Hubble Constant from LSST Strong lens time delays with microlensing systematics

Abstract

Strong lens time delays have been widely used in cosmological studies, especially to infer $H_0$. The upcoming LSST will provide several hundred well measured time delays from the light curves of lensed quasars. However, due to the inclination of the finite AGN accretion disc and the differential magnification of the coherent temperature fluctuations, the microlensing by the stars can lead to changes in the actual time delay on the light-crossing time scale of the emission region $\sim days$. We first study how this would change the uncertainty of $H_0$ in the LSST era, assuming the microlensing time delays can be well estimated. We adopt 1/3, 1 and 3 days respectively as the typical microlensing time delay uncertainties. The relative uncertainty of $H_0$ will be enlarged to $0.47\%$, $0.51\%$, $0.76\%$, respectively from the one without microlensing impact $0.45\%$. Then, due to the lack of understandings on the quasar models and microlensing patterns, we also test the reliability of the results if one neglects this effect in the analysis. The biases of $H_0$ will be $0.12\%$, $0.22\%$ and $0.70\%$, respectively, suggesting that 1 day is the cut-off for a robust $H_0$ estimate.