Improved Parameters and New Lensed Features for Q0957+561 from WFPC2 Imaging

Abstract

New HST WFPC2 observations of the lensed double QSO 0957+561 will allow improved constraints on the lens mass distribution and hence will improve the derived value of H 0 . We first present improved optical positions and photometry for the known components of this lens. The optical separation between the A and B quasar images agrees with VLBI data at the 10 mas level, and the optical center of the primary lensing galaxy G1 coincides with the VLBI source G ′ to within 10 mas. The best previous model for this lens (Grogin and Narayan 1996) is excluded by these data and must be reevaluated.Several new resolved features are found within 10 ′′ of G1, including an apparent fold arc with two bright knots. Several other small galaxies are detected, including two which may be multiple images of each other. We present positions and crude photometry of these objects.

Spectra of faint sources in crowded fields with FRODOSpec on the Liverpool Robotic Telescope

“…However, averaging over independent intervals of 400Å, red-arm fluxes of the galaxy are comparable to quasar fluxes. This result agrees with previous observations of the lens system (e.g., Bernstein et al 1997;Keeton et al 1998). The response function S(λ) in Eq.…”

“…The positions of the B image and the lensing galaxy (G) relative to the A image, and the de Vaucouleurs profile of G are set to accurate values from HST observations (Bernstein et al 1997;Keeton et al 1998;Kochanek et al 2013). Therefore, each monochromatic frame of q0957 is initially modelled by a ten-parameter distribution of light.…”

New aperture photometry of QSO 0957+561; application to time delay and microlensing

“…In order to find the color offset for the HST data, we simply looked for what gave the best agreement in the overlap area of the profiles. The "correction" V −R = 1.3 seemed to merge the profiles The position of the center and the orientation of the semimajor axis are calculated from the positions of the quasar images and from the relative astrometry of Bernstein et al (1997). To synthesize the galaxy we start by oversampling the pixels four times.…”

“…169 and 6. 175, respectively (Bernstein et al 1997). for the light contamination between the quasar components (crosstalk) or from the lens galaxy, see e.g. Schild & Cholfin (1986); Schild (1990); Kundic et al (1995).…”

Gravitational Lenses and the Hubble Constant: Present and Future

“…This estimate is 10% accurate at 1 (Oscoz et al 1997). We h o w ever remark that a new lens model satisfying the constraints from radio mapping as well as new optical constraints deduced from HST observations is now in progress (Bernstein et al 1997). Also, the approximation v = v (light) m ust be reconsidered (Mediavilla et al 1998).…”