In 1981, a faint radio source (G′) was detected near the center of the lensing galaxy of the famous "twin quasar" Q0957+561. It is still unknown whether this central radio source is a third quasar image or an active nucleus of the lensing galaxy, or a combination of both. In an attempt to resolve this ambiguity, we observed Q0957+561 at radio wavelengths of 13 cm, 18 cm, and 21 cm, using the Very Long Baseline Array in combination with the phased Very Large Array and the Green Bank Telescope. We measured the spectrum of G′ for the first time and found it to be significantly different from the spectra of the two bright quasar images. This finding suggests that the central component is primarily or entirely emission from the foreground lens galaxy, but the spectrum is also consistent with the hypothesis of a central quasar image suffering free-free absorption. In addition, we confirm the previously-reported Very Long Baseline Interferometry position of G′ just north of the optical center of the lens galaxy. The position slightly favors the hypothesis that G′ originates in the lens, but is not conclusive. We discuss the prospects for further clarification of this issue.
Haarsma, Deborah B.; Winn, Joshua N.; Shapiro, Irwin; and Lehár, Joseph, "The central component of gravitational lens Q0957+561" (2008). University Faculty Publications and Creative Works. 489.