The extraordinary SN 1997cy associated with GRB 970514 has been observed photometrically and spectroscopically for nearly 2 yr. At the time of discovery SN 1997cy was the brightest SN ever observed ( M _ { V } \leq - 20.1 , v _ { hel } = 19140 km s ^ { -1 } , H _ { 0 } = 65 km s ^ { -1 } Mpc ^ { -1 } ) .
Up to the last available observations ( 600 days after the GRB ) the total time-integrated flux was equal to or larger than that expected from the complete thermalization of the \gamma -rays produced by 2.3 M _ { \odot } of ^ { 56 } Co .
However , starting already on day 60 the luminosity decline is slower than the ^ { 56 } Co decay rate , indicating that the SN ejecta was interacting with circumstellar material ( CSM ) .
The interaction appeared to weaken around day 550 .
The spectra of SN 1997cy are dominated at all epochs by H \alpha emission , which shows at least 3 components of different widths , as in SN 1988Z .
Several other lines with different widths are also visible , especially at early epochs .
The entire light curve of SN 1997cy is reproduced by a model of the interaction of the very energetic ( E = 5 \times 10 ^ { 52 } ergs ) ejecta of a massive star ( 25 M _ { \odot } ) with the CSM , with some contribution from radioactive decays .
The CSM could have been ejected with a mass-loss rate of \dot { M } \approx 4 \times 10 ^ { -4 } M _ { \odot } yr ^ { -1 } as the progenitor star evolved from a BSG to a RSG about 10 ^ { 4 } yr before the explosion .
The lack of oxygen and magnesium lines in the spectra at nebular phases poses a problem for models requiring high mass progenitors .
The possibility that most of the core material of the progenitor has fallen onto a massive black hole so that the reverse shock dies at the inner edge of the H/He envelope is discussed .