Magnification, dust and time-delay constraints from the first resolved
  strongly lensed Type Ia supernova
  We report lensing magnifications, extinction, and time-delay estimates for
the first resolved, multiply-imaged Type Ia supernova iPTF16geu, at $z =
0.409$, using $Hubble\,Space\,Telescope$ ($HST$) observations in combination
with supporting ground-based data. Multi-band photometry of the resolved images
provides unique information about the differential dimming due to dust in the
lensing galaxy. Using $HST$ and Keck AO reference images taken after the SN
faded, we obtain a total lensing magnification for iPTF16geu of $\mu =
67.8^{+2.6}_{-2.9}$, accounting for extinction in the host and lensing galaxy.
As expected from the symmetry of the system, we measure very short time-delays
for the three fainter images with respect to the brightest one: -0.23 $\pm$
0.99, -1.43 $\pm$ 0.74 and 1.36 $\pm$ 1.07 days. Interestingly, we find large
differences between the magnifications of the four supernova images, even after
accounting for uncertainties in the extinction corrections: $\Delta m_1 =
-3.88^{+0.07}_{-0.06}$, $\Delta m_2 = -2.99^{+0.09}_{-0.08}$, $\Delta m_3 =
-2.19^{+0.14}_{-0.15}$ and $\Delta m_4 = -2.40^{+0.14}_{-0.12}$ mag, discrepant
with model predictions suggesting similar image brightnesses. A possible
explanation for the large differences is gravitational lensing by
substructures, micro- or millilensing, in addition to the large scale lens
causing the image separations. We find that the inferred magnification is
insensitive to the assumptions about the dust properties in the host and lens
galaxy.
