Day 1027

Friday.


1512.05591
Problems using ratios of galaxy shape moments on requirements for weak lensing surveys
Israel, Kitching, Massey

The shapes of galaxies can be quantified by ratios of their quadruple moments.  For faint galaxies, observational noise can make the denominator close to zero, so the ratios become ill-defined.  Knowledge of these ratios (i.e. their measured standard deviation) is commonly used to assess the efficiency of WL surveys.  Since the requirements cannot be formally tested for faint galaxies, explore two complementary mitigation strategies.  In many WL contexts, the most problematic sources can be removed by a cut in measured size.  Investigate how a size cuts affects the required precision of the charge transfer inefficiency model and find slightly wider tolerance margins compared to the full size distribution.  However, subtle cases in the data analysis chain may be introduced.  Instead, as the second strategy, propose requirements directly on the quadrupole moments themselves.  To optimally exploit a Stage-IV DE survey, find that the mean and standard deviation of a population of galaxies' quadrupole moments must be known to better than 1.4e-3 arcsec^2, or the Stokes parameters to 1.9e-3 arcsec^2.  This testable requirements can now form the basis for future performance validation, or for proportioning the requirements between subsystems to ensure unbiased cosmo parameter inference.


1512.05734
SN Refsdal : Photometry and time delay measurements of the first Einstein Cross supernova
Rodney, et al

Present the first year HST imaging of SN Refsdal, a gravitationally lensed SN at z=1.488±0.001 with multiple images behind the galaxy cluster MACS J1149.6+2223.  The first four observed images of SN Refsdal (images S1-S4) exhibited a slow rise (one ~150 days) to reach a broad peak brightness around 20 April 2015.  Using a set of light curve template constructed from the family of SN 1987A-like peculiar Type II SNe, measure time delays for the 4 images relative to S1 of 4±4 (for S2), 2±5 (S3), and 24±7 days (S4).  The measured magnification ratios relative to S1 are 1.15±0.05 (S2), 1.01±0.04 (S3), and 0.34±0.01 (S4).  Find that none of the template light curves fully captures the photometric behavior of SN Refsdal, so also derive complementary measurements for these parameters using polynomials to represent the intrinsic light curve shape.  These more flexible fits deliver fully consistent time delays of 7±2 days (S2), 0.6±3 days (S3), and 27±8 days (S4).  The lensing magnification ratios are similarly consistent, measured as 1.17±0.02 (S2), 1.00±0.01 (S3), and 0.38±0.02 (S4).  Compare these measurements against published predictions from lens models, and find that the majority of model predictions are in very good agreement with the measurements.  Finally, discuss avenues for future improvement of time delay measurements -- both for SN Refsdal and for other strongly lensed SNe yet to come.