MTRLI observations of the double QSO at 408 MHz

Abstract

The suggestion¹ that the twin QSOs 0957 + 561 A, B, which are separated by ∼6 arc s and have emission redshifts 1.41, are images of a single QSO formed by a gravitational lens now seems to be generally accepted. This is a result of a wide range of data including optical spectroscopy^2,3 and, more particularly, direct imaging by CCD camera⁴ and image-intensified photography (A. Stockton, personal communication) which reveal a galaxy, Gl, with m_r ∼ 18.5 within 1 arc s of the QSO B. G1 can account for the splitting of the images⁴. It is the brightest member of a cluster⁴, has a redshift 0.375 ± 0.005 (ref. 3) and IR observations confirm that it is a giant elliptical^5,6. The flux density ratio of the two QSO components is nearly constant, B/A ≃ 0.75, from ∼3,000 Å (ref. 7) to decimetre wavelengths^8,9, as expected for gravitational imaging. The first radio maps^10,11 showed a complex structure and their initial interpretation was ambiguous, but there now seems to be agreement on the overall interpretation^4,11,12 in terms of gravitational imaging. Early observations at 408 MHz (ref. 10) suggested the flux tensity ratio of the two QSO components may differ significantly from that at shorter wavelengths, which would cast doubt on the gravitational lens interpretation. Recent observations at 5 GHz (ref. 13) indicate a point source 1 arc s from QSO B, coinciding closely with the position of Gl. We present here a 408-MHz map which provides spectral information on all components of the radio source, removes the doubt raised by the earlier 408 MHz observations and sheds light on the nature of the source coincident with G1.