We present the first year of Hubble Space Telescope imaging of the unique supernova ( SN ) ’ Refsdal ’ , a gravitationally lensed SN at z = 1.488 \pm 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 ( over \sim 150 days ) to reach a broad peak brightness around 20 April , 2015 .
Using a set of light curve templates constructed from SN 1987A-like peculiar Type II SNe , we measure time delays for the four images relative to S1 of 4 \pm 4 ( for S2 ) , 2 \pm 5 ( S3 ) , and 24 \pm 7 days ( S4 ) .
The measured magnification ratios relative to S1 are 1.15 \pm 0.05 ( S2 ) , 1.01 \pm 0.04 ( S3 ) , and 0.34 \pm 0.02 ( S4 ) .
None of the template light curves fully captures the photometric behavior of SN Refsdal , so we 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 \pm 2 ( S2 ) , 0.6 \pm 3 ( S3 ) , and 27 \pm 8 days ( S4 ) .
The lensing magnification ratios are similarly consistent , measured as 1.17 \pm 0.02 ( S2 ) , 1.00 \pm 0.01 ( S3 ) , and 0.38 \pm 0.02 ( S4 ) .
We compare these measurements against published predictions from lens models , and find that the majority of model predictions are in very good agreement with our measurements .
Finally , we discuss avenues for future improvement of time delay measurements – both for SN Refsdal and for other strongly lensed SNe yet to come .