The Twin Quasar

QSO0957+561 was the first system discovered with characteristics consistent with those of a gravitational lens. It consists of two quasar images separated by approximately 6" with essentially identical optical spectra and redshifts (z=1.41). The quasar is multiply imaged by a gravitational lens consisting of a z=0.36 bright cluster galaxy and its associated cluster. The two images (A and B) have similar magnitudes that can vary by a tenth of a magnitude in a timescale of a week.

The 0957+561 lens system as seen in a F555W WFPC2 images, with the A and B quasar images and the primary lensing galaxy G1 apparent.

From Bernstein et al. (1997), ApJ 483, L79

In the IUE spectral image shown below the position angle from A to B is roughly aligned with the spectrograph long axis (perpendicular to the dispersion).

Damped Ly-alpha absorption at redshift z(abs) = 1.3911 is present in the spectra of both the A and B components.

The ratio of the flux density in image A to that in image B in late 1993 had a constant value, 1.02, in four different UV bandpass between 1400 A and 3040 A observer's frame. These results are consistent with the prediction of the gravitation lens interpretation that the photometric ratio of the images measured simultaneously should be independent of frequency.


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