iPTF16geu through the lens of thermonuclear explosion models

TL;DR

This study leverages the exceptional brightness of the gravitationally lensed SN Ia iPTF16geu at $z=0.409$ (magnified by $\mu_{TOT} \approx 67.8$) to test a wide range of Type Ia explosion scenarios using UV-sensitive diagnostics. By comparing unresolved host-lens-corrected photometry and host/lens-subtracted spectra to radiative-transfer predictions from pure detonations, double detonations, pure deflagrations, and delayed detonations (notably DDC6 and PDDEL1), the authors identify which explosion channels qualitatively best reproduce the data. They find that delayed-detonation models, particularly DDC6 and PDDEL1, provide the closest agreement with iPTF16geu’s light curves and many optical features, though none fully matches the observed colours and UV flux; the N10 delayed-detonation model can also fit portions of the light curves under revised magnification and extinction assumptions. The work highlights the potential of glSNe to constrain explosion physics and redshift evolution, while acknowledging limitations from microlensing, dust, and modeling uncertainties. Overall, early, high-S/N observations of glSNe will be key to discriminating explosion channels as sample sizes grow with surveys like LSST.

Abstract

The magnification resulting from strong gravitational lensing is a powerful tool to add new constraints to the cosmic evolution of supernova progenitors by enabling the study of distant supernovae that would otherwise not be observable. iPTF16geu is the most well-observed gravitationally lensed supernova (glSN) to date. At a redshift of $z = 0.409$ and magnified by a factor of $\sim$68, extensive photometric and spectroscopic observations have been obtained. The explosion mechanism producing this rare event and differences compared to lower redshift supernovae however have not been explored in detail. Here we compare observations of iPTF16geu to existing radiative transfer simulations of type Ia supernova explosion models selected from the literature. We find that overall the DDC6 and PDDEL1 models, specific variations of the delayed detonation explosion scenario, produce the closest match to the light curves and many absorption features, but struggle to replicate the observed colours and in particular the rest-frame UV. We also investigate the magnification and reddening values required to improve agreement with the selected models. We find some evidence in favour of a UV flux excess that may be intrinsic to iPTF16geu or due to external factors such as host galaxy subtraction and uncertainties in the reddening law. Upcoming surveys will significantly increase the samples of SNe discovered at high redshifts due to strong gravitational lensing. These glSNe will enable tighter constraints on the explosion physics of type Ia supernovae and how this has evolved throughout the Universe.

Figures (12)

• Figure 1: Light curves of iPTF16geu compared against different explosion models. Each panel shows the observer-frame band and its corresponding approximate rest-frame band in brackets. Delayed detonation models are represented by the solid lines, pure detonations are represented by dashed lines, double detonations by a dotted line, and pure deflagrations by a dash-dotted line. Times are shown with respect to peak $B-$band.
• Figure 2: Colour evolution of iPTF16geu and our comparison models. Each panel shows the observer-frame colour and its corresponding rest-frame band in brackets. Times are in reference to the $B$-band peak. Observed colours of iPTF16geu have been corrected for extinction.
• Figure 3: Spectra of iPTF16geu and comparison models at different phases relative to $B$-band maximum. Spectra are normalised by the median flux in the wavelength region shown and offset for clarity. Spectra of iPTF16geu have been corrected for extinction. The grey comparison spectra belong to SN2011fe. We note that N5def model spectra do not extend beyond $\approx$ 25 rest-frame days after $B$-band maximum, therefore only phases before this are shown.
• Figure 4: Zoom-in around the NUV region for iPTF16geu (black) and the explosion models and comparison SNe at phase $\approx$ +8.8 days after maximum $B$-band brigthness. The shaded region corresponds to the NUV region.
• Figure 5: Evolution of the NUV flux ratio for iPTF16geu, our comparison models, SN 2011fe, and PS1-10afx.