We present Swift UVOT and XRT observations , and visual wavelength spectroscopy of the Type IIb supernova ( SN ) 2009mg , discovered in the Sb galaxy ESO 121-G26 .
The observational properties of SN 2009mg are compared to the prototype Type IIb SNe 1993J and 2008ax , with which we find many similarities .
However , minor differences are discernible including SN 2009mg not exhibiting an initial fast decline or u -band upturn as observed in the comparison objects , and its rise to maximum is somewhat slower leading to slightly broader light curves .
The late-time temporal index of SN 2009mg , determined from 40 days post-explosion , is consistent with the decay rate of SN 1993J , but inconsistent with the decay of ^ { 56 } Co .
This suggests leakage of \gamma -rays out of the ejecta and a stellar mass on the small side of the mass distribution .
Our XRT non-detection provides an upper limit on the mass-loss rate of the progenitor of \dot { M } < 1.5 \times 10 ^ { -5 } M _ { \odot } { yr ^ { -1 } } .
Modelling of the SN light curve indicates a kinetic energy of 0.15 ^ { +0.02 } _ { -0.13 } \times 10 ^ { 51 } { erg } , an ejecta mass of 0.56 ^ { +0.10 } _ { -0.26 } M _ { \sun } and a ^ { 56 } Ni mass of 0.10 \pm 0.01 M _ { \sun } .