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/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.
