The importance of alternative methods for measuring the Hubble constant, such as timedelay cosmography, is highlighted by the recent Hubble tension. It is paramount to thoroughly investigate and rule out systematic biases in all measurement methods before we can accept new physics as the source of this tension. In this study, we perform a check for systematic biases in the lens modelling procedure of timedelay cosmography by comparing independent and blind timedelay predictions of the system WGD 2038−4008 from two teams using two different software programs:
 Award ID(s):
 1906976
 NSFPAR ID:
 10473663
 Publisher / Repository:
 AA
 Date Published:
 Journal Name:
 Astronomy & Astrophysics
 Volume:
 667
 ISSN:
 00046361
 Page Range / eLocation ID:
 A123
 Format(s):
 Medium: X
 Sponsoring Org:
 National Science Foundation
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Context. Timedelay cosmography uses strong gravitational lensing of a timevariable source to infer the Hubble constant. The measurement is independent from both traditional distance ladder and CMB measurements. An accurate measurement with this technique requires considering the effects of objects along the line of sight outside the primary lens, which is quantified by the external convergence (κ _{ext}). In absence of such corrections,H _{0}will be biased towards higher values in overdense fields and lower values in underdense fields.Aims. We discuss the current state of the methods used to account for environment effects. We present a new software package built for this kind of analysis and others that can leverage large astronomical survey datasets. We apply these techniques to the SDSS J0924+0219 strong lens field.Methods. We infer the relative density of the SDSS J0924+0219 field by computing weighted number counts for all galaxies in the field, and comparing to weighted number counts computed for a large number of fields in a reference survey. We then compute weighted number counts in the Millennium Simulation and compare these results to infer the external convergence of the lens field.Results. Our results show the SDSS J0924+0219 field is a fairly typical line of sight, with medianκ _{ext} = −0.012 and standard deviationσ _{κ} = 0.028. 
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