VLBI Observations of the double QSO, 0957 + 561 A, B

Abstract

Walsh et al.¹ originally suggested that the double QSO 0957 + 561 A, B might be images of the same object formed by a gravitational lens because of their remarkably similar optical spectra. Additional optical spectral data² strengthen the evidence for a gravitational lens. Radio data at 5 GHz have shown a complex structure^3,4. There are compact (<1 arc s) components closely coincident with the two QSOs; Pooley et al.³ suggested that the one near B may be extended ∼1.5 arc s, but the higher resolution data of Roberts et al.⁴ make it probable that B is, in fact, also compact. In addition, there are at least two extended components, one to the north-east producing more than half the total 5-GHz flux density, and a much weaker one to the west. Pooley et al. show that these two extended components have fairly steep spectral indices (−0.65) between 5 GHz and 408 MHz but the components close to the QSOs have relatively flat spectra. We have now tested the gravitational lens hypothesis using the greater resolution obtained by very long baseline interferometer (VLBI) measurements of the QSOs, with the European Network at 1,666 MHz. We have detected two apparently unresolved (≲20 marc s) sources with about the same separation as the optical objects and with flux densities of 44 and 32 mJy. These sources have the same flux density ratio as the small-diameter components detected in lower resolution 5-GHz maps but we see no evidence for the extended features in these maps. Our high resolution results are consistent with the gravitational lens hypothesis.