| Central images of gravitational lenses are unique probes of galaxy centers. They reveal the central surface density of
the lensing galaxy and place an upper bound on the mass of any central black hole. Over 20 years ago, a faint central
radio component was discovered between the two bright quasar images of Q0957+561, the rst known gravitational
lens. But even today, it is unknown whether that faint component is a central quasar image, or a faint AGN in
the lensing galaxy, or a combination of both. The most practical way to nd out is to compare the radio spectra
of the quasar and the central component over a broad frequency range. We have data ranging in frequency from
1.7 to 15 GHz, but the results are ambiguous, because of the possible e ect of absorption at low frequencies. The
ambiguity will be greatly reduced by using the High Sensitivity Array to extend the comparison to 1.4 GHz (4 hr
observation) and 22 GHz (12 hr). |
