Astrometric requirements for strong lensing time-delay cosmography

Understand

The time delay between the arrival of photons of multiple images of time variable sources can be used to constrain absolute distances in the Universe (Refsdal 1964), and in turn obtain a direct estimate of the Hubble constant and other cosmological parameters.

• To convert the time delay into distances, it is well known that the gravitational potential of the main deflector and the contribution of the matter along the line-of-sight need to be known to a sufficient level of precision.
• In this paper, we discuss a new astrometric requirement that is becoming important as time-delay cosmography improves in precision and accuracy with larger samples, and better data and modelling techniques.
• We derive an analytic expression for the propagation of astrometric uncertainties on the multiple image positions into the inference of the Hubble constant and derive requirements depending on image separation and relative time delay.