How much can shape degeneracies affect the Hubble constant measurement?

Abstract

Cluster lens models are constrained almost entirely by observed lensed image positions, leaving a large space of mass perturbations with arbitrary amplitudes such that they reproduce every image exactly. Such degeneracies are known as shape degeneracies (ShaDes). We show that these ShaDes shift the observed time delays, by the difference of the perturbing potential between the two images, without depending on the number of source/lens planes. Bounding their amplitude by preserving the positivity of the convergence, image multiplicity, and parity, in MACS J0138.0$-$2155, they can contribute at least $\simeq 7$ days to the delay between the detected images of SN-Encore (i.e., $17.5\%$ of the measured value of $39.8$ days) and $\simeq 112$ days (i.e., $2.8$--$3.7\%$ of predicted value of $3,000-4000$ days) to the reappearance intervals predicted for SN-Encore and SN-Requiem. These error bars transfer one-to-one to $H_0$, and remain invisible in comparisons of independent models.

8 pages, 2 figures. Comments are welcome!