A Speed Bump: SN 2021aefx Shows that Doppler Shift Alone can Explain Early-Excess Blue Flux in Some Type Ia Supernovae

TL;DR

This paper demonstrates that Doppler shift effects due to rapid velocity changes in early supernova spectra can explain the early blue flux excess observed in some Type Ia supernovae, challenging previous explanations.

Contribution

It introduces a Doppler shift-based explanation for early blue flux excess in SNe Ia, emphasizing velocity changes as a key factor.

Findings

Doppler shift from velocity changes can produce early blue flux excess.

High-velocity SNe Ia are more affected by this effect.

Additional parameters may influence early flux diversity.

Abstract

We present early-time photometric and spectroscopic observations of the Type Ia Supernova (SN Ia) 2021aefx. The early time u-band light curve shows an excess flux when compared to normal SNe Ia. We suggest that the early-excess blue flux may be due to a rapid change in spectral velocity in the first few days post explosion, produced by the emission of the Ca II H&K feature passing from the u to the B bands on the time scale of a few days. This effect could be dominant for all SNe Ia which have broad absorption features and early-time velocities over 25,000 km/s. It is likely to be one of the main causes of early-excess u-band flux in SNe Ia which have early-time high-velocities. This effect may also be dominant in the UV filters, as well as in places where the SN spectral energy distribution is quickly rising to longer wavelengths. The rapid change in velocity can only produce a monotonic change (in flux-space) in the u-band. For objects which explode at lower velocities, and have a more structured shape in the early-excess emission, there must also be an additional parameter producing the early-time diversity. More early time observations, in particular early spectra, are required to determine how prominent this effect is within SNe Ia.