THE MOST SLOWLY DECLINING TYPE Ia SUPERNOVA 2001ay

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 s^(–1), while Si III and S II showed velocities of 9000 km s^(–1). There is also evidence for some unburned carbon at 12,000 km s^(–1). SN 2001ay exhibited a decline-rate parameter of Δm_(15)(B) = 0.68 ± 0.05 mag; this and the B-band photometry at t ≳+25 day past maximum make it the most slowly declining Type Ia SN yet discovered. Three of the 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 2σ level at most. For a rise time of 18 days (explosion to bolometric maximum) the implied ^(56)Ni yield was (0.58 ± 0.15)/α M_☉, with α = L_(max)/E_(Ni) probably in the range 1.0-1.2. The ^(56)Ni yield is comparable to that of many Type Ia SNe. The "normal" 56Ni yield and the typical peak optical brightness suggest that the very broad optical light curve is explained by the trapping of γ rays in the inner regions.