Observations and modeling for the light curve ( LC ) and spectra of supernova ( SN ) 2005bf are reported .
This SN showed unique features : the LC had two maxima , and declined rapidly after the second maximum , while the spectra showed strengthening He lines whose velocity increased with time .
The double-peaked LC can be reproduced by a double-peaked ^ { 56 } Ni distribution , with most ^ { 56 } Ni at low velocity and a small amount at high velocity .
The rapid post-maximum decline requires a large fraction of the \gamma -rays to escape from the ^ { 56 } Ni-dominated region , possibly because of low-density “ holes ” .
The presence of Balmer lines in the spectrum suggests that the He layer of the progenitor was substantially intact .
Increasing \gamma -ray deposition in the He layer due to enhanced \gamma -ray escape from the ^ { 56 } Ni-dominated region may explain both the delayed strengthening and the increasing velocity of the He lines .
The SN has massive ejecta ( \sim 6 - 7 M _ { \odot } ) , normal kinetic energy ( \sim 1.0 - 1.5 \times 10 ^ { 51 } ergs ) , high peak bolometric luminosity ( \sim 5 \times 10 ^ { 42 } erg s ^ { -1 } ) for an epoch as late as \sim 40 days , and a large ^ { 56 } Ni mass ( \sim 0.32 M _ { \odot } ) .
These properties , and the presence of a small amount of H suggest that the progenitor was initially massive ( M \sim 25 - 30 M _ { \odot } ) and had lost most of its H envelope , and was possibly a WN star .
The double-peaked ^ { 56 } Ni distribution suggests that the explosion may have formed jets that did not reach the He layer .
The properties of SN 2005bf resemble those of the explosion of Cassiopeia A .