The Most Slowly Declining Type Ia Supernova 2001ay

TL;DR

SN 2001ay is the most slowly declining Type Ia supernova observed, with unique spectral features and a Ni-56 yield comparable to typical Type Ia supernovae, challenging assumptions about luminosity and decline rate correlation.

Contribution

This study provides detailed optical, near-infrared, and ultraviolet observations of SN 2001ay, revealing its unusually slow decline rate and normal Ni-56 production, offering new insights into Type Ia supernova diversity.

Findings

SN 2001ay has the slowest decline rate among Type Ia supernovae.

Its peak brightness is consistent with typical Type Ia supernovae after extinction correction.

The Ni-56 yield is comparable to standard Type Ia supernovae, despite its broad light curve.

Abstract

We present optical and near-infrared photometry, as well as ground-based optical spectra and Hubble Space Telescope ultraviolet spectra, of the Type Ia supernova (SN) 2001ay. At maximum light the Si II and Mg II lines indicated expansion velocities of 14,000 km/sec, while Si III and S II showed velocities of 9,000 km/sec There is also evidence for some unburned carbon at 12,000 km/sec. SN 2001ay exhibited a decline-rate parameter Delta m_15(B) = 0.68 \pm 0.05 mag; this and the B-band photometry at t > +25 d past maximum make it the most slowly declining Type Ia SN yet discovered. Three of four super-Chandrasekhar-mass candidates have decline rates almost as slow as this. After correction for Galactic and host-galaxy extinction, SN 2001ay had M_B = -19.19 and M_V = -19.17 mag at maximum light; thus, it was not overluminous in optical bands. In near-infrared bands it was overluminous only at the 2-sigma level at most. For a rise time of 18 d (explosion to bolometric maximum) the implied Ni-56 yield was (0.58 \pm 0.15)/alpha M_Sun, with alpha = L_max/E_Ni probably in the range 1.0 to 1.2. The Ni-56 yield is comparable to that of many Type Ia supernovae. The "normal" Ni-56 yield and the typical peak optical brightness suggest that the very broad optical light curve is explained by the trapping of the gamma rays in the inner regions.