The fast declining Type Ia supernova 2003gs, and evidence for a significant dispersion in near-infrared absolute magnitudes of fast decliners at maximum light

TL;DR

SN 2003gs, a fast-declining Type Ia supernova, challenges previous notions by showing normal near-infrared brightness at maximum light and suggests a bimodal distribution in near-IR absolute magnitudes among such supernovae.

Contribution

This study provides detailed optical and near-IR observations of SN 2003gs, revealing a bimodal distribution in near-IR absolute magnitudes and insights into the opacity changes affecting supernova decline rates.

Findings

SN 2003gs was not subluminous in near-IR at maximum light.

Evidence for a bimodal distribution in near-IR absolute magnitudes of Type Ia SNe.

Reduced outer layer opacities in SN 2003gs lead to faster gamma-ray escape.

Abstract

We obtained optical photometry of SN 2003gs on 49 nights, from 2 to 494 days after T(B_max). We also obtained near-IR photometry on 21 nights. SN 2003gs was the first fast declining Type Ia SN that has been well observed since SN 1999by. While it was subluminous in optical bands compared to more slowly declining Type Ia SNe, it was not subluminous at maximum light in the near-IR bands. There appears to be a bimodal distribution in the near-IR absolute magnitudes of Type Ia SNe at maximum light. Those that peak in the near-IR after T(B_max) are subluminous in the all bands. Those that peak in the near-IR prior to T(B_max), such as SN 2003gs, have effectively the same near-IR absolute magnitudes at maximum light regardless of the decline rate Delta m_15(B). Near-IR spectral evidence suggests that opacities in the outer layers of SN 2003gs are reduced much earlier than for normal Type Ia SNe. That may allow gamma rays that power the luminosity to escape more rapidly and accelerate the decline rate. This conclusion is consistent with the photometric behavior of SN 2003gs in the IR, which indicates a faster than normal decline from approximately normal peak brightness.