Turning Gravitationally Lensed Supernovae into Cosmological Probes
  Recently, there have been two landmark discoveries of gravitationally lensed
supernovae: the first multiply-imaged SN, "Refsdal", and the first Type Ia SN
resolved into multiple images, SN iPTF16geu. Fitting the multiple light curves
of such objects can deliver measurements of the lensing time delays, which are
the difference in arrival times for the separate images. These measurements
provide precise tests of lens models or constraints on the Hubble constant and
other cosmological parameters that are independent of the local distance
ladder. Over the next decade, accurate time delay measurements will be needed
for the tens to hundreds of lensed SNe to be found by wide-field time-domain
surveys such as LSST and WFIRST. We have developed an open source software
package for simulations and time delay measurements of multiply-imaged SNe,
including an improved characterization of the uncertainty caused by
microlensing. We describe simulations using the package that suggest a
before-peak detection of the leading image enables a more accurate and precise
time delay measurement (by ~1 and ~2 days, respectively), when compared to an
after-peak detection. We also conclude that fitting the effects of microlensing
without an accurate prior often leads to biases in the time delay measurement
and over-fitting to the data, but that employing a Gaussian Process Regression
(GPR) technique is sufficient for determining the uncertainty due to
microlensing.
