Central gravitational image detection is very important for the study of the mass distribution of the inner parts ($sim 100$ pc) of lens galaxies. However, the detection of such images is extremely rare and difficult. We present a 1.7-GHz High Sensitivity Array (HSA) observation of the double-image radio lens system B1030+074. The data are combined with archive VLBA and global-VLBI observations, and careful consideration is given to the effects of noise, {sc clean}ing and self-calibration. An upper limit is derived for the strength of the central image of 180 $mu$Jy (90% confidence level), considerably greater than would have been expected on the basis of a simple analysis. This gives a lower limit of $sim 10^3$ for the ratio of the brightest image to the central image. For cusped models of lens mass distributions, we have made use of this non-detection to constrain the relation between inner power-law slope $beta$ of the lensing galaxy mass profile, and its break radius $r_b$. For $r_b>130$ pc the power-law slope is required to be close to isothermal ($beta>1.8$). A flatter inner slope is allowed if a massive black hole is present at the centre of the lensing galaxy, but the effect of the black hole is small unless it is $sim 10$ times more massive than that implied by the relation between black hole mass and stellar velocity dispersion. By comparing four epochs of VLBI observations, we also detected possible superluminal motion in the jet in the brighter A image. The B jet remains unresolved, as expected from a simple lens model of the system.
Download