Magnification, dust and time-delay constraints from the first resolved strongly lensed Type Ia supernova iPTF16geu

Abstract

We report lensing magnifications, extinction, and time-delay estimates for the first resolved, multiply-imaged Type Ia supernova iPTF16geu, at z = 0.409, using Hubble Space Telescope (HST) observations in combination with supporting ground-based data. Multi-band photometry of the resolved images provides unique information about the differential dimming due to dust in the lensing galaxy. Using HST and Keck AO reference images taken after the SN faded, we obtain a total lensing magnification for iPTF16geu of µ = 67.8 +2.6 −2.9 , accounting for extinction in the host and lensing galaxy. As expected from the symmetry of the system, we measure very short time-delays for the three fainter images with respect to the brightest one: -0.23 ± 0.99, -1.43 ± 0.74 and 1.36 ± 1.07 days. Interestingly, we find large differences between the magnifications of the four supernova images, even after accounting for uncertainties in the extinction corrections: ∆m 1 = −3.88 +0.07 −0.06 , ∆m 2 = −2.99 +0.09 −0.08 , ∆m 3 = −2.19 +0.14 −0.15 and ∆m 4 = −2.40 +0.14 −0.12 mag, discrepant with model predictions suggesting similar image brightnesses. A possible explanation for the large differences is gravitational lensing by substructures, micro-or millilensing, in addition to the large scale lens causing the image separations. We find that the inferred magnification is insensitive to the assumptions about the dust properties in the host and lens galaxy.

Strong gravitational lensing of explosive transients

“…Type z s z l N img m peak µ tot θ max ∆t max PS1-10afx (Section 5. The observed light curves of iPTF16geu have been analyzed by Dhawan et al [363] to confirm very short time delays between images, 1 day. Specifically, time delays with respect to the brightest image are measured to −0.23 ± 0.99, −1.43 ± 0.74, and 1.36 ± 1.07 days.…”

Spectroscopy of the first resolved strongly lensed Type Ia supernova iPTF16geu

“…The main focus of this paper is the analysis of the ground-and space-based spectroscopic observations of iPTF16geu. In order to accurately account for the lens and host galaxy contribution to the observed SN spectra we will use the HST and ground-based photometry described in Goobar et al (2017) and Dhawan et al (2020).…”

“…Furthermore, the lensing analyses in More et al (2017) and Mörtsell et al (2019) indicate that iPTF16geu, especially the brightest image (Image 1), is likely affected by additional "microlensing" from sub-structures in the lens galaxy halo. We make use of the resolved HST and ground-based SN photometry presented in Goobar et al (2017) and Dhawan et al (2020). In Sec.…”

“…To do this, we use broad band photometric data of the lens+host system from the SDSS (ugriz filters), Pan-Starrs (grizy filters) and 2MASS (JHK filters) surveys predating the SN, as well as our HST template images (F390W, F475W, F625W, F814W, F110W and F160W filters, observed after the SN faded). We then use the models of the lens and host galaxy light in Dhawan et al (2020) to compute the fractional fluxes of the lens and host galaxies (blue and red symbols in Fig. 5, respectively) within a 1" diameter aperture for all filters.…”

“…The first case of a spatially resolved SN Ia is iPTF16geu (Goobar et al 2017), a four-image gravitational lens system and the subject of this work. In accompanying papers (Dhawan et al 2020;Mörtsell et al 2019) we report on the photometric measurements of time-delays, extinction and magnification of iPTF16geu as well as the lens model. In addition to the time-delay estimates from photometric measurements, the well known temporal spectral evolution of SNe Ia (Hsiao et al 2007) can also be used to constrain the timedelay between images, through spatially resolved spectroscopy.…”