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Astron. Astrophys. 318, L67-L70 (1997)

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2. Observations

SBS 1520+530 was identified as a QSO at z=1.85 during the follow-up spectroscopy at the 6m telescope on the 18th May 1993. A low-resolution spectrum was taken with the 1024-channel TV scaner (Drabek et al. 1985) under mediocre weather conditions. The spectrum revealed presence of some most prominent emission lines and a broad absorption feature in CIV [FORMULA] 1549Å. No assumptions were made regarding the multiplicity of QSO image.

The first direct images of SBS 1520+530 were obtained in May 1995 in the course of a photometric study of SBS QSOs with the SAO 1m telescope. The photometer at the Cassegrain focus was equipped with a CCD camera 530 [FORMULA] 580 pixels (Zin'kovskii et al. 1994) in combination with Johnson B and V filters. The element of the CCD corresponded to 0:0028 [FORMULA] 0:0037 in the focal plane.

The images were taken under good seeing (FWHM [FORMULA] 1:002) with exposures of 10 min and 5 min in B and V passbands, respectively. They revealed the apparent multiplicity of QSO image. It appears to be a triple system, aligned approximately in EW direction (see Fig. 1). Two bright components with V=18:m2 (component A) and 18:m6 (component B) are separated by [FORMULA] 1:005, the third one is located at [FORMULA] 2:005 from component A and is much fainter: its V magnitude was estimated as 19:m5. The identical component colours lead us to suspect SBS 1520+530 as a possible candidate for a gravitationally lensed system.

[FIGURE] Fig. 1. Finding chart for SBS 1520+530, obtained from B band image. Size of field corresponds to 1:03 [FORMULA] 1:03. Equatorial coordinates of the A component are : [FORMULA], [FORMULA].

2.1. Spectroscopy

Spectroscopy of the SBS 1520+530 components was carried out on 1996 June 12 with a long-slit spectrograph (Afanasiev at al. 1995), installed in the prime focus of the 6m telescope. A fast (f/1.1) Schmidt camera with the internal focus in combination with the CCD 530 [FORMULA] 580 pixels provided a spatial sampling of 0:004 per pixel. The weather was photometric with seeing about [FORMULA]. The observations were made with a grating 325 mm-1 and [FORMULA] entrance slit, resulting in a spectral resolution about 15Å over the wavelength range 3700-7000ÅA. The slit of the spectrograph was positioned to pass through all three components.

Two 1200 s exposures were taken, in order to accurately clean spectral acquisitions from cosmic particles. The shift of components along the slit between two exposures did not exceed 0:002 and allowed us to add both images with removal of particle hits without the loss of resolution. The resulting spatial resolution was equal to 1:001 and the spectra of the closest components A and B were clearly separated on the obtained image.

The reduction of spectral acquisitions was fulfilled using a software developed in SAO (Vlasyuk, 1993). The resulting spectra (shown in Fig. 2) reveal that both components are quasars having similar spectra with identical redshifts ([FORMULA] =1.855). Both spectra displayed broad absorption line (BAL) features of CIV [FORMULA] 1549Å line and two systems of narrow absorption lines.

[FIGURE] Fig. 2. Flux-calibrated spectra of the A & B components of SBS 1520+530 at [FORMULA] resolution.

2.2. Additional photometry

Since the nature of SBS 1520+530 was substantiated by spectral investigations, a decision was made to refine the photometric parameters. Direct imaging was performed at the SAO 1m telescope with the same instrumental set-up, as mentioned above, from 12th to 14th June 1996. Broad-band filters B, V, R, I were used. The total integration time was about 30-40 min in each band. The standard stars in NGC 7006 and M 92 fields were used for calibration. The photometric condition and seeing (FWHM [FORMULA] 0:008-1:002) allowed us to measure the brightness and relative positions of components as well as to detect some faint objects in the surrounding field.

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© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998