Improved mass constraints for two nearby strong-lensing elliptical
  galaxies from Hubble Space Telescope Imaging
  We analyse newly obtained Hubble Space Telescope (HST) imaging for two nearby
strong lensing elliptical galaxies, SNL-1 (z = 0.03) and SNL-2 (z = 0.05), in
order to improve the lensing mass constraints. The imaging reveals previously
unseen structure in both the lens galaxies and lensed images. For SNL-1 which
has a well resolved source, we break the mass-vs-shear degeneracy using the
relative magnification information, and measure a lensing mass of 9.49 $\pm$
0.15 $\times$ 10$^{10}$ M$_{\odot}$, a 7 per cent increase on the previous
estimate. For SNL-2 the imaging reveals a bright unresolved component to the
source and this presents additional complexity due to possible AGN microlensing
or variability. We tentatively use the relative magnification information to
constrain the contribution from SNL-2's nearby companion galaxy, measuring a
lensing mass of 12.59 $\pm$ 0.30 $\times$ 10$^{10}$ M$_{\odot}$, a 9 per cent
increase in mass. Our improved lens modelling reduces the mass uncertainty from
5 and 10 per cent to 2 and 3 per cent respectively. Our results support the
conclusions of the previous analysis, with newly measured mass excess
parameters of 1.17 $\pm$ 0.09 and 0.96 $\pm$ 0.10 for SNL-1 and SNL-2, relative
to a Milky-Way like (Kroupa) initial mass function.
