We present X-ray , UV/optical , and radio observations of the stripped-envelope , core-collapse supernova ( SN ) 2011ei , one of the least luminous SNe IIb or Ib observed to date .
Our observations begin with a discovery within \sim 1 day of explosion and span several months afterward .
Early optical spectra exhibit broad , Type II-like hydrogen Balmer profiles that subside rapidly and are replaced by Type Ib-like He-rich features on the timescale of one week .
High-cadence monitoring of this transition suggests that absorption attributable to a high velocity ( \gtrsim 12 , 000 km s ^ { -1 } ) H-rich shell is not rare in Type Ib events .
Radio observations imply a shock velocity of v \approx 0.13 c and a progenitor star mass-loss rate of \dot { M } \approx 1.4 \times 10 ^ { -5 } M _ { \odot } yr ^ { -1 } ( assuming wind velocity v _ { w } = 10 ^ { 3 } ~ { } km~ { } s ^ { -1 } ) .
This is consistent with independent constraints from deep X-ray observations with Swift -XRT and Chandra .
Overall , the multi-wavelength properties of SN 2011ei are consistent with the explosion of a lower-mass ( 3 - 4 M _ { \odot } ) , compact ( R _ { * } \lesssim 1 \times 10 ^ { 11 } cm ) , He core star .
The star retained a thin hydrogen envelope at the time of explosion , and was embedded in an inhomogeneous circumstellar wind suggestive of modest episodic mass-loss .
We conclude that SN 2011ei ’ s rapid spectral metamorphosis is indicative of time-dependent classifications that bias estimates of explosion rates for Type IIb and Ib objects , and that important information about a progenitor star ’ s evolutionary state and mass-loss immediately prior to SN explosion can be inferred from timely multi-wavelength observations .