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JWST Spectroscopy of SN H0pe: Classification and Time Delays of a Triply-imaged Type Ia Supernova at z = 1.78

Color image of the Arc 2 region of G165. This “Epoch 2” NIRCam image was taken on 2023 April 22, the same day the spectra were acquired. Green dashed circles in the main image are centered on the three SN images, which are labeled A, B, C from north to south. The three stamp images superposed on the main image show magnified images of the SN and its host galaxy with superposed green rectangles marking the open shutters in the NIRSpec MSA. The SN is visible as a blue point source situated at the central shutter of each slitlet. North is up and east is to the left in all images, and red represents the NIRCam F356W and F444W filters, green represents F200W and F277W, and blue represents F090W and F150W. The scale of the main image is indicated by the distance from A to C, which is 14''. Abstract: SN H0pe is a triply imaged supernova (SN) at redshift z=1.78 discovered using the James Webb Space Telescope (JWST). In order to classify the SN spectroscopically and measure the relative time delays of its three images (designated A, B, and C), we acquired NIRSpec follow-up spectroscopy spanning 0.6 to 5 microns. From the high signal-to-noise spectra of the two bright images B and C, we first classify the SN, whose spectra most closely match those of SN 1994D and SN 2013dy, as a Type Ia SN. We identify prominent blueshifted absorption features corresponding to Si II ?6355 and Ca II H ?3970 and K ?3935. We next measure the absolute phases of the three images from our spectra, which allows us to constrain their relative time delays. The absolute phases of the three images, determined by fitting the three spectra to Hsiao07 SN templates, are 6.5+2.4-1.8d, 24.3+3.9-3.9d, and 50.6+16.1-15.3d for the brightest to faintest images. These correspond to relative time delays between Image A and Image B and between Image B and Image C of -122.3+43.7-43.8d and 49.3+12.2-14.7d, respectively. The SALT3-NIR model yields phases and time delays consistent with these values. After unblinding, we additionally explored the effect of using Hsiao07 template spectra for simulations through eighty instead of sixty days past maximum, and found a small (11.5 and 1.0 days, respectively) yet statistically insignificant (~0.25s and ~0.1s) effect on the inferred image delays.