A year-long plateau in the late-time near-infrared light curves of type Ia supernovae

The light curves of type Ia supernovae are routinely used to constrain cosmology models. Driven by radioactive decay of ⁵⁶Ni, the light curves steadily decline over time, but after 150 d post-explosion the near-infrared portion is poorly characterized. We report a year-long plateau in the near-infrared light curve at 150-500 d, followed by a second decline phase accompanied by a possible appearance of [Fe I] emission lines. This near-infrared plateau contrasts sharply with type IIP plateaux and requires a new physical mechanism. We suggest a masking of the `near-infrared catastrophe'—a predicted, yet unobserved, sharp light-curve decline—by scattering of ultraviolet photons to longer wavelengths. The transition off the plateau could be due to a change in the dominant ionization state of the supernova ejecta. Our results help explain the complex radiative transfer processes that take place in type Ia supernovae and enhance their use as `standard candles'.