trace of the spectrum. The lower trace is the same spectrum divided by a factor of 6.

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 [FORMULA]4430, Na I D, and the [FORMULA]5780,5797 blend, respectively. From the calibrations of Herbig (1975) and Tüg & Schmidt-Kaler (1981) we estimate [FORMULA]. 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 [FORMULA] cm-2 in this direction. From the standard conversions [FORMULA] cm-2 mag-1 (Predehl & Schmitt 1995) and [FORMULA], one obtains [FORMULA]. Also, the reddening is less than determined from IRAS [FORMULA]m dust maps (Schlegel et al. 1998) which give [FORMULA].

Assuming that [FORMULA], the dereddened magnitudes from the spectrum become [FORMULA], [FORMULA] and [FORMULA]. 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 [FORMULA] is a realistic estimate. Also, if we assume an average [FORMULA] 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 [FORMULA]. None of these are coincident with the NFI error box.

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© European Southern Observatory (ESO) 2000

Online publication: September 5, 2000
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