Astron. Astrophys. 327, L1-L4 (1997)

3. FOS observations of J03.13

To definitely prove the spectral similarity between the J03.13 A and B images ("to be or not to be lensed"), we have taken on October 28, 1996 (17h31m - 25h06m UT) FOS spectra at the positions of these two components, and at a third position C, symmetric from B with respect to A. These FOS/RD spectra were obtained with the grating G650L (central wavelength of 4400 Å, FWHM 30.5 Å, COSTAR deployed) and the small 0.5 (actually 0.43 ) circular aperture. A four stage ACQ/PEAK acquisition of the bright A component has first been successfully achieved through the standard 4.3, 1.0, 0.5 and 0.3 FOS apertures. With the goal of reaching a signal-to-noise ratio of 20 per resolution element, integration times of 840 and 4860 sec. have been aimed for the spectra of J03.13 A (HST dataset Y2YK0406T) and B (datasets Y2YK0407T - Y2YK040AT). Since a total of 3 HST orbits was allocated for these FOS observations, there remained 1200 sec. for the integration of the spectrum of C (dataset Y2YK040BT). Two reference wavelength spectra (datasets Y2YK0405T and Y2YK040CT) were also obtained right before and after the target observations. No significant shift between the reference spectral lines is detectable. The spectra of J03.13 A and B are shown in Figure 2.

Fig. 2. FOS spectra of (A) J03.13 A, (B) J03.13 B after multiplication of its flux by 7.5 and a vertical offset by erg/sec/cm2 /Å, (C) a fiducial target at the symmetric position of B with respect to A for probing light contamination from the latter component and (D) the residual spectrum of . The QSO emission and two intervening absorption line systems are identified with vertical lines and labels at different redshifts (see text).

We find that, apart from a multiplicative constant of 7.5 0.7 (corresponding to a magnitude difference m = 2.2 0.1), the spectra of J03.13 A and B are identical. We have also reproduced in Fig. 2 the residual spectrum showing that apart from a few datapoints near the emission line peaks -which are undersampled by the FOS- and the MgII absorption lines at z = 1.085, the spectra of J03.13 A and B do indeed look quite similar. Spectrum of C probes for the possible contamination of component B by A, which turns out to be absolutely negligible. We confirm the emission-line redshift = 2.545 reported in Paper I as well as the two absorption line systems at = 2.344 and = 1.085 (the latter one being only detected from its MgII resonance lines in the spectrum of image A). Note that due to the somewhat larger spectral coverage of the present FOS data, we do also detect the Ly 1025 and OVI 1031, 1037 emission lines at = 2.545.

© European Southern Observatory (ESO) 1997

Online publication: April 6, 1998
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