In this paper we present the results of the radio light curve and X-ray observations of broad-lined Type Ic SN 2007bg .
The light curve shows three distinct phases of spectral and temporal evolution , implying that the SNe shock likely encountered at least 3 different circumstellar medium regimes .
We interpret this as the progenitor of SN 2007bg having at least two distinct mass-loss episodes ( i.e. , phases 1 and 3 ) during its final stages of evolution , yielding a highly-stratified circumstellar medium .
Modelling the phase 1 light curve as a freely-expanding , synchrotron-emitting shell , self-absorbed by its own radiating electrons , requires a progenitor mass-loss rate of \dot { M } \approx 1.9 \times 10 ^ { -6 } ( v _ { w } / 1000 km s ^ { -1 } ) M _ { \odot } yr ^ { -1 } for the last t \sim 20 ( v _ { w } / 1000 km s ^ { -1 } ) yr before explosion , and a total energy of the radio emitting ejecta of E \approx 1 \times 10 ^ { 48 } erg after 10 days from explosion .
This places SN 2007bg among the most energetic Type Ib/c events .
We interpret the second phase as a sparser ” gap ” region between the two winds stages .
Phase 3 shows a second absorption turn-on before rising to a peak luminosity 2.6 times higher than in phase 1 .
Assuming this luminosity jump is due to a circumstellar medium density enhancement from a faster previous mass-loss episode , we estimate that the phase 3 mass-loss rate could be as high as \dot { M } \la 4.3 \times 10 ^ { -4 } ( v _ { w } / 1000 km s ^ { -1 } ) M _ { \odot } yr ^ { -1 } .
The phase 3 wind would have transitioned directly into the phase 1 wind for a wind speed difference of \approx 2 .
In summary , the radio light curve provides robust evidence for dramatic global changes in at least some Ic-BL progenitors just prior ( \sim 10 - 1000 yr ) to explosion .
The observed luminosity of this SN is the highest observed for a non-gamma-ray-burst broad-lined Type Ic SN , reaching L _ { 8.46 GHz } \approx 1 \times 10 ^ { 29 } erg Hz ^ { -1 } s ^ { -1 } , \sim 567 days after explosion .