Time delay measurements with Broken Power Law model

Abstract: One of the key challenges in strong gravitational lensing cosmography is the accurate measurement of time delays between multiple lensed images, which are essential for constraining the Hubble constant ($H_0$). We investigate how lens mass-profile assumptions affect time delays. Specifically, we implement a Broken Power Law (BPL) mass model within the Lenstronomy framework (Birrer & Amara 2018), which introduces additional flexibility in the radial mass distribution and can phenomenologically capture deviations from a single power-law profile. This model is combined with a numerical approach to compute time delays at the image positions. We validate the BPL implementation using simulated lenses and compare the results with those obtained from the elliptical power-law (EPL) model. We then apply both model families to the quadruply imaged quasar WGD~2038-4008. Both models fit the imaging and kinematic data comparably well, yet the greater radial freedom in the BPL model shifts the inferred time-delay distance -- and thus $H_0$ -- by an amount comparable to the current discrepancy between early- and late-universe $H_0$ estimates. In a flat $Λ$CDM cosmology, the $H_0$ inferred using the BPL lens model is $75^{+23.1}_{-16.3} \ \mathrm{km \ s^{-1} \ Mpc^{-1}},$ while the EPL model gives $H_0 = 61^{+19.2}_{-13.2} \ \mathrm{km \ s^{-1} \ Mpc^{-1}}.$ This difference is largely due to uncertainties in the inner mass profile ($θ<0.2''$), a region where point spread function (PSF) reconstruction is a critical factor -- a finding consistent with results reported in Shajib et al. (2022). This highlights how time-delay cosmography remains sensitive to assumptions about the lens mass profile. With current precision, this difference does not favor one cosmological scenario over another, but rather underscores the importance of flexible mass modeling and PSF modeling.