The Magnificent Five Images of Supernova Refsdal: Time Delay and Magnification Measurements

Abstract

In late 2014, four images of supernova (SN) “Refsdal,” the first known example of a strongly lensed SN with multiple resolved images, were detected in the MACS J1149 galaxy-cluster field. Following the images’ discovery, the SN was predicted to reappear within hundreds of days at a new position ∼8″ away in the field. The observed reappearance in late 2015 makes it possible to carry out Refsdal’s original proposal to use a multiply imaged SN to measure the Hubble constant H 0, since the time delay between appearances should vary inversely with H 0. Moreover, the position, brightness, and timing of the reappearance enable a novel test of the blind predictions of galaxy-cluster models, which are typically constrained only by the positions of multiply imaged galaxies. We have developed a new photometry pipeline that uses DOLPHOT to measure the fluxes of the five images of SN Refsdal from difference images. We apply four separate techniques to perform a blind measurement of the relative time delays and magnification ratios between the last image SX and the earlier images S1–S4. We measure the relative time delay of SX–S1 to be 376.0 − 5.5 + 5.6 days and the relative magnification to be 0.30 − 0.3 + 0.5 . This corresponds to a 1.5% precision on the time delay and 17% precision for the magnification ratios and includes uncertainties due to millilensing and microlensing. In an accompanying paper, we place initial and blind constraints on the value of the Hubble constant.

Constraints on the Hubble constant from supernova Refsdal’s reappearance

“…The apparent F125W-F160W color of S4 is bluer than the other four images of the SN by 0.28±0.05 mag in the first 150 days after its peak brightness (in the observer frame) and 1.8±0.4 mag after 150 days [figure 12 of ( 8 )]. This bluer color implies that (i) the SN does not have a uniform color and (ii) it overlaps a region, called a caustic, where the magnification due to stars changes abruptly, which causes differential magnification ( 8 ). Simulations of lensing of an azimuthally symmetric synthetic SN, with properties similar to those of SN Refsdal ( 37 ), by objects in the foreground cluster lens do not predict microlensing events with color differences as large as that observed for S4 [figure 6 of ( 8 )].…”

“…To avoid human bias, we carried out our analysis without knowledge of the time-delay measurements and the implied value of H 0 . For the light curve (flux as a function of time) of each of the images S1 to SX, we selected a random number, whose value was stored but kept hidden, which was added to the dates associated with the flux measurements, shifting the light curve in time by an unknown amount ( 8 ). Before unblinding the time delay, we were only aware that the relative delay of SX and S1 was in the range of 320 to 380 days (68% confidence level) from a previously published analysis of two epochs of imaging ( 36 ).…”

“…The HST F125W and F160W light curves of image S4 have very strong evidence (>5σ) that the SN overlaps a region of high magnification because of one or more stars (or stellar remnants) in the foreground cluster lens (microlensing) ( 8 ). The apparent F125W-F160W color of S4 is bluer than the other four images of the SN by 0.28±0.05 mag in the first 150 days after its peak brightness (in the observer frame) and 1.8±0.4 mag after 150 days [figure 12 of ( 8 )]. This bluer color implies that (i) the SN does not have a uniform color and (ii) it overlaps a region, called a caustic, where the magnification due to stars changes abruptly, which causes differential magnification ( 8 ).…”

“…This bluer color implies that (i) the SN does not have a uniform color and (ii) it overlaps a region, called a caustic, where the magnification due to stars changes abruptly, which causes differential magnification ( 8 ). Simulations of lensing of an azimuthally symmetric synthetic SN, with properties similar to those of SN Refsdal ( 37 ), by objects in the foreground cluster lens do not predict microlensing events with color differences as large as that observed for S4 [figure 6 of ( 8 )]. Therefore, we exclude the magnification ratio of S4 to S1 because we cannot model accurately the extreme microlensing event, cannot correct its magnification, or assign an uncertainty.…”

“…S1B) ( 7 ). A companion paper ( 8 ) has measured the relative time delay between the images S1 to S4 and image SX as 376.0−5.5+5.6 days, a precision of 1.5%, using Hubble Space Telescope (HST) observations in the near-infrared F125W ( J -band) and F160W ( H -band) broadband filters. If the matter distribution in the foreground MACS J1149 cluster lens were known exactly, time-delay cosmography could provide a measurement of H 0 with equivalent 1.5% precision.…”