Astron. Astrophys. 354, L29-L32 (2000)

2. Observations

2.1. Optical spectroscopy

We observed the optical counterpart to 1WGA J1958.2+3232 on July 12, 1999, using the Intermediate Dispersion Spectroscopic and Imaging System (ISIS) on the 4.2-m William Herschel Telescope (WHT), located at the Observatorio del Roque de los Muchachos, La Palma, Spain. The blue arm was equipped with the R300B grating and the EEV#10 CCD, which gives a nominal dispersion of Å/pixel over Å. The red arm was equipped with the R1200R grating and the Tek4 CCD, which gives a nominal dispersion of Å/pixel at H. The exposure time was . The data were processed using the Starlink packages CCDPACK (Draper 1998) and FIGARO (Shortridge et al. 1997) The extracted spectra are displayed in Figs. 1 and 2.

Fig. 1. Blue spectrum of the optical counterpart to 1WGA J1958.2+3232, taken on July 12, 1999, with the WHT and ISIS. Emission lines are marked.

Fig. 2. H andHeI 6678Å emission lines in the optical counterpart to 1WGA J1958.2+3232. Spectrum taken with the WHT and ISIS on July 12, 1999.

We obtained lower resolution spectroscopy using the 1.3-m Telescope at the Skinakas Observatory (Crete, Greece) on July 26, 1999. The telescope is an f/7.7 Ritchey-Cretien and was equipped with a 2000 800 ISA SITe chip CCD. This camera has 15µm pixels and reaches maximum efficiency ( 90%) in the red, at around H. The spectrum (a 1800-s exposure) was taken with a 1300 line mm^-1 grating and a 320 µm width slit (6".7) which gave a dispersion of 1 Å pixel^-1. The spectrum, which is displayed in Fig. 3, was reduced using FIGARO .

Fig. 3. Low-resolution spectrum of the optical counterpart to 1WGA J1958.2+3232, taken on July 26, 1999, with the 1.3-m telescope at Skinakas.

2.2. Optical photometry

We obtained Strömgren photometry of the field using the 1.3-m Telescope at Skinakas Observatory on August 16, 1999 (JD 2,451,407). The telescope was equipped with a 1024 1024 pixel SITe CH360 CCD. The size of the pixels was 24µm, representing approximately 0".5 on the sky. The source was observed through standard u, v, b, y filters with exposure times of 1200, 900, 600 and 300 seconds, respectively. A sufficient number of standards were observed in order to compute the atmospheric extinction coefficients and allow the transformation to the standard system.

The results are displayed in Table 1. We have also obtained measurements for the only other star of similar brightness which was inside both the ASCA and ROSAT error circles - dubbed "candidate A" by Israel et al. (1999). As can be seen, the values of y for the proposed optical counterpart and candidate A are compatible with the V values obtained by Israel et al. (1999) - 15.70.2 and 15.40.2 respectively.

Table 1. Observational details of the optical photometry.

© European Southern Observatory (ESO) 2000

Online publication: January 31, 2000
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