Deep radio observation of the gravitational lens candidate QSO2345+007

The double QSO2345+007 comprises two optical components separated by 7.1
arcseconds and is the most prominent `dark matter’ gravitational lens candidate. Despite being known for more than a decade, optical spectroscopy and imaging have been unable to determine whether this double QSO is a binary QSO or a gravitational lens system. In this note we report a deep VLA observation of this system, yielding a map with a noise level of 8.5\ts $\mu$Jy per beam. We have a $4\sigma$ detection of a radio source within one arcsecond of the optical position of the brighter A-component of the QSO, but no significant detection of any radio counterpart of the B component. Given that the flux ratio in the optical waveband is $\sim$3–4, the gravitational lens hypothesis for this system would predict a radio flux of image B of $\lesssim 10$\ts $\mu$Jy. The non-detection of the B component is thus consistent with, but does not prove, the lens interpretation.