Lens modelling of the strongly lensed Type Ia supernova iPTF16geu

Improved observational constraints on the strongly lensed Type Ia supernova iPTF16geu, including the time delay between images, are used to decrease uncertainties in the lens model by a factor $\sim 7$ and to investigate the dependence on the universal expansion rate $H_0$. We constrain a combination of the dimensionless Hubble constant, $h\equiv H_0/(100\,{\rm km/s/Mpc})$, and the slope of the projected surface density of the lens galaxy, $\Sigma\propto r^{\alpha-2}$, at $r\sim 1\,{\rm kpc}$, to $h\gtrsim 2(1-2\alpha/3)$. This implies $\alpha\gtrsim 1$ using our current knowledge of the expansion rate, corresponding to a flatter surface density than an isothermal halo for which $\alpha=1$. Regardless of the slope, a smooth lens density fails to explain the iPTF16geu image fluxes, and additional sub-structure lensing is needed. Taking advantage of the standard candle nature of the source and including stellar microlensing, we show that the probability to obtain the observed fluxes is maximized for $\alpha=1.3$, confirming that $\alpha\gtrsim 1$. For $\alpha=1.3$, images 1 needs an additional $magnification$ from microlensing of $\Delta m_1\sim -0.7$, whereas images 3 and 4 require a $demagnification$ of $\Delta m_3\sim 0.8$ and $\Delta m_4\sim 1.0$, the total probability for which is $p_{\rm tot}\sim 26\,\%$. We conclude that the iPTF16geu flux "anomalies" are well within stellar microlensing predictions.